# Xiansheng 708B headphone tube amp



## drarthurwells

The Xian Sheng 708B is a real nice sound for low bucks (close to $200 USD). Available in the USA from:

http://www.pacificvalve.us/Headphones.html

 I got mine used from a fellow head-fier and was interested because I have hundreds of tubes, and many of them are type used in this amp. I thought it would be fun to play with in tube changing.

 It uses five tubes: 6n1, 6n3, 6Z4 and two 6DJ8/6922. The 6N1 is the front tube in the window, the 6Z4 rectifier is a lone tube in the back, while three tubes in a row are the front 6N3 the middle 6DJ8 left driver, and the rear 6DJ8 right driver. Russian equivalents are 6N1P (instead of the Chinese 6N1) and the 6N3P (instead of the Chinese 6N3). 

 It sounds OK with the Chinese tubes supplied but improves very much from tube changes.

 I first changed the 6N1 for a new Russian military 6N1P EB (from the 70s or 80s). The sound became cleaner and clearer - sounded better with just this one change.

 I next changed the Chinese drivers with two Sylvania JAN 6DJ8 and loved the sound with Mike Oldfield's Amorok - beautiful timbre.

 However, this CD sounds good with almost anything so later I tried some demanding but well recorded symphonic works.

 Started picking up some euphonic coloration in some tones here and there so I switched to some Amperex-Holland. This was a real nice sound and eliminated any euphonic coloration.

 However, I wanted to sharpen the soft tone edges slightly so I used one of my favorites - RCA 6DJ8 tubes, stamped UK on the tube, but actually made in Russia. I bought a bunch of these new and they work really well. The Russian 6n23p (EB or IV for military) made before 1985 are as good.

 Getting good sound now - you could spend more and not get any better sound.

 This is a great choice in a headphone amp at the price point. 

 The rectifier is a Chinese 6Z4 (not the old non-Chinese 6Z4), but you can't use the 6X4 instead. Don't try.

 I next will try some Toshiba 6DJ8, some Mullards (GB made), Russian 6n23p EB, and whatever else I can find. I also will try a Russian miltary 6n3p tube in place of the Chinese 6n3.

 The amp nicely drives the AKG K701 and Sony SA5000 BTW.

 All the amps I have owned offer good sound. You have to find the one that suits you. Some seem more of a bargain than others, offering sound that matches more expensive ones - at least in my tastes.

 The Xian Sheng is an outstanding bargain.

 P.S. Toshiba 6DJ8 tubes are good - sharp tone edge and transient attack bite, with good natural timbre. However, this means that if you have an "edgy" CD or source, you will hear the harshness more than with some other tubes (like Amperex-Holland). However the RCA 6DJ8, stamped UK, are the best.

 P.S. Works good as preamp in a speaker system. Much above average in comparison with the other headphone amps, that can serve as a preamp, that I have owned. Plenty of gain for my amp (a real problem for other headphone amps as preamps). Well worth the price for use as a home preamp alone.


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## tourmaline

Compared to bada ph 12?


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## drarthurwells

For me the Bada PH12 is the best I have owned - though I can see how others might prefer some of the amps I have owned to the Bada.

 Something about the soundstage, instrument separation, 3-D tone images pinpointed in empty space - just a certain transparency and realism that makes me there with the music more often than with other amps.

 The Bada is my loving wife while the other amps are brief flings - exciting for a short time but they quickly lose their luster - and then I am very happy to return to the Bada for the "real thing". I enjoy new amps for awhile, change tubes ad really get to know and like them. But after a few days I go back to the Bada and really appreciate its superiority - no matter how good the other amp sounded. The day, I don't discover that, is the day the new amp replaces the Bada.

 Some will say it is force of habit - that I am too accustomed to the Bada - and that if I only owned their amp I would realize the Bada was inferior to theirs. Might be true - send me your amp and let me hear (lol).

 As far as the Xiangsheng vs. Bada PH12, in addition to better 3-D imaging, the Bada PH12 has a better soundstage and more dynamc impact, with more power - greater realism and transparency overall.

 However, the Xiangsheng is very impressive in comparison to other lower cost all-tube amps, and is a cost-effective champ when its price is factored in.


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## tourmaline

Quote:


  Originally Posted by *drarthurwells* 
_
 Some will say it is force of habit - that I am too accustomed to the Bada - and that if I only owned their amp I would realize the Bada was inferior to theirs. Might be true - send me your amp and let me hear._

 

Well, i also owned an expensive maxed out stax system, wich was inferior to the bada ph12 and hd650 combo in a lot of respects! Especially timbral correctness and the rich full body is a treat. The electrostats sounds mostly very detailed but thin and dull. Also bass is a weakness of electrostats in general! The bada sounds natural, like the real instruments and give you a great live feeling, like really being there, an electrostat is more detailed(also depends on source/headphone combo) but fails in other departments.

 Until the bada, i never heard a heaphone amp that sounds so real! And i tried a couple!

 Others might have more detail, but they never have the timbre and natural, neutral sound as the bada and don't portrait the live feeling as i have with the bada! In other words, if you wanna listen to real livelike music, the bada is a great amp, if you want micro-detail, look elsewhere.

 Also heard alot of more expensive amps wich don't have the correct timbre and body as the bada, also poweramps for speakers!


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## Fang

How does this compare to the ming da 66?


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## drarthurwells

Quote:


  Originally Posted by *Fang* 
_How does this compare to the ming da 66?_

 

The Xian Sheng ($200), Ming Da MC66AE ($300), and G & W T2.6F ($355) are all good for the money.

 Xian Sheng - nice timbre, good detail, good dynamics, very pleasing to listen to, good preamp in a home system with plenty of gain (unusual for a preamp/headphone amp) - need to use the tubes I specified. Some slight bass shyness and soundstage restriction.

 Ming Da has a little fuller sound with more impact, richer timbre, limited tube options (the power tube is a 12AX7 and offers many change opportunities though, and the Chinese 6N6 drivers are surprisingly good)). Bass is a little tubby.

 G&W T2.6F offers plenty of tube changing (two 6DJ8/6922 and mosfets output) and is very clean, detailed, neutral ( tends to be lean but rich enough with two Sylvania JAN 6922), dynamic.

 All offer great listening pleasure for the money.

 A step up in price is the ASL MG Head III, and some would prefer this one to the others.

 I haven't heard the Dared but people say good things about it also.

 The $400 and under category offers some real good amps.

 There is so much overpriced and overhyped stuff around - buyer beware - high price does not always mean high sound quality. Since judging sound quaility can be very difficult and subjective, it is easy to believe the hype and think you have great sound quality when you actually don't.


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## titoyd

AfterI have own the 1st one of this model 708B...I got it smoke on the R-Core Transformer!Well I switching it off and wait till it cold.Open it! nothing strange but the R-Core almost Brown and Smell of Burnt...
 I put it on it still working but after 30 mn it again Smoke came from the R-Core...I send it back to China after that and still wait it will send me back but i can't wait then i bought the 2nd again!Well Same problem after hours it smoke again.!Now i'm sick what wrong with my these two amplifiers?
	

	
	
		
		

		
		
	


	



 By the way it sound incredible for suck a small machine.I also owned VAL E-10(138 $) that have with 2 inputs so i hook 2 PCs on it.(Mac/PC)But sound are not good as 708B.That why I bought 2 of them.But I never have a chance to get it to work as i wish too...Any one help me please what I should do!

*-I hook a pair of highspeakers from VAL M-20MKII(108$)on ebay! thought the out put behind not the earphone out put.Are this should be a problems?
 -I have changed the 6N11 to 6922 gold pin from Tesla Should this be a Problem too?*


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## drarthurwells

Quote:


  Originally Posted by *titoyd* 
_





 AfterI have own the 1st one of this model 708B...I got it smoke on the R-Core Transformer!Well I switching it off and wait till it cold.Open it! nothing strange but the R-Core almost Brown and Smell of Burnt...
 I put it on it still working but after 30 mn it again Smoke came from the R-Core...I send it back to China after that and still wait it will send me back but i can't wait then i bought the 2nd again!Well Same problem after hours it smoke again.!Now i'm sick what wrong with my these two amplifiers?
	

	
	
		
		

		
		
	


	



 By the way it sound incredible for suck a small machine.I also owned VAL E-10(138 $) that have with 2 inputs so i hook 2 PCs on it.(Mac/PC)But sound are not good as 708B.That why I bought 2 of them.But I never have a chance to get it to work as i wish too...Any one help me please what I should do!

*-I hook a pair of highspeakers from VAL M-20MKII(108$)on ebay! thought the out put behind not the earphone out put.Are this should be a problems?
 -I have changed the 6N11 to 6922 gold pin from Tesla Should this be a Problem too?*_

 


 Does it do the same with 6DJ8 tubes? I have only used 6DJ8 tubes.

 6922 tube draws slightly more power but this shouldn't be a problem.

 I have used mine as a preamp in my home video system for over 12 hours at a time with no problems - no sign of overheating.


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## drarthurwells

deleted duplicate post


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## mirumu

I bought a Xiang Sheng 708b myself recently and my experience is similar, the sound is great for the money. I like the sound better than my Xin Supermacro IV personally, I feel it has more impact, timbre and better treble extension. So far I have swapped the Chinese 6N11 pair for a matched and balanced pair of JJ E88CC tubes. I realise opinions of JJ tubes vary but I personally felt it was quite an improvement over the original ones, the sound became cleaner and smoother, especially for classical music. I was mainly just trying to see if current production tubes would sound acceptable since they are very easy and cheap to obtain. I also plan to pick up some NOS tubes to try, the way I see it, things should only improve. I'd like to shield the transformer more, there is a tiny bit of hum that creeps into the headphone output, I believe because the wire is so close to the transformer. I'm also considering upgrading the large capactiors although that's more of a long term plan.

 titoyd, I think you were the person who emailed me earlier via ebay regarding your amplifiers. As I mentioned in the email, I haven't seen any smoke from mine even after many hours of use. There is a definite smell that is a little like burning but to me that is a smell I associate with tubes, I don't believe there is anything actually wrong with that smell. I hope you find an answer to the smoking because this really is a lovely sounding amp.


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## drarthurwells

My 708B is dead silent and runs cool.

 Hum is usually tube related - some tubes are noisier than others even though otherwise good.

 Replacing the front Chinese 6N1 tube (in the window) with a Russian 6N1P EB military tube is important also for good sound.


 You can have all kinds of problems if you put the wrong tube in the wrong socket.

 The front window tube is a 6N1 (Chinese) or 6N1P (Russian).

 The single rear tube is a Chinese minature (not old USA) 6Z4.

 the three tubes in a row are 6N3 or 6N3P (in front) followed by two 6DJ8 or 6922 (in the middle and rear positions).

 Be sure you get the tubes in the correct positions.


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## Fang

Thanks for the comparing the amps, one more question though, which one is more suited for beyers?


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## titoyd

Quote:


  Originally Posted by *mirumu* 
_I bought a Xiang Sheng 708b myself recently and my experience is similar, the sound is great for the money. I like the sound better than my Xin Supermacro IV personally, I feel it has more impact, timbre and better treble extension. So far I have swapped the Chinese 6N11 pair for a matched and balanced pair of JJ E88CC tubes. I realise opinions of JJ tubes vary but I personally felt it was quite an improvement over the original ones, the sound became cleaner and smoother, especially for classical music. I was mainly just trying to see if current production tubes would sound acceptable since they are very easy and cheap to obtain. I also plan to pick up some NOS tubes to try, the way I see it, things should only improve. I'd like to shield the transformer more, there is a tiny bit of hum that creeps into the headphone output, I believe because the wire is so close to the transformer. I'm also considering upgrading the large capactiors although that's more of a long term plan.

 titoyd, I think you were the person who emailed me earlier via ebay regarding your amplifiers. As I mentioned in the email, I haven't seen any smoke from mine even after many hours of use. There is a definite smell that is a little like burning but to me that is a smell I associate with tubes, I don't believe there is anything actually wrong with that smell. I hope you find an answer to the smoking because this really is a lovely sounding amp._

 

Hello Mirumu and drarthurwell,
 Yes it's me who wrote to you and aalso many buyer in the ebay that bought this 708B from jennychan...!She is good seller and also sold such cheap price for the good amplifier event the big amplifier...Don't know if she won some money back after that?
 Nobody had same problem like me...as i told your guy I had two of this 708B and they all smoked!I prefer to get it smell than the smoke and heating from the R-Core one.every new tube amplifier will smell after you turn it on at the first time but not longer than 4-5 hours listening it will not smeel any.But smoke seem it make me worry about that i still not see any problems;since i have wait till it cold and checking inside there is nothing strange except the r-core that gettng brown like burning.The VAL E-10 i had didn't use the same transformer like 708B...But it use only one tube (6922)ECC88...The sound are ok for who want to listen just for a pop musics...I love the sound of 708B...
 But i may be try the pre-amplifier from the Xiansheng 728 III Vacuum Tube Preamp one cause i don't listen too much the headphone...so i can hook my VAL M-20MKII on it...also alots of inputs...But i still afraid to get the same problems like 708B...Because they look like same build...
 I beleive i'm not mistake on the positions of all the tubes even the second one i didn't event open the cap to change the tubes but it still gave me same problems...
 I'm living in france and here we use 230 volts like some europeane countries here too.the 708B it wrote that it 's 220-240 Volts so there should not be the problems...The 6922 tubes also should not be the problem...!
	

	
	
		
		

		
		
	


	




but seem the r-core sending and working too much to drive all the tubes (5 tubes) don't your guy thing so?
 I also owned the Mr'Liung 2002 EL34but I didn't turn it on often because after the fews hours of using the sound balance are gond ;..the left chanel getting lower than the right chanel.This may cause from the heat of the tubes and i have found many user have same problems like me from the mr,Liung 2002.But the 708B no one have same problems with me.If it happen with only one machine I still get it but all the two of mine theay had the same problems...so let say may be my electric circuit should give this problem ? hmmm!but why not the other Head phone like VAL E-10 i got one..It run just fine and so far so good!But i want the 708bB run fine like the other...


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## Hirsch

Quote:


  Originally Posted by *drarthurwells* 
_I first changed the 6N1 for a new Russian military 6N1P EB (from the 70s or 80s). The sound became cleaner and clearer - sounded better with just this one change._

 

A 5670 is usable in the driver sockets, unless the circuit has changed dramatically from 708A, whch I owned. Amperex are very good. However, the best of them, the Westinghouse 396A is not. It exposes too much of the weaknesses of the amp.

  Quote:


 I next changed the Chinese drivers with two Sylvania JAN 6DJ8 and loved the sound with Mike Oldfield's Amorok - beautiful timbre. 
 

I found that staying within limits was big help. Sylvania's are a good choice. I used some RSD PCC88's from Eastern Europe that worked particularly well. This is another slot in which you don't want to get out of hand. Premium tubes, such as Siemens 6922's did not sound as good as the cheaper ones.

  Quote:


 
 The rectifier is a Chinese 6Z4 (not the old non-Chinese 6Z4), but you can't use the 6X4 instead. Don't try. 
 

To use a 6X4, you need to rewire the rectifier socket pin out. This is not difficult to do. The problem comes later, when you try and put the cover on, and discover that the 6X4 is taller than the case 
	

	
	
		
		

		
		
	


	




 Has the 708B eliminated that awful turn-on hum that plagued the 708A? It didn't hurt anything, but you didn't want to be wearing headphones until the hum had passed.

 I found the Xiang Sheng to be a beautiful example of synergistic design. Stay within the limits of what it's trying to do, and you get beautiful harmonic structure. Try to go overboard, and improve too much, and you lose what the amp does well.


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## drarthurwells

Thanks for your valuable input Hirsch.

 Do you know what the front window tube 6N1 (or 6N1P) does?

 Is it a "preamp" part of the input gain for the drivers?

 How about the 6N3 (or 6N3P) located in front of the two 6DJ8 drivers? Is this 6N3 also a part of the input gain that feeds the drivers?

 Is switching from the 6Z4 to a 6X4 simply a matter of re-soldering some socket pins? No other circuit changes required?

 Although the Bada PH12 is more of a realistic sound, and is my main headphone amp, I intend to keep the 708B as a spare in case my home speaker preamp (or the Bada) fails. I may one day convert the 6Z4 to a 6X4 as I have plenty of 6X4s on hand.


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## mirumu

Quote:


  Originally Posted by *titoyd* 
_I'm living in france and here we use 230 volts like some europeane countries here too.the 708B it wrote that it 's 220-240 Volts so there should not be the problems...The 6922 tubes also should not be the problem...!
	

	
	
		
		

		
			





but seem the r-core sending and working too much to drive all the tubes (5 tubes) don't your guy thing so?_

 

I had a look in some old electronics books I have around and it would seem that a smoking transformer usually means the input voltage is too high. This seems a little strange to me because where I live in New Zealand we use 230V, 50HZ much like France, and the supply may vary between 220V and 240V depending on the location. This would make me think that if voltage was the problem I would have a smoking transformer too. I wonder if it is possible that maybe the actual voltage on your wall socket might be above 240V? I don't know about France, but in Australia, South Africa and some parts of New Zealand 250V is not unheard of. If that was the problem, a power conditioner might help although they can be expensive.


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## mirumu

Quote:


  Originally Posted by *drarthurwells* 
_My 708B is dead silent and runs cool.

 Hum is usually tube related - some tubes are noisier than others even though otherwise good._

 

In my case I'm pretty sure it's not the tubes, I've tried a few different ones but the hum is the same. It also doesn't increase when you turn the volume up (so it's getting in after the signal has been amplified) and mostly isn't really that noticable, it's only when the music is totally silent that I can just pick it. I might be wrong but I think it is a 50Hz hum, and the power frequency here is 50Hz which makes me suspect it's the power supply circuitry. I know the power input is clean, I have it plugged into a power smoothing box with RFI filtration. Next time I open it up I might try to find if my suspicion is right about the transformer and the signal wire being too close.


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## titoyd

Quote:


  Originally Posted by *mirumu* 
_In my case I'm pretty sure it's not the tubes, I've tried a few different ones but the hum is the same. It also doesn't increase when you turn the volume up (so it's getting in after the signal has been amplified) and mostly isn't really that noticable, it's only when the music is totally silent that I can just pick it. I might be wrong but I think it is a 50Hz hum, and the power frequency here is 50Hz which makes me suspect it's the power supply circuitry. I know the power input is clean, I have it plugged into a power smoothing box with RFI filtration. Next time I open it up I might try to find if my suspicion is right about the transformer and the signal wire being too close._

 

Have you tried different head phones?Or your RCA cables that conect from the CD player If it's the right signal..Some cable use only one direction signal flow...So i have been over a month for wong signal and didn't notice about that and all that time i have hum!but since i checking the cables directions well!it's.

 Today I have received my 708B back from china!+ A Shengya CD-10 (470 $ included shipping UPS 7 days arrived from china If your guys would like to buy some try to contact the guy name acount on ebay's HOTPRODUCTSTORE he's very kind and serius person Ask for the best price that he could offer,Don't worry about he will stole your money it will not happen I bought alots of time with him aready.It you would like to be sure tell him it's from me "mr,Dech TT" introduce about this store for you he will be very please to serve you as well.Sorry not the advertise but once i have found this person, someone goods and cheapest price for such a good products like this i prefer to present if your guy interesting too.He's Not writing too much because he's not know too much english but he know what you want so you will not miss what you will get.Also the 708B i didn't bought from him but he replace it for me for free!Cool guy and i just send the machine back to him that's.Well I still not turn that 708B on yet because i afraid to burn it again so i will wait till i found the right answer or solved my problems first.I have a Fr'eind's Father he know about this electric and do like a hobbie for his retry...so I will give it to him the one that burnt to take alook...May be it will solved after all so I will keep in touch to tell your guys all.
 For the moment I just have fun with Shengya CD-10...Can make me forget a problem for a whiles...So! Sweet sound.


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## Hirsch

Quote:


  Originally Posted by *drarthurwells* 
_Thanks for your valuable input Hirsch.

 Do you know what the front window tube 6N1 (or 6N1P) does?

 Is it a "preamp" part of the input gain for the drivers?

 How about the 6N3 (or 6N3P) located in front of the two 6DJ8 drivers? Is this 6N3 also a part of the input gain that feeds the drivers?_

 

The tube in the window, and the front tube on the side are the gain tubes. 6DJ8's are the outputs. There is a separate one of each for each channel.

  Quote:


 
 Is switching from the 6Z4 to a 6X4 simply a matter of re-soldering some socket pins? No other circuit changes required? 
 

Yep. This is easier said than done, as the socket attaches to pads on the board, IIRC. However, it was a pretty simple matter to bend the affected pins upward, and run wires to the correct pads for the 6X4 pinout. No other circuit changes were needed. However, the Mullard 6X4 is simply too tall for the case, so I had to change it back to stock to close the cover.


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## mirumu

Quote:


  Originally Posted by *Hirsch* 
_The tube in the window, and the front tube on the side are the gain tubes. 6DJ8's are the outputs. There is a separate one of each for each channel._

 

Ah, thank you for that information, I wondered if maybe the 6N1 and 6N3 were being used that way since from their appearance they are twin triodes capable of handling two signals. I would assume then that these two gain tubes would most likely have the largest effect on the sound?


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## drarthurwells

Quote:


  Originally Posted by *mirumu* 
_Ah, thank you for that information, I wondered if maybe the 6N1 and 6N3 were being used that way since from their appearance they are twin triodes capable of handling two signals. I would assume then that these two gain tubes would most likely have the largest effect on the sound?_

 

Yes.

 Big impact on sound.

 Use a 6N1P EB (make sure it is a EB military tube and not the modern Russian 6N1P) in place of the 6N1.

 This alone only will improve the sound some.

 As drivers, use two RCA 6DJ8 stamped UK (which I have for $10 each shipped anywhere) or two Russian 6N23P EB (make sure it is a EB military tube and not the modern Russian 6N23P). 

 Use a 6N3P EB (make sure it is a EB military tube and not the modern Russian 6N3P) in place of the 6N3. As the one in front of the two drivers.

 Those old Russian military tubes are great - the modern ones suck.


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## Robfow

I've just bought this amp from someone. After some listening, I've realised that there's a hissing sound that comes on in the right channel every minute or so. I'm wonder if any of us have this same problem as well? 

 cheers!
 Robfow


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## Hirsch

Quote:


  Originally Posted by *Robfow* 
_I've just bought this amp from someone. After some listening, I've realised that there's a hissing sound that comes on in the right channel every minute or so. I'm wonder if any of us have this same problem as well? 

 cheers!
 Robfow_

 

This is likely to be tube noise. To diagnose:

 1) Exchange the gain tubes (the one in the window and the one closest to the front on the side). If the noise changes sides, you need to replace the noisy tube. (Note, check the tubes to make sure that the ones you're exchanging are the same tube type, just to be sure).

 2) If Step 1 didn't find the tube, exchange the output tubes and listen again.

 If the sound switches sides on one of these steps, you've located a tube to change. If the sound stays put, then there is likely to be another problem that's going to be harder to find.


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## Robfow

Quote:


  Originally Posted by *Hirsch* 
_This is likely to be tube noise. To diagnose:

 1) Exchange the gain tubes (the one in the window and the one closest to the front on the side). If the noise changes sides, you need to replace the noisy tube. (Note, check the tubes to make sure that the ones you're exchanging are the same tube type, just to be sure).

 2) If Step 1 didn't find the tube, exchange the output tubes and listen again.

 If the sound switches sides on one of these steps, you've located a tube to change. If the sound stays put, then there is likely to be another problem that's going to be harder to find._

 

Somehow after swapping the tubes around, the noise seems to be gone already. Might be the connection that caused it? Keeping my fingers crossed that it won't come back.

 Thanks!


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## Hirsch

Quote:


  Originally Posted by *Robfow* 
_Somehow after swapping the tubes around, the noise seems to be gone already. Might be the connection that caused it? Keeping my fingers crossed that it won't come back.

 Thanks!_

 

Yep. A tube making a poor socket connection can be noisy. One way to clean tube pins is to put the tube in and out of a socket a couple of times. You may never hear that noise again. 

 If it comes back, you can try cleaning the pins with something like Caig DeOxit. I do that, and then follow with a silver treatment (I use Quicksilver AVExtreme). That seems to clean up any issues caused by poor contact that I've run into. No real need to do that if it's already sounding good (although I do it anyway). 

 Note: silver treatments are highly conductive, so if you use them on tube pins, you've got to use great care not to accidentally create an electrical connection between two pins, thereby causing a short.


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## Robfow

The noise came back. I opened up the casing and took the tubes in and out of the sockets a few times. Think it is alright again now.


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## nakedtoes

yeah the 708B is a value for money tube amp.. it actually has very good synergy with grado phones.. It sound even better then the stock Cayin HA-1A except the soundstage.. After i rolled the tubes in the Cayin with Mullard CDV4004, CV4003, 2 x JJ EL84... It supress the 708B way ahead with even wider soundstage, better instruments seperation, bass impact...


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## mirumu

I just though I'd add an update regarding the hum I was experiencing with the 708B. I've been trying out some Sennheiser HD595s and I can't hear the hum at all, in fact with these headphones I can't even hear any background hiss until the volume knob reaches 12 o'clock. This makes me think that it was my Shure E500s exposing the hum and that possibly it is just inaudible on larger headphones. I would not think the impedance difference is a significant factor since the E500s are 32 ohm and these HD595s are 50 ohm.

 Also, I don't know if anyone else with one of these amps was worried about the 6Z4 having no direct substitutes, it's quite a hard hard tube to find online. After much searching I tracked down two places selling it, Audiotriodes in France and Diyclub.biz in Hong Kong. I ordered a bunch of them from Audiotriodes myself and they arrived with no problems. It kind of goes without saying but I'm not associated with either of these places, just passing it on incase it helps someone find some spares of this seemingly hard to find tube.


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## ymngzhou

A hongkonger builds it, am I right? sounds like very good for the price. by how much tubes could affect the sound traits?


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## mirumu

Quote:


  Originally Posted by *ymngzhou* 
_A hongkonger builds it, am I right? sounds like very good for the price. by how much tubes could affect the sound traits?_

 

Xiang Sheng is a company in China but they seem to have many internet dealers in Hong Kong.

 In my experience the tubes have quite a noticeable effect on the sound, especially the 6N3 and the 6N1.


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## drarthurwells

Quote:


  Originally Posted by *mirumu* 
_Xiang Sheng is a company in China but they seem to have many internet dealers in Hong Kong.

 In my experience the tubes have quite a noticeable effect on the sound, especially the 6N3 and the 6N1._

 

I listened once with the stock set up to be sure it was working, and immediately changed the 6N1 to a Russian 6N1P EB and it seemed to improve the sound - but this was based on just a few minutes experience.

 Then I changed the Chinese 6N23 (6DJ8) to the Amperex-Holland 6DJ8 and found an improvement. Over the next few days I tried a variety of 6DJ8 and settled on the RCA stamped made in UK.

 I am still waiting on the 6N3P EB tubes from Russia which I suspect will significantly change the sound as did chaging out the two 6DJ8.

 Now, just got a message from j214 who advised me the front 6N1 tube is just cosmetic and not part of the signal circuit.

 I initially suspected it was just cosmetic but it is powered so I wasn't sure. Changing it had seemed to improve the sound but I only checked this for a few minutes before changing the other tubes.

 There are wires running to the circuit but these are likely just heater wires. So I simply pulled the 6N1P tube out and the amp worked without it - proof that this front window tubes is for looks but not function.


----------



## Nugget

Just put in a bid on a 708B, and realized I have no idea how one would play with my DT770/80s. Any synergy there, or notable problems?


----------



## mirumu

Quote:


  Originally Posted by *drarthurwells* 
_Now, just got a message from j214 who advised me the front 6N1 tube is just cosmetic and not part of the signal circuit.

 I initially suspected it was just cosmetic but it is powered so I wasn't sure. Changing it had seemed to improve the sound but I only checked this for a few minutes before changing the other tubes.

 There are wires running to the circuit but these are likely just heater wires. So I simply pulled the 6N1P tube out and the amp worked without it - proof that this front window tubes is for looks but not function._

 

Haha, I had seen that there were a bunch of yellow/orange LEDs around the outside edge of the window just to give a good looking tube-style glow but it's hilarious that the 6N1 is only there for the same purpose. Glad I didn't spend any money buying quality replacements. On the plus side though, I'm very glad that those wires going to the 6N1 aren't signal wires, it didn't seem a good idea to carry the signal all the way out to the front window along those thin wires.

 Nugget, I haven't heard the 708B with the DT770/80s but in my experience, this amp provides a lot of driving power. From this perspective I would be surprised if it had a problem driving 32 ohm headphones like those. I can't otherwise comment on the synergy though.

 Edit: My bad, I see the DT770/80 is 80ohms. I still don't think that load would give the 708B any problems, I have driven up to 110ohm headphones from mine without issue.


----------



## titoyd

Huh! I just get back and ready to run my 708B..since it was burned and now I'm not afraid any more..i change some the tubes.
 -6N3 to 5670 sylvania
 -6N1,6N11 to 6922 Gold pins TESLA and amperex...
 I just surprise all the sounds it's so wide and big improvement after change these tubes.
 Very Happy now no more smoke and my transformer not over heat anymore...
 I use it as Pre-Amplifier...


----------



## drarthurwells

That's good news.

 The stock Chinese tubes are less than ideal.

 I am still waiting on the 6N3P EB tubes (old Russian military) to replace the Chinese 6N3


----------



## Robfow

Is the 6N1 really just cosmetic? I wouldn't want to spend money on a replacement that's not going to improve the sound


----------



## drarthurwells

Quote:


  Originally Posted by *Robfow* 
_Is the 6N1 really just cosmetic? I wouldn't want to spend money on a replacement that's not going to improve the sound 
	

	
	
		
		

		
		
	


	


_

 

Yes.

 I took mine out.


 The 5670 is a sub for the 6N3 but has slightly different values but should improve the sound over the Chinese 6N3.


 The 6N11 is the same as the 6DJ8 and this change makes a big difference.


----------



## mirumu

Quote:


  Originally Posted by *drarthurwells* 
_The 5670 is a sub for the 6N3 but has slightly different values but should improve the sound over the Chinese 6N3.


 The 6N11 is the same as the 6DJ8 and this change makes a big difference._

 

From what I have found, the 6N3 tube is the hardest one to find a good replacement for. I tried a General Electric JAN 5670W but it was more noisy than the original 6N3. I then tried a 6N3P-DR which is military grade but different from the 6N3P-EB. This introduced quite a bit of noise and hum so I ended up switching back to the original 6N3 for now. 

 For the 6N11s, I am currently using a matched pair of TESLA PCC88 tubes. I wasn't that impressed with them initially but they are very transparent and uncolored.


----------



## edwinywh

Quote:


  Originally Posted by *drarthurwells* 
_I listened once with the stock set up to be sure it was working, and immediately changed the 6N1 to a Russian 6N1P EB and it seemed to improve the sound - but this was based on just a few minutes experience.

 Then I changed the Chinese 6N23 (6DJ8) to the Amperex-Holland 6DJ8 and found an improvement. Over the next few days I tried a variety of 6DJ8 and settled on the RCA stamped made in UK.

 I am still waiting on the 6N3P EB tubes from Russia which I suspect will significantly change the sound as did chaging out the two 6DJ8.

 Now, just got a message from j214 who advised me the front 6N1 tube is just cosmetic and not part of the signal circuit.

 I initially suspected it was just cosmetic but it is powered so I wasn't sure. Changing it had seemed to improve the sound but I only checked this for a few minutes before changing the other tubes.

 There are wires running to the circuit but these are likely just heater wires. So I simply pulled the 6N1P tube out and the amp worked without it - proof that this front window tubes is for looks but not function._

 


 Hi,I would say if 6N1 is purely there for cosmetic purposes,by changing it how would u notice the difference in sounds quality???

 I noticed that the connections comes from the power supply section only and nothing goes out from that tube.


----------



## edwinywh

Quote:


  Originally Posted by *mirumu* 
_From what I have found, the 6N3 tube is the hardest one to find a good replacement for. I tried a General Electric JAN 5670W but it was more noisy than the original 6N3. I then tried a 6N3P-DR which is military grade but different from the 6N3P-EB. This introduced quite a bit of noise and hum so I ended up switching back to the original 6N3 for now. 

 For the 6N11s, I am currently using a matched pair of TESLA PCC88 tubes. I wasn't that impressed with them initially but they are very transparent and uncolored._

 


 HI,I do agree with u that 6n3 is the hardest. I replaced mine with Western Electric 2c51 and it has increased the noise quite alot i would say.The noise is quite noticeable at 11clock onwards already. However,the 2c51 do bring out alot of details in my songs and I love it.

 Since I normally listen at volume lower than 10 oclock,so i'm ok with this big gain tube, hehee....


----------



## mirumu

Quote:


  Originally Posted by *edwinywh* 
_Hi,I would say if 6N1 is purely there for cosmetic purposes,by changing it how would u notice the difference in sounds quality???_

 

From my reading drarthurwells was saying that he initially thought there was a difference in the sound but that he had only listened for a few minutes and that really it was having no effect.

  Quote:


  Originally Posted by *edwinywh* 
_HI,I do agree with u that 6n3 is the hardest. I replaced mine with Western Electric 2c51 and it has increased the noise quite alot i would say.The noise is quite noticeable at 11clock onwards already. However,the 2c51 do bring out alot of details in my songs and I love it.

 Since I normally listen at volume lower than 10 oclock,so i'm ok with this big gain tube, hehee...._

 

That does sound like my initial impression of the GE 5670W. I felt that it might have been a bit more detailed too but I found the noise was bothering me a bit too much so I took it out quite quickly. I'd like to put it in again some time and spend more time listening with some headphones that don't expose the noise as much as my Shure E500s.

 I'd quite like to track down a Sylvania 5670 or some other Russian 6N3P variants to try in that socket as well.

 I know what you mean about the gain, I think I'm normally listening at less than 9 o'clock.


----------



## edwinywh

Quote:


  Originally Posted by *mirumu* 
_From my reading drarthurwells was saying that he initially thought there was a difference in the sound but that he had only listened for a few minutes and that really it was having no effect.


 That does sound like my initial impression of the GE 5670W. I felt that it might have been a bit more detailed too but I found the noise was bothering me a bit too much so I took it out quite quickly. I'd like to put it in again some time and spend more time listening with some headphones that don't expose the noise as much as my Shure E500s.

 I'd quite like to track down a Sylvania 5670 or some other Russian 6N3P variants to try in that socket as well.

 I know what you mean about the gain, I think I'm normally listening at less than 9 o'clock. 
	

	
	
		
		

		
		
	


	


_

 


 Hi!
 Thanks for clarifying with wat drarthurwells was saying. =) I might have interpret it wrongly.

 Yah,do try the amp with diff headphones.U might enjoy it?? 
	

	
	
		
		

		
		
	


	




 I'm currently driving a pair of Grado's SR80.I feel that the bass is quite weak or rather,not as punchy and loud like i wan it to be. Any tips to enhance the bass?

 Cheers,
 Edwin.


----------



## drarthurwells

Quote:


  Originally Posted by *edwinywh* 
_Hi!
 Thanks for clarifying with wat drarthurwells was saying. =) I might have interpret it wrongly.

 Yah,do try the amp with diff headphones.U might enjoy it?? 
	

	
	
		
		

		
		
	


	




 I'm currently driving a pair of Grado's SR80.I feel that the bass is quite weak or rather,not as punchy and loud like i wan it to be. Any tips to enhance the bass?

 Cheers,
 Edwin._

 

The bass is weaker, and dynamic impact is less, with the 708B compared to my Bada PH12 (which cost twice as much as the 708B).

 At its price level the 708B is excellent though.


----------



## Robfow

Quote:


  Originally Posted by *drarthurwells* 
_The bass is weaker, and dynamic impact is less, with the 708B compared to my Bada PH12 (which cost twice as much as the 708B).

 At its price level the 708B is excellent though._

 


 Hi, I take it that you are referring to stock Bada PH12?

 Because for the XS 708b usually it has gone through some tubes rolling and/or power cords replacement.


----------



## mirumu

Quote:


  Originally Posted by *edwinywh* 
_Hi!I'm currently driving a pair of Grado's SR80.I feel that the bass is quite weak or rather,not as punchy and loud like i wan it to be. Any tips to enhance the bass?_

 

That's much my experience with the Grado SR80s and the XS 708B as well. I haven't noticed this with other headphones. I find EQ is the only way to make this combination sound right to me although I find cutting better than boosting otherwise the distortion becomes a problem.


----------



## edwinywh

Quote:


  Originally Posted by *mirumu* 
_That's much my experience with the Grado SR80s and the XS 708B as well. I haven't noticed this with other headphones. I find EQ is the only way to make this combination sound right to me although I find cutting better than boosting otherwise the distortion becomes a problem._

 


 Hmm,sounds quite hopeful that by changing the headphones will improve my experience better???

 AKG701? Or SR225?? Or Senn 650??? Hmmm....


----------



## micha42

Hey everyone,

 well, I'm slowly moving up in the world of audio fidelity, and after headphones (Audio Technica A900 Ltd) and DAC (Diyeden GM Modded by Pacific Valve) I've gone and bought the missing component: the amp, the 708B being discussed here, which should arrive in about a week.

 If I've understood correctly, the 6N1 should be left alone (or does removing it somehow benefit the system?), the 6N3 should be replaced, and the 6N11s should both be replaced. The 6Z4 is theoretically replaceable, but can and should be left alone until replacement with another 6Z4.

 I'm totally new to tube amps, and so at least some of my questions will scream my utter ignorance to the world.

 1) What can I replace the 6N3 with? Is there something encoded in that which indicates all things compatible and incompatible? Same goes for the 6N11s. I've gathered that these are compatible with 6DJ8s, which are the same as 6922 (E88CC) and 7308 (E188CC)? Is this equivalence always true, or only under special conditions?

 2) Is there anything that needs special attention when taking out and putting in tubes? I'm assuming they're simply stuck in, and should be removed/replaced without skin-contact to the tubes (like lightbulbs). Any other treatment required, e.g. of the pins?

 Thanks for your help!


----------



## mirumu

Quote:


  Originally Posted by *micha42* 
_If I've understood correctly, the 6N1 should be left alone (or does removing it somehow benefit the system?)_

 

As long as the stability of the circuit isn't affected, removing the 6N1 should have no affect on the sound, it might save some power with it removed but wouldn't look as cool. 
	

	
	
		
		

		
			





  Quote:


 , the 6N3 should be replaced, and the 6N11s should both be replaced. The 6Z4 is theoretically replaceable, but can and should be left alone until replacement with another 6Z4. 
 

I think you'd only want to replace the 6Z4 if the tube stops working or the amp started acting oddly. As far as I'm aware, there are no replacements that are directly compatible with it. The 6Z4 is not an easy tube to track down but an earlier post I made in this thread has some links to online stores that stock them some if you need any.

  Quote:


 1) What can I replace the 6N3 with? Is there something encoded in that which indicates all things compatible and incompatible? Same goes for the 6N11s. I've gathered that these are compatible with 6DJ8s, which are the same as 6922 (E88CC) and 7308 (E188CC)? Is this equivalence always true, or only under special conditions? 
 

For the 6N3 ------> 6N3P, 5670, ECC42, 2C51 and 396A

 For the 6N11 -----> 6DJ8, 6922, E88CC, ECC88, PCC88, E188CC, 7308, 7DJ8 and 6N23

 There's probably a few others as well but these ones are commonly known to match.

  Quote:


 2) Is there anything that needs special attention when taking out and putting in tubes? I'm assuming they're simply stuck in, and should be removed/replaced without skin-contact to the tubes (like lightbulbs). Any other treatment required, e.g. of the pins? 
 

The way to remove them is gently roll them in a circular motion around the socket until they will just slide out. Clearly you shouldn't be putting a lot of pressure on them to do this otherwise the glass might break or the pins might be damaged. I don't know how it could be done without touching the glass, I try to use a clean cloth myself. There's a good sticky tube FAQ thread in the amplification forum that has a lot of this sort of information.


----------



## mirumu

Quote:


  Originally Posted by *edwinywh* 
_Hmm,sounds quite hopeful that by changing the headphones will improve my experience better???

 AKG701? Or SR225?? Or Senn 650??? Hmmm...._

 

I haven't heard any of those with the 708B myself but my HD595s certainly sound a lot different through it than my SR80s. I'd almost say they are opposite extremes, the HD595 stops just short of being what I'd consider dark, the bass is good but the upper midrange and treble is a bit too recessed to me (or maybe it's that the midrange is more dominant in general with the HD595). A friend of mine with a good ear feels that the HD595s have good synergy with the 708B and I would tend to agree but it is still a bit too dark for my liking. The AT W5000s sounded somewhere in between the two, more bass than the SR80s but a bit more bright than the HD595. The amp drives them well. Of the headphones I have heard with it though, the Shure E500 IEMs sound the best through this amp in my opinion even if they are a bit more sensitive to the tube hum.


----------



## micha42

Thanks for the clarification mirumu. I've just bought a few tubes of eBay now, which will arrive sometime in the next few days, as will the 708b hopefully. The ones I got are 2 matched pairs of 6N23P-EBs, 1 pair of Siemens E88CC Goldpins, 1 pair of 6N3P-DR (don't know how this compares with the EB, but feedback on it seems good), and finally a pair of RCA 6Z4. This should keep me stocked for a bit and let me do some minor testing between the Russian and Siemens tubes at the output, as well as comparing to the original tubes included. Will post again in 2-3 weeks with my findings


----------



## mirumu

Quote:


  Originally Posted by *micha42* 
_Thanks for the clarification mirumu. I've just bought a few tubes of eBay now, which will arrive sometime in the next few days, as will the 708b hopefully. The ones I got are 2 matched pairs of 6N23P-EBs, 1 pair of Siemens E88CC Goldpins,_

 

I'll definitely be interested to see what you think of those. 
	

	
	
		
		

		
		
	


	




  Quote:


 1 pair of 6N3P-DR (don't know how this compares with the EB, but feedback on it seems good), 
 

I did try some of those but there was a little too much noise for my taste. Some of that was probably due to my E500s IEMs though.

  Quote:


 and finally a pair of RCA 6Z4. 
 

Unfortunately the RCA 6Z4 is a different tube, sorry I should have gone into more detail, I think I did in another Xiang Sheng thread. The RCA 6Z4 is a large 5-pin tube. The 708B amp uses the minature 7-pin 6Z4 made by Shuguang in China that is totally different. It would have been so much better if they had picked a different name. 
	

	
	
		
		

		
		
	


	




 They are very cheap but hard to find. I could only find two stores online that sell them, Audiotriodes in France and Diyclub.biz in Hong Kong. I have ordered things from both stores and they arrived fine. There are some Chinese 6Z4s on ebay but you have to buy a power supply kit to get them which ends up costing more and you only get one.

  Quote:


 This should keep me stocked for a bit and let me do some minor testing between the Russian and Siemens tubes at the output, as well as comparing to the original tubes included. Will post again in 2-3 weeks with my findings 
	

	
	
		
		

		
		
	


	



 

I'll definitely look forward to that.


----------



## micha42

Quote:


  Originally Posted by *mirumu* 
_Unfortunately the RCA 6Z4 is a different tube, ..._

 

Doh! Oh well, didn't put me back that much, I guess I'll survive. Maybe they'll come in handy some other time.

 Are there any other good choices for alternatives to the 6N3 besides the difficult-to-find 6N3P-EB?


----------



## drarthurwells

Quote:


  Originally Posted by *micha42* 
_Doh! Oh well, didn't put me back that much, I guess I'll survive. Maybe they'll come in handy some other time.

 Are there any other good choices for alternatives to the 6N3 besides the difficult-to-find 6N3P-EB?_

 

I would not buy the 6N3P DR on ebay - much too expensive - don't think cyro treatment of tubes does anything other than what people imagine - if these are modern manufacture 6N3P then i wouldn't want them.

 Stick to the old (1989 or before) 6N3P which can be guaranteed if you get the military version.

 As Mirumu earlier said about subs: For the 6N3 ------> 6N3P, 5670, ECC42, 2C51 and 396A

 However, only the 6N3P is a direct sub - the rest have some differences but may work out OK as subs.


----------



## edwinywh

Quote:


  Originally Posted by *micha42* 
_Thanks for the clarification mirumu. I've just bought a few tubes of eBay now, which will arrive sometime in the next few days, as will the 708b hopefully. The ones I got are 2 matched pairs of 6N23P-EBs, 1 pair of Siemens E88CC Goldpins, 1 pair of 6N3P-DR (don't know how this compares with the EB, but feedback on it seems good), and finally a pair of RCA 6Z4. This should keep me stocked for a bit and let me do some minor testing between the Russian and Siemens tubes at the output, as well as comparing to the original tubes included. Will post again in 2-3 weeks with my findings 
	

	
	
		
		

		
		
	


	


_

 

Wow! U certainly have spent quite an amount to get these tubes.

 I look forward to ur review of ur amp again after u rolled the tubes. =)
 Especially ur Siemens E88cc goldpins together with 623P-EBs. 
	

	
	
		
		

		
		
	


	




 Hear from u soon,pal!


----------



## mirumu

Quote:


  Originally Posted by *drarthurwells* 
_As Mirumu earlier said about subs: For the 6N3 ------> 6N3P, 5670, ECC42, 2C51 and 396A

 However, only the 6N3P is a direct sub - the rest have some differences but may work out OK as subs._

 

Yes, I don't know about the ECC42 but the 5670, 2C51 and 396A are only half as tall for a start, you have to bend the wire tube holder to fit these shorter tubes so that they don't fly around and hit live parts of the circuit board. No doubt these tubes are different electrically too but they do seem to be close enough to work well as replacements for the 6N3s.


----------



## Nugget

I'm loving the 708B (and not just because the glowing tubes earned a delicious spousal eye-roll).

 Anyway, for the time being I'm running an X-Fi as my primary source, and I'd like to be able to switch easily back and forth between the Xiangsheng and my Logitech Z-2300s. Being dedicated computer speakers, the 2300s are meant to run on a sound card's weak signal; so I'm guessing it's a bad idea to run them off of the Xiangsheng (i.e. using the 708B as a preamp in a system that's designed not to need one).

 Any other way to rig everything up so I don't have to crawl behind my desk to switch cables?


----------



## dcheming

Well I finally got my 708B this morning. I plugged it in for a quick listen totally stock just to see what it's capable of. The sound was ok, roughly the same as my soundcards 1/8" line out that I've been using. The left input jack on the 708 was undersized enough to let my cable wobble a bit. Considering how much it cost and where it came from this is understandable, but still annoying. The stock RCA jacks are total crap anyway, so they had to go. The top case was a very tight fit and will be a pain to remove every time I want to roll a tube. After looking at the low quality input cable inside and the truly horrible soldering job on the RCA jacks I ended up replacing the jacks, the cable to and from the potentiometer. I just used some Calrad teflon RCA jacks I had in my parts bin and some teflon Cat6 (Belden 7882A) twisted pair wire. This wire is unshielded but it didn't add any noise. When I finished those mods I swapped in the RCA 6DJ8's that I got from drarthurwells. Thanks Again for these! 
 I'm listening to it now as I type and I will say that these RCA tubes definitely sound a lot better than the stock Chinese 6N11's. At this point I like the sound better than what my soundcards 1/8" stereo out was doing. I'm going to wait a while to post my overall impressions to give some burn in time. I also plan on replacing a lot of other stuff inside these next few weeks.


----------



## nakedtoes

Change to a better power cord... the XS708B greatly benifit from a powercord upgrade


----------



## Nugget

Keep us posted on the process, I've already got my eye on a few mods I'd like to do. One benefit of a cheaper amp is that there's less of a psychological barrier to cracking it open!

  Quote:


  Originally Posted by *dcheming* 
_Well I finally got my 708B this morning. I plugged it in for a quick listen totally stock just to see what it's capable of. The sound was ok, roughly the same as my soundcards 1/8" line out that I've been using. The left input jack on the 708 was undersized enough to let my cable wobble a bit. Considering how much it cost and where it came from this is understandable, but still annoying. The stock RCA jacks are total crap anyway, so they had to go. The top case was a very tight fit and will be a pain to remove every time I want to roll a tube. After looking at the low quality input cable inside and the truly horrible soldering job on the RCA jacks I ended up replacing the jacks, the cable to and from the potentiometer. I just used some Calrad teflon RCA jacks I had in my parts bin and some teflon Cat6 (Belden 7882A) twisted pair wire. This wire is unshielded but it didn't add any noise. When I finished those mods I swapped in the RCA 6DJ8's that I got from drarthurwells. Thanks Again for these! 
 I'm listening to it now as I type and I will say that these RCA tubes definitely sound a lot better than the stock Chinese 6N11's. At this point I like the sound better than what my soundcards 1/8" stereo out was doing. I'm going to wait a while to post my overall impressions to give some burn in time. I also plan on replacing a lot of other stuff inside these next few weeks. 
	

	
	
		
		

		
		
	


	


_


----------



## drarthurwells

The RCA 6DJ8 stamped made in U.K. are good all around with a high definition tone.

 The Amperex Holland smooth out the edges some. You lose some tone edge resolution/imgaing but this will sound smoother with a harsh source like in many digital (but not all) sources.


----------



## dcheming

Yeah the power cord that comes with the 708 is one of the smallest/cheapest that I've ever seen. I threw it in the trash! I found some nice 14ga heavily shielded 6ft IEC power cords at an awesome electronics store when I lived in Houston. These cords were surplus and cost $2.99 ea so I bought about 10 of them! Well today I did some more mods. I completely removed the output RCA's and the cheap wire going to them. I will replace these eventually, but I don't really need the preamp function right now and I was just getting tired of looking at them. Then I used the same Belden Teflon wire to replace the headphone wire to the jack. While I had the board that the power switch and the 1/4" jack are on removed, I noticed that there are some 10ohm resisters in series to the jack. I just happened to have some NTE 2W metal oxides so I replaced these. On this note, a lot of the resistors and caps in my amp are kind of damaged/scratched/dented. I plan on replacing all of the resistors that are in the signal path first with something nice like RIKEN carbons. For coupling and preamp out, my 708 uses the red WIMA MKS caps. According to the opinion of many, these metallized polyester caps are crap and shouldn't be used for audio. Some WIMA FKP polypropylene film/foils would be a lot nicer and are the same shape/style. I would much rather try some Mundorf M-Cap Zn or Multicap RTX instead though. Changing these WIMA's and some of the resistors are going to be my next mod. At this point I would definitely say that this amp blows away my previous method of powering my 580's. My amp only has about 10 hours on it so I'm not going to be very critical about the sound right now, but I am very impressed with it so far for the money! Nugget, I totally agree about not feeling bad about cracking this cheap amp open and pulling stuff out. I want to see how far I can take this little amp, within a reasonable budget of course. Modding is great stress relief from classes! 
	

	
	
		
		

		
			





 I'm taking pictures of every step for before and after comparisons. If any one is interested I will try to post some.


----------



## Robfow

Quote:


  Originally Posted by *dcheming* 
_Modding is great stress relief from classes! 
	

	
	
		
		

		
		
	


	




 I'm taking pictures of every step for before and after comparisons. If any one is interested I will try to post some._

 

Yes please post them!


----------



## Nugget

Damn right I want to see some pictures! Any of the mods make a particularly large difference? I can see starting with some new RCA jacks and a new power cord. And some new tubes, o/c.

 Now if I could only find an electronics shop in New York that sells this stuff; mail-order is a PITA with my microscopic mailbox and work schedule.


----------



## dcheming

Well as far as what made the biggest change, I'd say it was the tubes so far. The new wire, RCA's, and resistor seemed to have helped too, but since I'm not familiar with this amps sound yet I can't say by how much. Plus it's barely even started the break-in period. I want to get about 50 hours on it before I do any other mods. From here on out the mods will start making a much bigger difference on the sound so I want to be able to decipher if it is better or worse. 

 Nugget, couldn't you just get your mail-order parts sent to you at work? Ok, so I have plenty of photos and I can use Photoshop to resize them from 2.5MB to ?? I read somewhere that pics here need to be scaled to 25kB but that there was a way around this. Also how do I post the pics directly in the post, instead of as a link? I've never really posted pics before so I'm new at this. Could someone help me with these, I would appreciate it.
	

	
	
		
		

		
		
	


	




 jamato8 started a great mod thread for the Wooaudio 3 and it has a lot of great info in it. Definitely worth checking out.
http://www6.head-fi.org/forums/showt...ight=wooaudio3


----------



## Nugget

You can upload pics to ImageShack and then direct-link them in your posts.


----------



## dcheming

Thanks Nugget! I'm taking a few more right now so I try it out in a few minutes.


----------



## dcheming




----------



## dcheming

I just realized that I have some 1.25uF 400V mylar caps in my parts bin. I can't hold back; I'm going to bypass the headphone output caps with them. The stock output caps are _polar_ electrolytics by the way! Not exactly the best choice to pass audio through.


----------



## dcheming

Well it's not a huge difference, but these cheap Mylar caps opened up the sound a bit. A bit more detail from the midrange on up. I'm not sure if these are metallized or are film/foil, but they are only temporary anyway. They cost me $1.25 each surplus. One of these caps is about a 1/4" away from the back 6922 so I hope heat will not be a problem, but I'll keep an eye on it.


----------



## Nugget

Very clean work. I like. I figure I'll let mine burn in before I really start whacking away at it, though.

 Incidentally, where do you find your tubes?


----------



## dcheming

Thank you; doing quality work brings me satisfaction, which is why I find it so hard to leave things alone. 
	

	
	
		
		

		
		
	


	




 I'm also going to try to let it burn in more before doing anything else. However, the 15% off sale at Parts Connexion ends on the 30th so I might at least order some parts.

 The 6N3 is still the stock tube. The RCA 6922's I got from drarthurwells. I'm in the process of looking for 6N3 replacements so I'll let you know if I find any good looking deals.


----------



## micha42

Hey everyone!

 Okay, my own little 708B has made it to London from China (thanks Leshele), and although I only had a couple of hours on the weekend to play with it, here are my initial impressions and tests.

 The package I got was nicely wrapped, was the right voltage, and also contained the correct power cord to plug it in immediately. The cable/plug might not be audiophile quiality, but I can't estimate yet of what magnitude sound can improve through a better cable (I'm thinking not much). I do have a Belkin mains filter which does a few rudimentary things like surge protection and a bit of isolation of the attached components, and that's good enough for now.

 So, I unscrewed the amp to make sure all the tubes are nicely flush in their sockets, left the cover off (since I wanted to do some tube rolling), and then switched the whole thing on and gave it a listen. First impression: balance is off 
	

	
	
		
		

		
		
	


	




 everything sounded like it was too far left. A quick switch of the two driver tubes and the balance flipped to the other side. This probably means the tubes aren't well matched, which isn't great (if you're looking for the amp to work well out of the box) but no biggie (you could always adjust balance in the player, or change the tubes which I wanted to do anyways).

 So, out went the two Chinese 6N11s to be replaced by a matched pair of 6N23P-EBs. These worked flawlessly with perfect balance from the start, which was the main improvement I was listening for, and the sound seemed to fill out a bit more as well. As a side note, please don't expect any fancy audiophile lingo for describing sound here. The sound before was a bit thinner before, and became a bit fuller with the Russian tubes, in fact noticeably so.

 Since the other 6DJ8esque tubes I"m trying to get haven't arrived yet, the next thing to be played with was the 6N3 sitting in front of the two drivers. This was already doing a good job I felt, but I had 3 more different types of tubes to play with here: a 6N3P-E, a 6N3P-DR, and an RCA 5670 from 1961. The first of these tubes was worse than the Chinese original; it created a massive hum, and quickly exited the amp. The other two tubes are pretty good, I didn't get any hum out of them until I turned up the amp to more than half maximum volume (which is too loud for listening for > 10 seconds with my headphones). In hum, there was a minute advantage for the RCA tube, though it might just be my imagination. The more noticeable difference was that the RCA tube seems comparatively neutral, while the DR one emphasizes the highs a bit too much and sacrifices bass. It could well be that I should just give it 10-20 hours to burn in, but I haven't had that time yet, so my current favourite is the RCA tube. Hopefully I'll have some more time next weekend to do more listening and said burning in. I cleaned all the tube contacts with deoxit before socketing them, which probably has all of 0.03% effect on sound, but gives my mind a nice cosy feeling anyways 
	

	
	
		
		

		
		
	


	




 Compared to the headphone-out from the Diyeden GM modified, I've noticed two improvements (and might see more as time progresses). The first is that highs no longer all have a slight reverberation to them (minor but noticeable), and the second is that bass sounds a lot more abrupt then before (I think this is also referred to as a tighter or drier bass). 

 My listening was done with some piano music (Mitsuko Uchida playing the Mozart Sonatas, Dick Hyman playing Duke Ellington), Vivaldi's Four Seasons (Deutsche Grammophon), Pink Floyd - The Wall (MFSL), and a bit of 2RaumWohnung. I'll try out a few more flavours of music when I get the chance to see how things have changed from before


----------



## dcheming

Hey Nugget, I was wondering if you could please take some pics of the inside of your amp and post them. I was just looking at the pic that drarthurwells posted at the beginning of this thread and I noticed that the main board with the tubes on it has a much different layout of it's caps. The headphone output caps look much larger than the ones in mine. The pre out caps are also different from the WIMA's in mine. The other electrolytics are also layed out differently. The big red resistors in his are not in mine either. My 708 just uses metal oxides throughout. I would like to know if your unit is like mine since we got them around the same time. 

 I'm looking forward to when you start doing mods on yours. It will be nice to have someone to bounce ideas off of. Having two sets of ears listening to the same mod will be helpful.


----------



## mirumu

I'm keen to do some modding to my 708B, but I have my brother's kids staying currently so I won't be opening the covers on my 708B again for a few days at least.


----------



## dcheming

Quote:


  Originally Posted by *mirumu* 
_I'm keen to do some modding to my 708B, but I have my brother's kids staying currently so I won't be opening the covers off my 708B again for a few days at least._

 

How long have you had your 708 for? It'd be great to have a small group of people doing mods. We could cover much more ground this way as far a variety in parts goes.
 My unit does not have any hum, but it does have a small amount of hiss that grows with volume. I know this is common in cheap tube equipment, but I'm researching the various ways to make it dead quite. I also noticed in drarthurwells pic that his unit has a ground wire from the IEC jack to the case. Mine doesn't have any ground wire at all! Only the signal ground goes to the case. I'm going to check into this more as it kind of worries me if this amp is not grounded properly.


----------



## mirumu

Quote:


  Originally Posted by *dcheming* 
_How long have you had your 708 for?_

 

I believe I got mine around June.

  Quote:


 It'd be great to have a small group of people doing mods. We could cover much more ground this way as far a variety in parts goes. 
 

The main thing I wanted to replace was the internal signal cabling since it has some quite long runs over what seems to be very cheap cable. Also any caps in the signal path I'd be keen to upgrade. I replaced the IEC cable as soon as it was out of the box with something much more heavy duty, I needed a different plug on the end anyway for the local power sockets.

  Quote:


 My unit does not have any hum, but it does have a small amount of hiss that grows with volume. I know this is common in cheap tube equipment, but I'm researching the various ways to make it dead quite. 
 

Well at this point I'm happy that it's my Shure E500s letting me hear the hum. I've tried quite a few other headphones and can't hear it at all with them. It is louder or quieter though depending on what tube I replace the 6N3 with. It really doesn't bother me that much since the only time I can hear it is when there's no music playing, even on very low volumes it's inaudible. I have a little hiss too but I never get the volumes high enough that I notice it without listening for it.

  Quote:


 I also noticed in drarthurwells pic that his unit has a ground wire from the IEC jack to the case. Mine doesn't have any ground wire at all! Only the signal ground goes to the case. I'm going to check into this more as it kind of worries me if this amp is not grounded properly.
	

	
	
		
		

		
			




 

That's a bit of a concern, I don't recall how that looks on mine, I'll have to check next time I open it up.


----------



## dcheming

During breakin was there any point that the sound suddenly opened up, like I've read about with some amps?

 Yeah the stock signal cable was the cheapest that I've ever seen. The solder job on the shielding is horrible too; check this out underneath the heat shrink when you pull it out. The wire I used probably isn't the best, but it's all I have on hand right now. I'd also recommend changing all the RCA jacks too, even if it's just with some $1.99 ones. The ones on mine were garbage!

 Changing the power wire is probably a good idea. It looks like one 18ga and one 16ga on mine. If I do this I'll probably get a new IEC with a separate fuse holder. I really don't like those IEC jacks with the fuse holder molded in. What ga of wire did you go with? I figure some nice OFC 12 ga soldered as close to the input of the transformer as possible will do it.

 I really can't hear the hiss at the levels that I listen to music at, but it would be nice to have a dead quite amp. I'm going to look into the possibility of converting the heaters to regulated DC to see if this will help.

 Just out of curiosity, how many uF are your headphone output caps? Mine are 150 uF at 200V and unfortunately they are also polar. I'm looking for some affordable non-polars around 400 to 500uF. Blackgates cost too much to justify using in this cheap amp.


----------



## Nugget

Quote:


  Originally Posted by *dcheming* 
_Hey Nugget, I was wondering if you could please take some pics of the inside of your amp and post them. I was just looking at the pic that drarthurwells posted at the beginning of this thread and I noticed that the main board with the tubes on it has a much different layout of it's caps. The headphone output caps look much larger than the ones in mine. The pre out caps are also different from the WIMA's in mine. The other electrolytics are also layed out differently. The big red resistors in his are not in mine either. My 708 just uses metal oxides throughout. I would like to know if your unit is like mine since we got them around the same time. 

 I'm looking forward to when you start doing mods on yours. It will be nice to have someone to bounce ideas off of. Having two sets of ears listening to the same mod will be helpful.
	

	
	
		
		

		
		
	


	


_

 

Well, it's past midnight here and with work in the morning it'd be ridiculous to pop the top off this amp at this hour, so of course that's exactly what I'm going to do. Be back in a bit with some shots of the internals.


----------



## mirumu

Quote:


  Originally Posted by *dcheming* 
_During breakin was there any point that the sound suddenly opened up, like I've read about with some amps?_

 

I can't say I noticed that, swapping out the pair of 6N11 tubes had the biggest effect on the sound to me and it's become slightly more "refined" as they've burned in.

  Quote:


 Yeah the stock signal cable was the cheapest that I've ever seen. The solder job on the shielding is horrible too; check this out underneath the heat shrink when you pull it out. The wire I used probably isn't the best, but it's all I have on hand right now. I'd also recommend changing all the RCA jacks too, even if it's just with some $1.99 ones. The ones on mine were garbage! 
 

Yes, definitely. The RCA jacks on mine didn't seem to bad but I can easily get better.

  Quote:


 Changing the power wire is probably a good idea. It looks like one 18ga and one 16ga on mine. If I do this I'll probably get a new IEC with a separate fuse holder. I really don't like those IEC jacks with the fuse holder molded in. What ga of wire did you go with? I figure some nice OFC 12 ga soldered as close to the input of the transformer as possible will do it. 
 

I don't know what the ga of mine was but it was a brand new cable and is thicker than most of the IEC cables available here in NZ. It's rated for 15A whereas most here are 10A.

  Quote:


 Just out of curiosity, how many uF are your headphone output caps? Mine are 150 uF at 200V and unfortunately they are also polar. I'm looking for some affordable non-polars around 400 to 500uF. Blackgates cost too much to justify using in this cheap amp. 
 

I'll have to wait till next time I open my amp up, I don't recall what the values of the caps are.


----------



## Nugget

Here you go. I've got high-res versions here if you need to see anything in more detail.

 Oh, and I found out another reason not to do this late at night other than being tired at work -- I accidentally did my first tube rolling! Knocked the sucker right out trying to cram the poorly-machined casing back on. All works fine, though. Phew.


----------



## drarthurwells

I've been listening to the 708B with the Rega Saturn and K701 (my fancy ICs and Tice Micro power conditioner also).

 The source is so important. Nothing better than the Saturn as a source - the best CDP available IMO.

 I like the RCA 6DJ8 tubes better than the Mullards, Amperex-Holland A frames, Toshiba 6922, Sylvania 6DJ8 or JAN 6922, Philips JAN 6922, or Brimars.

 Russian 6N23P EB are also good - close to the RCA 6DJ8.

 Still impressed with the 708B sound for the bucks.


----------



## dcheming

Thanks for the pics. Our two units look pretty much identical, the only difference is the color of some of your resistors and the 22uF PS bypass caps. I can only imagine where they get these cheap parts from! 
	

	
	
		
		

		
		
	


	




 It came to my attention that drarthurwells 708B is the 220V version, which is why things are different. I don't know why they didn't ground the 110V version though. I can only think that they did this as an easy way to avoid ground loops.

 Yeah the outer case on mine was a PITA to get off. They did a very poor job forming the cover indeed. I leave mine off to make it easier on myself because I'm going to be needing access to the insides quite often. Helps it run much cooler too. I would love to fabricate a new one from aluminum. I would make either an access window above the tubes or just make the whole top removable separately from the sides, which would be nicer. Where's a CNC mill when you need one!
	

	
	
		
		

		
		
	


	




 I'm looking into bypassing the cathode bias resistors with the proper size electrolytics, but I would like to talk to my repair tech friend about this first to see if this is the right track to reduce hiss. Modding this amp will definitely be a learning process for me. I've always wanted to get into the finer points of designing and modifying tube circuits, but never had the motivation to do so. This is one of the main reasons why I bought an amp that I new would have some problems: hands on work is the quickest way to learn something. The money I saved by going cheap on the amp can go right into parts to experiment with to find out what makes improvements and what doesn't. Hopefully we can take this amp to new levels!

 I'm not above even going to the level of making new circuit boards if I decide to make changes to the layout or design of the circuit itself. I think it'd be kind of cool to make a new pcb layout that would allow the tube sockets to be mounted on the bottom so that the whole board could then be mounted upside down under the bottom of the top cover. This would let the tubes be exposed through holes in the top case and also give plenty of space underneath for the big film and electrolytic caps. This would necessitate the relocation of the transformer and the PS board, but it'd be a fun project. Like I said this will be a learning experience in tube circuits so I don't know how far I will take this humble little amp in the process. I'm just going to take it one step at a time and of course I'll share my opinions of all the mods along the way. Projects like this excite me!


----------



## dcheming

Nugget, I didn't realize how late it was, especially on your side of the country. Thanks for staying up and taking the pics anyways! Head-Fi surely is more important than work anyhow though, right?


----------



## Gradofan2

Quote:


  Originally Posted by *drarthurwells* 
_I've been listening to the 708B with the Rega Saturn and K701 (my fancy ICs and Tice Micro power conditioner also).

 The source is so important. Nothing better than the Saturn as a source - the best CDP available IMO.

 I like the RCA 6DJ8 tubes better than the Mullards, Amperex-Holland A frames, Toshiba 6922, Sylvania 6DJ8 or JAN 6922, Philips JAN 6922, or Brimars.

 Russian 6N23P EB are also good - close to the RCA 6DJ8.

 Still impressed with the 708B sound for the bucks._

 

It's a bit confusing... is the E5 + BADA + SA5000's still the "best there is"... or is that now the Saturn + BADA + K701's... or what?

 I know you've ranked your amps and gear for us... but... can you assign a relative point value to them (i.e. 10, 9, 8, etc.) to give us a sense of how great the difference is??? I mean is the BADA a 10, and the 708B a 6, and the Ming Da a 5, or what? I'm sure that would be helpful to a lot of us "vicarious audiophiles."

 Also - do you know where a "vicarious audiophile" can pick up the CT Silver Reference II's for a bargain price (i.e. cheap)??? $550 for IC's is a bit ridiculous - no cable can possibly make that much difference - which is precisely why I have the CT Silver IC's.

 Thanks,
 GF2


----------



## Nugget

Quote:


  Originally Posted by *dcheming* 
_Nugget, I didn't realize how late it was, especially on your side of the country. Thanks for staying up and taking the pics anyways! Head-Fi surely is more important than work anyhow though, right? 
	

	
	
		
		

		
		
	


	


_

 

Damn right.

 OK, I'm going to get started playing with this thing. I want to do a lot of work, little by little -- new RCAs, new caps, new tubes, new wiring, maybe even new tube sockets. I also want to pull out the LEDs mounted near the front window.

 Problem is that I'm a law student and a history major. My electronics experience has been limited to building and modding PCs, so while I can solder and use a multimeter, that's about it.

 So I'm warning you: I may (scratch that, _will_) ask you a ton of very basic questions along the way.

 First up, is there any problem with the 4% silver/96% tin Radio Shack solder I've got lying around? Second, I've been thinking of making some DIY fine-silver speaker wire as per this guide -- is there any reason not to use wire made to those specifications to rewire the 708B?

 Thanks, man. I know this thing has a lot of potential, and I'm looking forward to getting my hands dirty tweaking it.

 -Nuggs-


----------



## dcheming

Well I just got back from talking to my tech friend about the 708. Bypassing the cathode bias resistors would do more harm than good in this circuit topology, so that's out. Replacing the critical resistors and the caps is the next round of mods, so I'm going to put in an order with Parts Connexion for these parts before their sale ends on Saturday.

 Yeah, I'm going to pull out the whole LED pcb today because they make a lot of noise. I've been considering removing the whole reflector assembly with the tube as well since it is only for show. This will also free up some current in the heater circuit by removing the 6N1. Speaking of heaters, I am going to build a board to convert the heaters over to regulated DC. The space that the reflector assembly takes up now would be a perfect place to mount this board. The only thing is that I'm not sure what to do with this window. It will just be looking inside the amp to the components at that point. Any ideas? I was thinking last night that it would be cool to convert the volume control knob to be a source selector or maybe to switch between different output caps. Then the volume pot would be moved to the center of a metal plate replacing the glass in the window. I would get a local machine shop to turn a custom 2.35" aluminum knob to fill in this huge circle. I think that would be a lot nicer than the lame tube window and would definitely be one of a kind.

 I don't mind a lot of questions Nugget, that's why I'm posting so much here. I want to share as much info as possible as I go through this. Plus tinkering with audio equipment is a great retreat from school.
	

	
	
		
		

		
		
	


	




 Those DIY silver ICs look very promising. I read about them about a month ago and they seem like a great idea. I imagine they would be killer inside of a head amp, especially if you made some of the ICs to go to your source. It would be interesting to also make some HP cables in that style also, then your entire system would have the same wiring.

 I really can't comment on the quality of Rat Shack's silver solder. I just use Kester 4% silver and it's served me well for years. Your solder is probably okay for this level of work. I just hate supporting RS and I avoid them as much as possible. Unfortunately they are the only electronics store in this whole town!
	

	
	
		
		

		
		
	


	




 So all my stuff will have to be mail order.

 I'm going to make some little spring loaded jumpers to short the input caps. I'm pretty sure my soundcard doesn't put out DC, but I want to make it a temp mod for now just in case. I'm also going to build some some Cat6 XLR to RCA cables today. Right now I'm using some XLR to RCA adapters going to some DIY shielded coax. I'll post some pics when I'm done of course.


----------



## Nugget

Nice idea on replacing the front window. (When my wife first saw the amp she said, "But why does it have to _glow_?")

 If you're really going balls-out, you might also consider having Front Panel Express make you a custom faceplate. Fitz did that for his completely overhauled Darkvoice, and IMO it's stunning:

 [edit: I can't find the source offhand, but I believe Fitz said the job cost him around $60.]


----------



## dcheming

I was just thinking to myself as I'm removing the reflector assembly that I should let you guys know that the HV caps don't have any bleeder resistors on them. So they stay at dangerous voltage levels for hours, if not days. Just be careful around these when you are poking around inside, especially when you are handling the boards by hand. It's very easy to have your finger tips touch the traces underneath and receive a nasty shock. Even the 150uF output caps keep about 110V on them after unplugging power.

 I made a useful little tool by soldering 5 5.6k 1/2w resistors close together in series. I then cut one of my jumper wires that have alligator clips on them in half and soldered one side to the resistor chain. I put the resistors inside a BIC pen that I removed the ink tube from. I made a hole in the end cap so the wire runs through and I did the same on the other end. So this gives me a 35k resistor protected in plastic with clips on it that I connect across the various HV caps for about a minute each to safely drain the stored charge.

 When I take the PS board out I'm going to solder a 47k, or so, 2W resistor across the terminals of each 330uf cap to do this automatically at power-down.


----------



## dcheming

Yeah Fitz did do a very impressive job rebuilding that Darkvoice. Changing the faceplate on an amp is like putting rims on a car: it can totally change the look for the better, if done right. I think his custom faceplate turned out extremely well. He has good taste.


----------



## dcheming

Well I'm not sure if it's due to removing the 6N1 and its wiring, the LEDs, or shorting the input caps, but the sound is noticeably clearer. The LEDs were wired straight into the heater supply coming from the 6Z4 without any kind of isolation. I'm pretty sure the clarity is more from the shorted input caps though. If everything operates okay with them shorted for a few days, I'm going to replace them with soldered jumpers.


----------



## dcheming




----------



## titoyd

More more up up...very interesting what are you doing right now!
 Can you make the full photo inside again...please!


----------



## dcheming

Well I finally decided on the parts for my PC order:

RIKEN carbon films for the input resistors and all the cathode bias resistors

Auricaps for the interstage coupling caps

Mundorf M-Caps for the power supply bypass caps

ALPS Blue Velvet potentiometer

 I spent a bit more money than I wanted to, but I gotta take advantage of the sale right.
	

	
	
		
		

		
		
	


	




 Going to phone it in tomorrow at lunch.


----------



## drarthurwells

Quote:


  Originally Posted by *Gradofan2* 
_It's a bit confusing... is the E5 + BADA + SA5000's still the "best there is"... or is that now the Saturn + BADA + K701's... or what?

 I know you've ranked your amps and gear for us... but... can you assign a relative point value to them (i.e. 10, 9, 8, etc.) to give us a sense of how great the difference is??? I mean is the BADA a 10, and the 708B a 6, and the Ming Da a 5, or what? I'm sure that would be helpful to a lot of us "vicarious audiophiles."

 Also - do you know where a "vicarious audiophile" can pick up the CT Silver Reference II's for a bargain price (i.e. cheap)??? $550 for IC's is a bit ridiculous - no cable can possibly make that much difference - which is precisely why I have the CT Silver IC's.

 Thanks,
 GF2_

 


 First, let me correct a previous evaluation of the Sylvania JAN 6DJ8 or 6922 tube. They are slightly richer in tone than the RCA I recommended, but have slightly less detail and clarity. Some will prefer one and some the other, but they are good with the 708B. 

 The SA5000 was superior (to me) than the HD650 - particularly in detail, imaging, and instrument separation.

 Then I got the K701, which not better than the SA5000 in some ways, was better overall than either the HD650 and SA5000. Yes the K701 is the best headphone currently available and produced. Now someone will remind me of this statement when I say, in the future, the HD700 are the best headphones.

 Then people told me of trouble driving the K701s with some amps - never had this problem with the amps I owned.

 The E5 remains a great high resolution CDP. More detail and beter imaging and image separation than the Rega Apollo, but the Apollo is not as lean in tone and is smoother. Then I got the Saturn which is better than either the E5 or the Apollo.

 ICs are important. Have heard good things about DH Labs (very inexpensive) but never owned them. The TweekGeek CT Silver ICs are great for a low cost - can't beat them for the bucks. My Silver Reference II ICs are great for higher cost, and yes, give slightly more transparency than the CT Silvers. They are no longer available but you can buy them as the Acoustic zen Silver Reference II at Audiogon at times for $500 used but excellent. Are they worth $500? The WTB connectors alone cost $50 each ($200 total for the one meter pair of ICs). The audible improvement is definite but slight. Wolfe Audio uses connectors that cost maybe $200 or so each. He sold out his stock of 8 WTB connectors to me for use in making my ICs, as he went with the higher priced ones. I am sure his current cables are fantastic.

 I still prefer the Bada PH12 - have blown mosfets from tube changing but these are cheap and not hard to replace. Some people will prefer other amps at the same or even lower price level.

 The Chinese amps I have owned generally rank in sound quality with their price. But some may not agree. The MC66AE was very lush and enjoyable - great sound, but somewhat un-natural in timbre. Some people will prefer the rich timbre to a natural timbre. Some people will prefer the MC66AE to the Bada PH12 - I don't.

 The 708B is great for $200 - but lacks the weighty fullness of the either the MC66AE or the Bada. The G&W T2.6F has great detail, imaging, and image separation, but has some leanness of tone (the Sylvania JAN 6922 tubes help here). The T2.6F reminds me of the Raptor but not quite as lean as the Raptor. The ASL MG III has some real good qualities which I did not explore in tube changes so I don't know this amp really well.

 I could be happy with any of these amps - I just keep going back to the Bada as more natural and realistic overall. This is not a big difference and others will prefer some of the other amps that I have owned to the Bada.

 So, did I fail to answer your question to the degree you expected?


----------



## Gradofan2

That helps...

 I realize it's moreless what you've said before... but, it seems to be a little more clearly articulated.

 Although only the perspective of one reviewer, I think what really helps in such comparisons is a rating scale for various qualities - because it provides a relative context for differences (e.g. how much more leaness, or weight, bass, treble, soundstage, etc. 1 to 10 scale). A slight difference may be difficult to discern and may not justify a much higher price (e.g. between the CT Silver cables and the CT Silver Reference II cables, etc. - perhaps the CTS's are an 8 and the CTSR II's are a 10, etc.)

 But... your clarification does provide some context and clarification.

 Of course... I realize... in the end... the only way for others to judge... is to try each themselves.

 Thanks!


----------



## dcheming

As we all know there are very few replacement choices when it come to the mini 6Z4 rectifier tube. Whereas the mini 6X4 has the normal range of NOS and current-production to chose from. The 6X4 uses the same heater voltage as the 6Z4, but two of the pins would need to be switched on the 708B's PCB to use it. The reason I'm posting this is that I'm going to design a new PCB to use the 6X4 instead and I'm curious if any other 708B owners here would be interested in going in on this with me to reduce the cost. I would prefer to get this professionally made for me somewhere instead of using prototyping board. I plan on reusing all the parts that are already on the stock PS board; the only change would be the rectifier tube. Let me know if you're interested.

 Since the transformer is made for 110VAC instead of 120VAC 
	

	
	
		
		

		
		
	


	




, my heater voltage is running pretty high. I measure 7.54V instead of the proper 6.3V. I really don't want to shorten the life of my tubes, so now I really want to build a regulated DC heater supply for this amp. I'm looking into the various ways of doing this and all the pros/cons of each method. It would also be nice to build this with a "standby" setting to keep the heaters idling at ~4V to significantly reduce warm-up time and more importantly avoid both cathode poisoning and stripping. I would probably replace the push-button power switch with a 3 position rotary switch. The first position would be "Off", the second would be "Standby", and the third would be "On". This setup would avoid having to drill a hole for a second toggle and would eliminate turning things on in the wrong order.

 It would be great to get both of these PCB's made together which is the route I will probably go.


----------



## dcheming

Well I just hooked my 580's back directly to my soundcard out and I don't know if I like what the 708B has been doing to the sound. The amp does have better tone and much more impactful bass, but there is a harshness in the treble that fatigues me quite quickly, especially on cymbals. I don't have this problem with my soundcard out and I've listened through it for 8 hours straight many times before with very little fatigue. I'm not sure if it's the tubes I'm using or the actual circuit design itself. I still need to change the 6N3 for something better. The RCA 6922's help, but I imagine the gain tube would make a bigger difference. Can any 708 owners make a comment on there amps treble properties? 
 I really like what tubes do to the sound and I would love to get this amp sounding the way I think it ought to, but I'm just trying to figure out if it's worth it making a bunch of expensive mods to _maybe_ get it there if it's not the tubes that are the problem. Should I just move on to a fundamentally better better design? Any comments from more experienced tube-headfiers?


----------



## Hirsch

Quote:


  Originally Posted by *dcheming* 
_As we all know there are very few replacement choices when it come to the mini 6Z4 rectifier tube. Whereas the mini 6X4 has the normal range of NOS and current-production to chose from. The 6X4 uses the same heater voltage as the 6Z4, but two of the pins would need to be switched on the 708B's PCB to use it. The reason I'm posting this is that I'm going to design a new PCB to use the 6X4 instead and I'm curious if any other 708B owners here would be interested in going in on this with me to reduce the cost. I would prefer to get this professionally made for me somewhere instead of using prototyping board. I plan on reusing all the parts that are already on the stock PS board; the only change would be the rectifier tube. Let me know if you're interested._

 

Unless you're willing to drill a hole in the top of the case and have a tube sticking out, you're going to need to mount the 6X4 horizontally (at least if you're going to use a Mullard). The 6X4 is a taller tube, and won't fit in the case. It's going to be a more complex operation than simply redoing the existing board with different pinouts.


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## mirumu

I don't think I'd be wanting to do those power supply mods myself, I've got about ten 6Z4s (no 6X4s) and my amp is running on 240V. Mostly planning cheaper mods as I wanted to spend my money on other gear.


----------



## drarthurwells

Quote:


  Originally Posted by *dcheming* 
_Well I just hooked my 580's back directly to my soundcard out and I don't know if I like what the 708B has been doing to the sound. The amp does have better tone and much more impactful bass, but there is a harshness in the treble that fatigues me quite quickly, especially on cymbals. I don't have this problem with my soundcard out and I've listened through it for 8 hours straight many times before with very little fatigue. I'm not sure if it's the tubes I'm using or the actual circuit design itself. I still need to change the 6N3 for something better. The RCA 6922's help, but I imagine the gain tube would make a bigger difference. Can any 708 owners make a comment on there amps treble properties? 
 I really like what tubes do to the sound and I would love to get this amp sounding the way I think it ought to, but I'm just trying to figure out if it's worth it making a bunch of expensive mods to maybe get it there if it's not the tubes that are the problem. Should I just move on to a fundamentally better better design? Any comments from more experienced tube-headfiers?_

 

Are you talking about the 708B unmodded?

 Your soundcard may have some treble rolloff that reduces distortion from your digital source.

 I changed my 6N3 stock tube for a Russian 6N3P EB and it seemed to have a slight improvement - not as much as I expected though and the improvement may have been just imagined - need more time to tell for sure. One odd thing is that the stock 6N3 has no markings on it at all - not sure it is a Chinese tube - though the other stock tubes are Chinese. The construction of the 6N3 that came with the amp looks like that of the Russian 6N3P EB.

 Botom line: My stock 6N3 tube may in fact be a Russian 6N3P tube which is why it sounds close to (or the same as) the Russian 6N3P EB.

 BTW - I tried 5 new 6N3P EB tubes I just got and three had hum problems. Others have noted hum problems with replacing the 6N3 with other tubes. Very unusual.

 One fault of the 708B is some glare in the timbre of tones in the upper midrange and some harshness with high energy treble material. This is no problem most of the time - just with some CDs or some program material on a CD. Better caps could help here I believe.


----------



## dcheming

Thank guys for your replies. It's all too easy to get frustrated with audio equipment, and I definitely was last night. 

 The diagram that I saw for the 6X4 showed it being a 1/4" taller than the 6Z4. If I move the wires to the top of the PS PCB then I might be able to lower the board down enough to make it fit under the top cover. I might also be able to find a lower profile socket to gain a bit more clearance. I guess the best thing to do would be to just order a cheap 6X4 and check to see if I could make it work.

 Yeah my 708B is pretty much stock right now. Just the signal wire and a pair of resistors have been changed. I don't think my sound card has any treble rolloff as I don't notice any on my monitors or when using the same HP directly from it.

 The 6N3 in mine does have what looks like Chinese letters on it and also a 6N3-J designation under them. I have some other types on they way from my sister so I'll see how these sound instead. Hopefully there will be no hum.

 The glare problem is not fully present on every song, but a few songs really are noticeably harsh. Makes me turn down the volume when it happens. I'm really hoping the caps that are on the way will help alleviate this. I guess my only option right now is to just sit and wait for the parts to get here and try them out. I am going to install them in steps instead of all at once to see if I can narrow down this glare to one part or area. I'll report my impressions when the parts get here next week. Man, waiting on mail-order sucks!


----------



## drarthurwells

I hope you will upgrade in stages and evaluate in each stage:

 Stage one: most important basic upgrades - describe what these are and the results

 Stage two: additional upgrades and describe per above

 Stage three: whole hog suggestions


 If you change out the 6Z4 for the 6X4 you need only drill a 6 mm hole on the top cover to allow the tube tip to clear - likely will extend not much above the level of the top - poses no electrical danger and ventilates the heat for better cooling.

 I think this could improve the sound as the 6X4 rectifier is likley better, and rectifiers do indeed influence the sound quality even though some people say they shouldn't. I changed the Chinese rectifier tube in my home preamp to a NOS RCA 5Z4 and got an improvement, and then to a Ken Rad 5Z4 and got even more improvement.


----------



## mirumu

Quote:


  Originally Posted by *dcheming* 
_The 6N3 in mine does have what looks like Chinese letters on it and also a 6N3-J designation under them. I have some other types on they way from my sister so I'll see how these sound instead. Hopefully there will be no hum._

 

All the tubes that came in my 708b except the 6Z4 have the same logo and are all labeled with the type code, i.e. 6N1, 6N3, 6N11. The original 6Z4 looked cloudy and old and had no markings on it. The extra 6Z4s I own look more like the others in the amp and have the same logo, these are made by Shuguang in China.


----------



## dcheming

Well I just did an experiment with my input cable setup. My sound-card has balanced XLR outs and I've just been using cables that have the "hot" signal and the ground going to an RCA plug. I have some cheap in-line impedance converting transformers that convert bal XLR's to unbalanced RCA's. Well when I used these converters and a RCA cable to go to the 708, the hiss that I normally hear with nothing playing was reduced in level by about four time! These also seemed to help with the high frequency problems as well. However since they are cheap transformers they really colored the mids and made the bass extremely hollow sounding. Nevertheless, this has shown me that the impedance miss-match that I'm getting by using my simple XLR to RCA cables is causing a lot of problems in my setup. I should have known better!
	

	
	
		
		

		
		
	


	




 So I'm going to need to either build a pair of active BAL-to-SE kits or use some nice transformers like Lundahls. I would much rather prefer to go with the transformers, but I think this route might cost too much. Can anybody here recommend a high quality active BAL/SE kit? Even some links to some schematics for a circuit would be helpful too. 

 Damn I wish I had a CD player to try the 708 out SE, but all I have is an X-Box which is _way_ to loud when turned on.


----------



## dcheming

Just finished putting in a 15 amp toggle switch and some 12ga power wire:























 I also mounted the transformer on some 3/8" spacers to get more airflow underneath, but mostly did it because I think it looks better raised up a bit. I used stainless steel fasteners for this:






 Like I said, I love upgrading things!


----------



## Robfow

Great work! 

 did you find any sonic changes after replacing the power cables?


----------



## dcheming

It might have tightened up the very deep frequencies a tiny bit. I don't really notice much of a difference, but I'll take any little bit I can. I wasn't expecting much improvement considering this amp's transformer is only rated for 31 watts max. I mostly did this because I like toggle switches much better than cheap-feeling push buttons. They didn't have any stranded 14ga at ACE so I just went with 12ga instead. If you are considering doing this though, I'd probably go with 14ga because one of the little solder tabs that stick out the back of the IEC connector isn't quite big enough for 12ga. I had to carve the oval hole in it a bit longer to fit the wire through. The metal is soft though, so this is easily done with an X-acto knife, but going with 14ga will eliminate this step if you don't want to deal with it.


----------



## Nugget

After realizing how little free time I actually have, the best I'm going to be able to do in the short run is some tube rolling. I'll pick something up based on the recommendations in this thread, but I have a specific question: Right now I'm only using the DT770/80, and the gain is way too high. The zero point on the volume know is around 7:30, and the highest volume I can stand is around 10. Any way to tame that a bit with different tubes?


----------



## mirumu

Quote:


  Originally Posted by *Nugget* 
_After realizing how little free time I actually have, the best I'm going to be able to do in the short run is some tube rolling. I'll pick something up based on the recommendations in this thread, but I have a specific question: Right now I'm only using the DT770/80, and the gain is way too high. The zero point on the volume know is around 7:30, and the highest volume I can stand is around 10. Any way to tame that a bit with different tubes?_

 

I don't think you'll find any compatible tubes that will lower the gain, that would require circuit changes elsewhere.


----------



## Robfow

Anyone pairing the Xiangsheng 708B with a Senn HD580? 

 My XS 708b stock tubes has been replaced with Electro Harmonix 6922 tubes ( the 2 rear tubes) and the Western Electric 2C51 ( replacing the 6N3).

 I used to be happy with this combination until one day I plugged in a Beyer DT880. Listening to some jazz tunes, i found that vocals on the HD580 sounded muddy. The bass was kind of like boomy.

 On the DT880 it was much more enjoyable.


----------



## dcheming

I'm currently using the 580's with the 708B. Since I really don't have any other decent 'phones right now I really can't make a direct comparison. But I do have a small group replacements for the input tube on the way and I'll give my feedback concerning muddiness and boominess when they get here.

 By the way, as I was drawing the schematic for 708B out today I realized that it is the exact same circuit from the Morgan Jones Amplifier! That amp uses all 6DJ8/6922's however. I'm considering making a tube adapter to run a 6922 in the 6N3's spot in addition to changing out all the various resistors to bring it up to the Optimized MJ specs. Although I think I'd rather just build a MJ properly with nice components.

 Here's the link for comparisons: 

http://www.headwize.com/projects/sho...=cmoy5_prj.htm


----------



## Nugget

I've been running through my tube options, and I've come to realize that I'm, well, a tube noob. There are literally dozens of options just for the 6DJ8/6922 tube, and I've no idea which would sound decent. I liked the good Doctor's description of the Amperex Hollands, but I can't see the wisdom in spending as much on tubes as I did on the amp, especially when I haven't yet figured out the sound I like.

 So, figuring that you've gotta start somewhere, recommend me some tubes and I'll buy the suckers. I'm looking for something that'll smooth out the top end, b/c with the stock tubes I get fatigued fairly quickly. Something that can tolerate a less-than-perfect source would help as well, because I'm running an X-Fi as source to this point and I haven't yet reripped most of my collection to FLAC just yet (most tracks are encoded as the highest quality EAC/LAME VBR). And maybe most important, I'm not looking to spend more than $15-25 per tube ATM. Doubtless I'll upgrade later.

 So: What are my best options?


----------



## dcheming

I would like to know some as well. I'm looking for 5670's, 2C51's, and 396A's online right now. I really need to replace the 6N3 as it is whining at a very high pitch sometimes and if I lightly bump my desk, I can here the metal structure inside resonate like a tuning fork.

 I am also getting fatigued fairly quickly when listening. I am really hoping the Auricaps are going to help with the high frequencies. Cymbals sound light splashes of white noise on a lot of songs right now. At the quite low volumes I mostly listen at, the bass isn't as strong as it is at higher levels.

 Have you found any good looking deals for tubes anywhere online? I also don't really want to spend much more than $25 each right now.


----------



## m8o

Quote:


  Originally Posted by *dcheming* 
_Just finished putting in a 15 amp toggle switch and some 12ga power wire:
 ....
 Like I said, I love upgrading things!
	

	
	
		
		

		
			



_

 

 That's nice wire .... Question. Have you ever looked at the wire gauge of the fuse -all- your power passes through? 
	

	
	
		
		

		
		
	


	




 (edit: assuming you run one)


----------



## drarthurwells

Quote:


  Originally Posted by *Robfow* 
_Anyone pairing the Xiangsheng 708B with a Senn HD580? 

 My XS 708b stock tubes has been replaced with Electro Harmonix 6922 tubes ( the 2 rear tubes) and the Western Electric 2C51 ( replacing the 6N3).

 I used to be happy with this combination until one day I plugged in a Beyer DT880. Listening to some jazz tunes, i found that vocals on the HD580 sounded muddy. The bass was kind of like boomy.

 On the DT880 it was much more enjoyable._

 

I don't like the EH 6922 at all - in the 708B or in the G&W T2.6F

 The EH *Gold* 6922 was very good in my preamp (but I like some others better). 

 I like the tubes I have mentioned earlier in this thread.


----------



## drarthurwells

Quote:


  Originally Posted by *Nugget* 
_There are literally dozens of options just for the 6DJ8/6922 tube, and I've no idea which would sound decent.... I can't see the wisdom in spending as much on tubes as I did on the amp, especially when I haven't yet figured out the sound I like.
_

 

I will sell $10 each total shipped paypal to drarthurwells@hotmail.com. All available in pairs except the 6N3P - have only one to sell (using the other one myself) now - more on order.

 All are fairly well matched and balanced. the most highly matched and balanced pairs is $4 more per tube:


 RCA new old stock 6DJ8

 Amperex Holland test good

 Sylvania JAN 6DJ8 or JAN 6922 test good

 Toshiba 6922 new old stock

 Russian 6N23P EB - new old stock

 For the power (input) tube located in front of the two 6DJ8 drivers (output) tubes:

 Russian 6N3P EB - new old stock for $12 (I bought a few and about half have bad hum and were thrown away so the few that have no hum over just audible levels are the ones I sell - don't know why this input tube is subject to hum so much but I test mine in use to be sure they have no noises).


----------



## dcheming

Quote:


  Originally Posted by *m8o* 
_That's nice wire .... Question. Have you ever looked at the wire gauge of the fuse -all- your power passes through? 
	

	
	
		
		

		
		
	


	




 (edit: assuming you run one)_

 

Yeah, I know the stock power wire was more than sufficient for the minimal current draw of this unit. Since I was already swapping in a new switch, I figured WTH, it'll at least help with cosmetics when the case is off!


----------



## drarthurwells

Quote:


  Originally Posted by *dcheming* 
_I'm currently using the 580's with the 708B. Since I really don't have any other decent 'phones right now I really can't make a direct comparison. But I do have a small group replacements for the input tube on the way and I'll give my feedback concerning muddiness and boominess when they get here.

 By the way, as I was drawing the schematic for 708B out today I realized that it is the exact same circuit from the Morgan Jones Amplifier! That amp uses all 6DJ8/6922's however. I'm considering making a tube adapter to run a 6922 in the 6N3's spot in addition to changing out all the various resistors to bring it up to the Optimized MJ specs. Although I think I'd rather just build a MJ properly with nice components.

 Here's the link for comparisons: 

http://www.headwize.com/projects/sho...=cmoy5_prj.htm_

 

Very interesting. I would prefer a version that used the Russian 6N1P EB tube - a favorite of mine:
http://headwize.com/projects/showpro...bender_prj.htm


----------



## dcheming

Just arrived today.


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## Robfow

dcheming, how much did you pay for those?
 Do update us on the changes they make to the sound


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## dcheming

Total in that pic was about $88. This was ordered from PC during the 15% off sale and we don't have sales tax here in OR(
	

	
	
		
		

		
			





), so it would be a bit more than this in reality.

 I'm doing this round of mods in stages and taking notes in between. The Auricaps went in first and then I listened for a hour and a half. These helped a lot with the treble harshness. Distorted guitars and cymbals were a lot more clear. They also seemed to tighten up the bass a bit, which surprised me. Overall the sound was more detailed, but also smoother which is a nice combo.

 As of right now the Riken's are also in and I'm listening as I type. They seem to have brought a bit of an edge back to the treble unfortunately, but I want to give them some time to burn in more before I make more comments.

 I'm going to wait a few days until I put the ALPS in to see what happens with the Riken's. I just realized that I'm almost out of solder! I need to drive to the nearest electronics store 30 miles away in Salem this weekend to get more.

 So far in this entire modding project I would say the Auricaps have made the biggest difference. My recommendations at this point in the game would be to replace the RCA's, the signal wiring, the inter-stage coupling caps, and the tubes. For me that totals to about $42 in parts. All the other stuff that I've done so far I can't say made a worthwhile difference. I'll post further impressions in time.


----------



## Robfow

Thanks DCheming. Wow seems that this little amp after some extensive mods gonna cost a lot!


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## dcheming

Just finished putting everything in. First things first; the three part swaps that made the biggest difference were the ALPS potentiometer, the Auricap inter-stage capacitors, and the ELNA headphone output capacitors.

 I feel the ALPS made the most noticeable difference, but the Auricaps were in as well at that point, so some of the improvement was the combination of the two. The ALPS helped a lot not only with overall resolution, but also with soundstage clarity. The instruments are easier to locate, sound more natural, and on songs that have little details such as echos that are way off to the side are much more audible now. Reverb sounds much sweeter too.

 Both the Auricaps and the ELNA's made the sound more detailed and smoother. Distorted electric guitars and cymbals are way less harsh than they were before. The Auricaps seemed to tighten up the bass some too.

 I'm pretty happy with how it's sounding right now. This is what I spent on the parts that I'd recommend for the biggest improvement:

 ALPS 100k - $18 ea
 Auricap 0.22uF - $15 pr


 I got the ELNA 100uF 250V caps for about $3 a piece. I'm using two in parallel for each channel. I also swapped the Riken input resistors out for NTE metal films. I liked the more organic sound of the Riken carbon films, but they where adding an edge to the treble which was bothering me. So I'd either leave in the stock input resistors or go with some nice metal films. 

 I also changed the 22uF power supply bypass electrolytics for some 33uF Gemcons. I did this because the caps that came with my amp where dented and scratched almost through the aluminum case. I didn't notice any improvement with these in though.

 I also rotated the power supply board around 180 degrees. This works out better for wiring and I think it makes the layout of the inside look more balanced. I re-soldered all the power wires to the top of the boards instead of underneath.

 Be aware that the ALPS is not a direct drop in. It will require the mounting hole to be enlarged slightly. I did this carefully with a round file because I didn't have the proper size drill bit. Also the little stub that prevents the pots body from rotating is further out than the one on the stock pot, so a new hole will have to be drilled for this. I mounted my ALPS with the pins facing up, so this little hole was drilled on the right side of the shaft.

 So overall the clarity has significantly improved, the sound is a lot smoother, and most of the harshness is gone. I will be listening all night so I'll see how fatigue compares to how it used to be. With the two crucial mods I mention above as well as new headphone output caps, RCA jacks, signal wiring, and tubes, one can get a huge improvement in sound for around $70. This just reiterates to me how important quality components are, especially when listening with something as sensitive as headphones.


----------



## dcheming




----------



## Nugget

That's some gorgeous work. Next might as well try building one from scratch! 
	

	
	
		
		

		
		
	


	




 I'm awaiting some new tubes from drarthurwells, will drop those in ASA they arrive. Other than that, I've come to the painful realization that my schedule won't allow for too much DIY goodness in the immediate future....


----------



## Nugget

Two new Russian 6N23P-EB and a Soviet-era 6N3P-EB just rolled in, and hot damn are they an improvement. My current setup has more weaknesses than I can count, so I was worried that I wouldn't notice a difference. Boy, was I wrong. The amp is now more musical, faster, more detailed, and more refined, especially in the treble. I don't have the advanced audiophile vocabulary that some of you folks do, so the best way I can describe it is that stock the sound came _at_ you and with the new tubes it surrounds you.

 Thanks to Doc Wells for the tubes. Now to (eventually) replace some of the other parts!


----------



## dcheming

Man, I really need to replace the cheap 6N3 in my amp. It's extremely microphonic and I'm getting a a high pitched whining sound sometimes. I put together a cheap isolation table using two 2"x2"x12" step-stones with a 12" inner-tube between them and a 13"x13" piece of carpet to go under the bottom stone. This helps a lot with the flimsy computer desk that I'm using right now. So you got the 6N3P-EB from drarthurwells too? I need to get some kind of replacement asap.

 I'm going to take the 708B to an audio shop down the street and listen to it with a NAD C542 this weekend. I want to see how good it sounds with a nice source feeding it. I've been letting it play as much as possible these last couple days to burn in all the new parts and I'm hoping to get another 25 to 35 hours on it before listening to the NAD.

 Nugget, it's a shame you don't live closer to me, I'd be more than happy to do some surgery on your amp as I have quite a bit of free time right now. I'm glad to hear you got an nice improvement with the new tubes.


----------



## Nugget

Yeah, I'm waiting for a chance to break out the soldering iron, but I just don't see it happening for a while. As my wife put it when I was swapping the tubes last night, and managed to short a charged capacitor with a piece of the stock bent-wire "tube-restraining-system" leading to a loud pop, "Why are you always trying to break things or kill yourself?"

 I didn't have any microphonics problems, but one of the stock 6922s had a crackling problem at anything approaching higher volumes. Very noticeable in quieter passages, completely destroying the dynamics of the music.


----------



## dcheming

I did the same thing with one of the bent-wire restrainers when I first put the tubes back in and got a big spark. I just took them all out after that, but I'll probably install them back in when I finally put the cover back on.


----------



## Robfow

dcheming, may I ask what were the original caps and resistors that were inside the 708b?


----------



## dcheming

Robfow, all the resistors that I've replaced were just cheap, generic metal-film types; 1k for the input, 1k for the 6N3 cathode bias, and 10R for the headphone jack.

 The power supply bypass caps were SHL 22uF 400V electrolytics.

 The interstage coupling caps were WIMA 0.22uF 400V metallized polyester. 

 The preamp output also uses these exact same WIMA caps too.

 The headphone output used KME 150uF 200V electrolytics.

 The volume pot was a cheap WL 100k carbon type.


----------



## Robfow

Thanks dcheming. I'm deciding what to get to replace those. I bought mine second hand and the previous owner had already replaced the volume pot with a Alps 100k.


----------



## zer061zer0

Hi guys, I am now currently modding my Xiang Sheng, after hugely being motivated by what you guys had done. 
	

	
	
		
		

		
		
	


	




 I am a real newbie to this, and was wondering if any of you have the schematic for Xiang Sheng.

 Appreciate it a lot. Thanks!


----------



## dcheming

zer061zer0, I do have a schematic drawn out, but I don't have a scanner. I could try to take a picture of it and see if it is readable. The 708B uses pretty much the same layout as the Morgan Jones amp, but most of the component values are different. Here's the link to the MJ amp for reference:

http://www.headwize.com/projects/sho...=cmoy5_prj.htm

 I'm pretty happy now with the overall sound I'm getting from the amp. I still need to swap the 6N3 though. I just picked up a NAD C521BEE cd player this weekend and I'm going to mod it of course. I just downloaded the service manual to help with that. The sound of this player is not bad, but I know it can do better. So this will be an on-going project these next few weeks.

 It will be nice to have someone else do some mods to the 708B also to get a second opinion. If you have any questions I might be able help and I have a ton of pics that I'd be able to use to help explain things with. So what are you thinking of doing first?


----------



## zer061zer0

Thanks Dcheming! I would appreciate that greatly!

 Actually, I have not done any modding to my 708B. I do not have any experience in DIY modding, and have no background in electronics. 

 I do have some one who is helping me with my modding. Was thinking of changing some of the in and output caps. Also, my right channel output suddenly went kaput yesterday, so i will also be changing the RCA plugs for it? Would like to change the on/off switch to maybe a solid toggle switch, and some of the internal wires too.


----------



## dcheming

Does your friend have experience working with the high voltages that are present in tube amps? Just wondering, since modding tube amps really isn't the safest project to start with if not. The big power supply caps in this amp don't have bleeder resistors on them so they remain charged for hours after turning off the power. I always use a resistor of around 35k with some jumper wires to drain these caps before I work on the amp.

 It might be that the right channel tube has gone bad. Swap the positions of the two 6922/6DJ8's to see if it is the tube. Could also be a loose connection on the signal wiring somewhere; I know that on my amp the stock signal wiring was very poorly soldered to the RCA jacks. Slight chance it could be that the cheap volume pot has broken internally.

 I personally prefer toggle switches to push buttons, especially on tube equipment. I used a medium duty toggle from ACE. I would definitely put in new signal wiring as well. I just used Belden CAT6 Teflon twisted pair. I was worried about hum since this wire is not shielded, but haven't had a problem with this.

 The headphone output caps will have to be electrolytic type due to the uF size. The voltage handling on these caps will need to be rated to at least 200 volts. The ELNA's that I used are 250V and 100uF each, with two in parallel for each channel. I took out my input caps since my CD player already has output caps. Only do this though if you are sure that your source doesn't put out large amounts of DC. You are probably better off just leaving them in. If you want to upgrade them, they are 0.1uF 50V. Due to the small value you could go with something nice like a PP film and Zn foil cap. I'd do the same for the other four 0.22uf coupling caps inside if you can afford it. Mundorf M-Cap ZN, Multicap PPFX, and Audiocap RT are all nice options that are relatively affordable. Of course there are many other choices for caps too.


----------



## zer061zer0

Quote:


  Originally Posted by *dcheming* 
_Does your friend have experience working with the high voltages that are present in tube amps? Just wondering, since modding tube amps really isn't the safest project to start with if not. The big power supply caps in this amp don't have bleeder resistors on them so they remain charged for hours after turning off the power. I always use a resistor of around 35k with some jumper wires to drain these caps before I work on the amp.

 It might be that the right channel tube has gone bad. Swap the positions of the two 6922/6DJ8's to see if it is the tube. Could also be a loose connection on the signal wiring somewhere; I know that on my amp the stock signal wiring was very poorly soldered to the RCA jacks. Slight chance it could be that the cheap volume pot has broken internally.

 I personally prefer toggle switches to push buttons, especially on tube equipment. I used a medium duty toggle from ACE. I would definitely put in new signal wiring as well. I just used Belden CAT6 Teflon twisted pair. I was worried about hum since this wire is not shielded, but haven't had a problem with this.

 The headphone output caps will have to be electrolytic type due to the uF size. The voltage handling on these caps will need to be rated to at least 200 volts. The ELNA's that I used are 250V and 100uF each, with two in parallel for each channel. I took out my input caps since my CD player already has output caps. Only do this though if you are sure that your source doesn't put out large amounts of DC. You are probably better off just leaving them in. If you want to upgrade them, they are 0.1uF 50V. Due to the small value you could go with something nice like a PP film and Zn foil cap. I'd do the same for the other four 0.22uf coupling caps inside if you can afford it. Mundorf M-Cap ZN, Multicap PPFX, and Audiocap RT are all nice options that are relatively affordable. Of course there are many other choices for caps too. 
	

	
	
		
		

		
		
	


	


_

 

My friend is a pretty well trained technician and i think heis having some plans to redo the circuit alittle, cos i am using it as a preamp, but i think he pretty much needs the schematic. 

 What would u suggest to mod if say we were to use it as a full fledge pre amp instead of a head amp.

 Regards,
 Marcus


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## Robfow

If we were to add bleeder resistors to the caps, will it degrade the SQ of the amp?


----------



## dcheming

zer061zer0, I just emailed you the requested pics. If you are only going to be using it as a preamp then you don't have to worry about the big headphone output caps. I would upgrade the RCA jacks, the signal wiring, the volume pot, the input caps, the input resistors, all of the 0.22uF caps, and of course the tubes. You might also want to replace the 22uF caps since the ones in mine were damaged. I didn't notice an improvement with these, just piece of mind since they are carrying ~220V.

 Robfow, I really don't think that there will be any problems with adding a bleeder resistor. Many companies do this with tube amps. Morgan Jones recommended a value of 220K at 2W for this purpose in one of his books. I've been meaning to do this to my amp.


----------



## zer061zer0

Quote:


  Originally Posted by *dcheming* 
_zer061zer0, I just emailed you the requested pics. If you are only going to be using it as a preamp then you don't have to worry about the big headphone output caps. I would upgrade the RCA jacks, the signal wiring, the volume pot, the input caps, the input resistors, all of the 0.22uF caps, and of course the tubes. You might also want to replace the 22uF caps since the ones in mine were damaged. I didn't notice an improvement with these, just piece of mind since they are carrying ~220V.

 Robfow, I really don't think that there will be any problems with adding a bleeder resistor. Many companies do this with tube amps. Morgan Jones recommended a value of 220K at 2W for this purpose in one of his books. I've been meaning to do this to my amp._

 

dcheming, i have gotten ur mail and have since forwarded to my frds, thanks for all the help. i will try to pose some pictures when the things is up.


----------



## dcheming

Did a few more mods tonight. The 2W resistors in the power supply were getting pretty damn hot so I increased their wattage. I just used some NTE's here. I decided to use four 1W resistors in series/parallel so that they would fit in between the PS caps and still get good circulation. The big 3W type that I used for the other position would of been too tight a fit here. 

 I also finally found a metal TRS jack made by Switchcraft that fits the 3/8" front panel. I think this matches the toggle switch much better.

 I picked up a Tripp-Lite isobar ultra when I was at the parts store. The noise filtration in it has made the background noticeably quieter; reverb goes a bit deeper now. I also put a snap-on ferrite on the amp's power cord.

 I picked up some 220K resistors to use as bleeders, but I think this value is way too conservative. The voltage on the caps was only going down by about a millivolt per second with these. When I use 35k, the voltage drops at a rate of a few volts per second. So I think I'm going to go with something like 60K instead next time I go to the parts store.


----------



## dcheming

Just put these babies in my NAD last night:






 They are just connected to some euro-style terminal blocks that I soldered on the board. I'll mount them properly later if I like the sound.


----------



## zer061zer0

Quote:


  Originally Posted by *dcheming* 
_Just put these babies in my NAD last night:






 They are just connected to some euro-style terminal blocks that I soldered on the board. I'll mount them properly later if I like the sound._

 


 Hi dcheming, 

 how did ur nad sound now with those new inputs.

 My XS is somewhat modded now and i am enjoying it as a preamp now. What my frd have done for me was changing the caps to polyproplene ones with basically the inputs, feedback and output caps, and also changing all the 4 rca plugs.

 The background is really quieter in a way with lesser noise. I have also tried the 6dj8 slyvannia tubes for the left and right channel. Much more detailed then the default tubes and slightly bright though, but i like it. 

 I am now wondering how it would sound if i were to more upgrade on it, i am wondering how well it go.

 What do u think.

 Regards.


----------



## dcheming

With the Mundorf M-Caps the bass tightened up a lot and the midrange sounded more natural. Something wasn't quite right with the upper end of the spectrum though. I took them out and have been using some non-polar NTE caps bypassed with the stock 0.22uF film caps. This setup with the bypassed NTE's sounds better than the stock setup, but not nearly as tight as with the Mundorf's. I'm going to try the M-Caps again when I can get some 0.22uF film and foil caps to bypass them with. I'm hoping that this will fill in the top end to match how good they sounded everywhere else. Better opamps mounted in sockets will be the next mod after that.

 I'm glad the PP caps helped clean up the sound. What brand did you end up going with? Did you use the same kind for all of the positions? Have you played around with different 6N3's yet? From what I understand this tube will make the biggest difference since it is the gain stage. Concerning noise, I found that I can turn the volume all the way up when on pause and not hear any hiss since I put in the Tripp-Lite isobar ultra. In my system without the isobar I hear hiss starting around 8:00 on the volume.

 IMHO, a good next step would be replacing the signal wiring and putting in a Blue Velvet. As I wrote a while back, I feel that the pot made the single biggest improvement.


----------



## zer061zer0

Quote:


  Originally Posted by *dcheming* 
_With the Mundorf M-Caps the bass tightened up a lot and the midrange sounded more natural. Something wasn't quite right with the upper end of the spectrum though. I took them out and have been using some non-polar NTE caps bypassed with the stock 0.22uF film caps. This setup with the bypassed NTE's sounds better than the stock setup, but not nearly as tight as with the Mundorf's. I'm going to try the M-Caps again when I can get some 0.22uF film and foil caps to bypass them with. I'm hoping that this will fill in the top end to match how good they sounded everywhere else. Better opamps mounted in sockets will be the next mod after that.

 I'm glad the PP caps helped clean up the sound. What brand did you end up going with? Did you use the same kind for all of the positions? Have you played around with different 6N3's yet? From what I understand this tube will make the biggest difference since it is the gain stage. Concerning noise, I found that I can turn the volume all the way up when on pause and not hear any hiss since I put in the Tripp-Lite isobar ultra. In my system without the isobar I hear hiss starting around 8:00 on the volume.

 IMHO, a good next step would be replacing the signal wiring and putting in a Blue Velvet. As I wrote a while back, I feel that the pot made the single biggest improvement._

 


 I have tried a 2c51 to replace the 6n3, the brand is colomor but i get a very loud hum from it. Still looking for replace for the 6n3. the pp caps are all of different brand, vishay, philips and one more which i can't see. 

 I am planning to change the pots and swtiches maybe at a later stage, maybe a recase even, when i start to itch again and change all the caps to the mundorf silver/oil ones. 

 however i do note that i got this strange sound when the amp is on and nothing is played, any experience on this.?


----------



## dcheming

That sucks about the 2C51 hum. Hopefully I will avoid that when I finally get a replacement.

 I wouldn't wait till later for the pot if I were you. If you can afford it I'd do it asap as well as new signal wiring. I don't know if this will make as big a difference in your system as it did for me since you are only using it as a preamp, but if I was only going to do one mod it would be the pot for sure.

 Man I can only imagine how all Mundorf Silver/Oil caps would sound! Putting it into a new case would be nice, but I'd rather build something much better DIY or just get a kit.

 Concerning the strange sound, what does it sound like? Does it remain when you unplug your source from it with the amp on?


----------



## Robfow

I had the same hum problem with a 5670, can't remember the brand. I later exchanged it for a 2c51 and it worked fine.


----------



## zer061zer0

Quote:


  Originally Posted by *dcheming* 
_That sucks about the 2C51 hum. Hopefully I will avoid that when I finally get a replacement.

 I wouldn't wait till later for the pot if I were you. If you can afford it I'd do it asap as well as new signal wiring. I don't know if this will make as big a difference in your system as it did for me since you are only using it as a preamp, but if I was only going to do one mod it would be the pot for sure.

 Man I can only imagine how all Mundorf Silver/Oil caps would sound! Putting it into a new case would be nice, but I'd rather build something much better DIY or just get a kit.

 Concerning the strange sound, what does it sound like? Does it remain when you unplug your source from it with the amp on?_

 

I think the strange sound was caused by the use of a fan on the same power socket. The sound is gone once i switch off the fan.

 Robfow what 2c51 did u get.


----------



## Robfow

i got a Western Electric 2c51.


----------



## dcheming

I finally got around to adding the bleeder resistors. I went with a value of 82k at 2w. I measured about a minute and a half to go from the operating voltage of 260v down to 10 volts when I turned off the power. Each resistor dissipates around 0.8w, so they should be fine. I raised the PS pcb up a tiny bit to give some clearance underneath. The top of the 6Z4 still has plenty of room.







 I also shortened the leads on the Auricaps since they are staying in for good. When I use boutique parts I like to leave the leads as long as possible so that if I end up taking them out I can use them easily in something else.


----------



## dcheming

zer061zer0, could you post some pics of your modded 708B?

 I'm going to add a source selector toggle switch later this week. I'm debating either putting it on the back panel beside the RCA jacks or putting it centered underneath the volume knob on the front panel. If I put it under the volume knob, I'd set it up to switch left/right to give better clearance. 

 What do you guys think? If it was you, which way would you go?


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## Robfow

Personally i'd put it on the front panel just for convenience sake.

 I'm outstationed for work currently and havent had the chance to do more mods yet.


----------



## dcheming

Yeah, on the front is where I think it should be too. I want to drill and tap the thick front panel to mount the switch since it works out to be the perfect depth. Only problem is that I'm having trouble finding a 1/4" 40 tap. I might just have to order one.


 Since I can't get the switch until Friday due to Turkey Day, I figured might as well get the hard part out of the way tonight:

















 I'm getting really tired of swapping cables between my CD player and computer; this is going to be really nice.


----------



## Miller-8

I just got a 708B recently and here's some things I've observed:

 1. The gain must be really high, using it as a pre-amp a tiny movement of the volume knob changes the sound dramatically. I cant even get to 9 o'clock it goes too loud.

 2. The volume pot seems dodgy, I was getting sound just through one channel but as I increased volume both channels kicked in. This problem seems to have decreased now. I was getting sound out the left when the volume knob was at zero. Cheap pot?

 3. When I plug my headphones in I need to make sure it's switched off and the power amp is off otherwise I get massive noise through the speakers. Even when it's all switched off I get a loud pop noises through the headphones when I plug the jack in.

 Can anyone suggest remedies for these problems? Currently tube changes are not on my mind til I get these things sorted.


----------



## dcheming

Yeah the gain is pretty high. I'm always in the 8 to 9 o'clock range when using my CD player. With my computer though I reduce the output level which lets me use much more range on the pot. I don't think it's very easy to mod the circuit to get less gain.

 The stock volume pot is absolute crap, both in sound quality and channel tracking precision. I measured the differences between channels on the stock pot at 6 different locations. The differences ranged from 1900Ω to about 2800Ω. I was having horrible balance issues at the 7 to 8 o'clock range. When I got the ALPS RK27 I did the same measurements. For 8 locations, the differences ranged from 200Ω to a max of 700Ω with most locations being in the 200Ω to 300Ω range. I get superb balance now at the low volumes that I listen at. The ALPS is also much more transparent and made a very nice improvement to the sound. One of the best mods to do IMO.

 I was getting the loud pops too whenever I plugged my cans into the powered up amp. So I built a dummy load into a 1/4" TRS plug. It's just a 300Ω resistor on each channel to ground. I leave it in as I turn on the unit to let it warm up, remove it when the headphones are finally plugged in, and then put it back in before I power the amp down. This practice has totally eliminated pops upon headphone insertion. I'm thinking it has to do with the large headphone output caps, but I could be wrong. It might be something with the tubes. Either way though, it's a cheap work around.


----------



## Miller-8

Does the volume knob just pull off?


----------



## dcheming

Yeah the stock plastic volume knob is just pressed on. The shaft of the volume pot has splines to get a good grip.


----------



## drarthurwells

Quote:


  Originally Posted by *zer061zer0* /img/forum/go_quote.gif 
_I have tried a 2c51 to replace the 6n3, the brand is colomor but i get a very loud hum from it. Still looking for replace for the 6n3._

 


 I think the 6N3P EB Russian military tube is the best replacement for the Chinese 6N3.

 I bought a bunch, tried them all out in my 708B, and found nearly half had a hum problem of varying degrees. I threw out the bad ones and have the good ones for sell (see Tubes Tubes Tubes in the amps for sell section) with refund if you don't like them.

 BTW to dcheming : You ought to offer a mod service for various mod stages as I bet they offer worthwhile SQ improvement.


----------



## F155mph

Quote:


  Originally Posted by *dcheming* /img/forum/go_quote.gif 
_Yeah the stock plastic volume knob is just pressed on. The shaft of the volume pot has splines to get a good grip._

 

Dcheming,

 Where did you buy your parts from and what spec Alps pot did you get?

 Thanks!


----------



## dcheming

drarthurwells, I'm going to have to take you up on your offer and get a pair of your 6N3P EB Russian's. I can't find any suitable replacements around here. 

 F155mph, sorry for taking so long to reply. Most of the boutique parts I got from parts conneXion when they had their last sale. The rest of the normal parts I just got locally. You could get them pretty much anywhere though, but I'm going with Michael Percy Audio simply because they have exactly what I need for my next order.

 The pot I went with was an ALPS Blue Velvet 100K with the smooth shaft. The knob is from rat-shack and just happend to fit nicely with the circular milled pocket on the front panel. It doesn't quite fit inside, but the chamfer on the pocket allows the knob to sit a little bit recessed in the front panel. I think the old school style knob matches the toggle switch and the fact that it's a tube amp. Only problem though is that the ALPS is a 6mm shaft and the knob I got is for 1/4" so it isn't quite centered. I just used some thin strips of tin foil wrapped on the shaft to increase its size a bit.

 Be warned though that it's not a simple swap with the Blue Velvet. Not only does the mounting hole have to be enlarged some, but a small hole has to be drilled accurately for the locating pin to fit into, which prevents the pot itself from rotating. It's definitely doable, but take your time with measurements.


----------



## F155mph

Dcheming,

 Can you please list the parts and specs of the parts you got? I want to try to do the mod myself but I don't want to buy the wrong parts. If I use Hovland caps instead of Auricap, would that make much of a difference? I can't find the Elna for the headphone output on PC.

 Thanks!


----------



## dcheming

All right, so first of all I'm assuming that you want to replace all the parts in the signal path. So starting from the input:

RCA Jacks - the mounting holes are 3/8" so any of the generic replacement kind will work here. 
 Something like P#: CONNEX- 53452 from parts connexion for $6/pair would be nice.

Signal Wiring - I used a twisted pair out of some Belden 7882A CAT 6 cable and I chose the pair that had the tightest twist, orange, out of the four twisted pairs inside. This wire has FEP insulation which is closely related to Teflon, but it melts at a lower temp so it's easier to coat wire with it. This wire is cheap and since it's 24ga it fits easily into the PCB holes. However, I just used it because it was the best small gage wire that I have at the moment. This type of twist has high capacitance so I would use something else. You could probably go as big as 20ga, but if you don't mind using a small drill bit in places you could use 18ga by slightly enlarging the holes in the PCB. I've had to do this in a few places. If you just got some nice 18ga to 24ga wire you could do a three-wire Litz braid in the three runs required inside the 708B. You could go with either solid core or stranded, it's up to you. Personally I like solid because it stays where you put it and some say it sounds better. Maybe some head-fiers could make some suggestions here. I know that the Cardas wire is a bitch to work with and so should be avoided. Maybe something like:
 FIM 68755 20 awg Mono-Crystal design with PVC dielectric for $0.66/ft or

 JUPITER 68824 28awg, 99.999% pure solid-core COPPER with unwaxed cotton insulation for $1/ft or

 KIMBER 519xx TCSS 19 awg hook-up wire for $0.86/ft.

 All of these P# are from parts connexion; I'm trying to stick with one supplier to save on shipping. 

Volume Pot - ALPS 64605 Blue Velvet STEREO 100K for $17.95 

Volume Knob - The inner diameter of the milled pocket measures 1.307" or 33.20mm on my amp. So a 1-1/4" knob should work here. Right now I'm using the Rat-Shack #274-407 knob, but as I said it will rotate a bit off-center. I had to cut a bit of the shaft off to make it short enough to get the knob flush with the front panel.

Input Caps - C101,102 - These are 0.1uF, but I'd probably go with a bigger uF here to cause less phase distortion. Since these caps are just passing line level signals you can use low voltage film here. I'll do some more research on this to get an actual value, but I think 1uF will be okay. You could even use the long leads on these caps to go directly from the output pins of the volume pot to the PCB. You would avoid some four inches of signal wire and also have two less solder joints in the signal path this way. The Hovland FF/Polypropylene that you suggested would be a very nice cap to use if you have the budget.
 HOVLAND 58903 1.0uF 200V for $14.45ea.

Input Resistors - R107,207 - These are 1kΩ. Something like the Holco/PRP Precision Metal Film would be nice. 1/4W should be fine since it's line level, but I'm using 1W since they where only a bit more. Vishay/Dale resistors would be nice too.

Cathode Bias Resistors - R103,203 - These are also 1kΩ at 1W. I don't know how important these are but it couldn't hurt.

Interstage Coupling Caps - C104,204 - These are 0.22uF and are passing high voltages so go with at least a 400V rating.
 HOVLAND 55980 0.22uF 400V for $10.20 would work. 

Pre-out Caps - C105,205 - Exact same as the above 0.22uF 400V here. If you aren't going to use the 708B as a preamp then skip these to save money.

Headphone Output Caps - C6,7 - You can use anything from 200uf to 470uF here rated to at least 250V with bigger capacitance giving a lower bass roll-off frequency. I got these at a local electronics store for about $3 each. They are ELNA 100uF 250V polar electrolytics, two in parallel per channel. If you have trouble finding something for these I could ship you some.

Power Supply Bypass Caps - C8,9 - These are 22uF 450V polar electrolytics. I just used some GEMCON's that I got at the electronics store. I replaced these because the ones in my amp where pretty beat up looking.

Power Supply Resistor - R2 - This is 2kΩ. The stock one is rated at 2W and was getting very hot so I put in a NTE 3W metal film which now runs much cooler.

PS Filter Cap Bleeder Resistors - These are the ones that I soldered underneath each of the two large caps on the PS board. They are 82kΩ at 2W. This isn't really required, but makes the amp a lot safer to work on.

Headphone Jack - The one I'm using is made by Switchcraft and it is the only one I've found that will handle the thick front panel. If need be, I could pick you up one the next time I go to that store and ship it with the ELNA's.

Headphone Output Resistors - R1,2 - These are 10Ω and 1W rating should be fine. Use the same brand resistors that you use for the input resistors. 

Toggle Switch - Just a medium duty toggle from ACE Hardware. It has a really small body that's about 1/2" by 3/4" and is about 3/4" with the terminals, which are screw type. The heavy duty one was way too big.

 That's pretty much everything. Of course you could go with whatever brand you'd like for these parts. Looking back on it I should have used a FF/PP cap instead of the Auricaps, but they still made an improvement so I can't complain. I have plenty of pics that I took so I'll be able to help you out if you need it for the harder parts.


----------



## F155mph

Dude,

 You rock!!! Thank. I am going order the parts and get this started. I almost shock myself yesterday swapping tubes.

 I am sure I got more questions for you once i get the parts.

 Thanks!


----------



## F155mph

Quote:


  Originally Posted by *dcheming* /img/forum/go_quote.gif 
_
Input Resistors - R107,207 - These are 1kΩ. Something like the Holco/PRP Precision Metal Film would be nice. 1/4W should be fine since it's line level, but I'm using 1W since they where only a bit more. Vishay/Dale resistors would be nice too.

Cathode Bias Resistors - R103,203 - These are also 1kΩ at 1W. I don't know how important these are but it couldn't hurt.


Power Supply Resistor - R2 - This is 2kΩ. The stock one is rated at 2W and was getting very hot so I put in a NTE 3W metal film which now runs much cooler.

PS Filter Cap Bleeder Resistors - These are the ones that I soldered underneath each of the two large caps on the PS board. They are 82kΩ at 2W. This isn't really required, but makes the amp a lot safer to work on.

Headphone Output Resistors - R1,2 - These are 10Ω and 1W rating should be fine. Use the same brand resistors that you use for the input resistors. _

 


 Ok I got some question now. I am not sure what to get for the input and cathode bias resistors. Some of the resistors have extra numbers after the value and I don't know what that mean. (ie PRP-70168 1K2 PRP 1Watt)

 Secondly I can't seem to find the right resistors for the power supply resistors, bleeder resistors and headphone resistors.

 Thanks for the help.


----------



## dcheming

For the input and cathode bias resistors you'd probably be better off going with a metal film style. They are typically cleaner sounding and have less noise than carbon types, but some people like the sound of carbons better.

 The PRP or HOLCO would be a good place to start. You could always solder in some terminal blocks so that it would only take a screw driver to quickly swap resistors. You could then pick up a few different types, listen to them all, and then make a final decision. I tried some Riken carbon films first and while I loved what they did to the overall sound, I wasn't completely happy with them. Looking back I would have liked have installed terminal blocks to experiment more with these.

 Ok, so in the example 100R, the "R" is basically taking the place of ohms (Ω), and so means 100Ω. For 1K2, this means 1.2K or 1200Ω. The "K" means 1000 and is also representing a period between the 1 and 2. Putting it in between the 1 and 2 makes it harder to make a mistake in reading the value since a big "K" is much more visible than a little period.

 So for the headphone resistors you'd need two 10R. The bleeders can be anything from 75K to 120K; the value of 82K is not set in stone here. The 2K power supply resistor can be anything really as long as it's rated for at least 3W and up. I didn't really want to go with a 5W sand cast resistor since that's a bit too big and I really don't like how they look. The NTE 3W metal films I got were only about $3 for a two pack and fit nicely.


----------



## F155mph

So how much sound improvement should I get out of this? The total so far is $153.65 not including the ELNA caps that I need to get from you. I am debating if this is worth it since the amp cost about the same price.
	

	
	
		
		

		
			





 Please pm so I can get those ELNA from you.

 Thanks!


----------



## dcheming

You really don't have to do everything mentioned to improve the sound. I feel that the two biggest improvements were the ALPS pot and the Auricaps.

 I've put about $110 worth of parts in mine and don't regret doing so. A few of these where just for cosmetic reasons, but if you just want to get the biggest improvement per dollar spent then maybe you could just start with the important stuff first like the pot, resistors, wire, RCA jacks, headphone output caps, and the interstage caps. These are the weakest links and should be around $80 total. I know that this is a good fraction of the cost of the amp, but one of the reasons why the 708B is so cheap is due to the low cost of the internals. You can always just do this in small steps to see if you think it's worth it.

 If you measure a low DC output on your source then you can get rid of the input caps completely. This is a gamble though because your source might develop higher DC someday and that would then destroy your cans, but the best cap is no cap.


----------



## dcheming

3 Hours and a six pack later...I've finally added the input selector switch. I just went with a GC 3PDT mini toggle switch instead of the TOCOS one I was planning on using; I didn't want to wait two more weeks for shipping. All I have to do now is order a 1/4"-40 tap from ICS Cutting Tools and then I'll be able to mount the switch underneath the volume knob.











 This was by far the best mod I've done so far since I go back and forth between my CD player and computer/Xbox daily.


----------



## zer061zer0

Quote:


  Originally Posted by *dcheming* /img/forum/go_quote.gif 
_3 Hours and a six pack later...I've finally added the input selector switch. I just went with a GC 3PDT mini toggle switch instead of the TOCOS one I was planning on using; I didn't want to wait two more weeks for shipping. All I have to do now is order a 1/4"-40 tap from ICS Cutting Tools and then I'll be able to mount the switch underneath the volume knob.











 This was by far the best mod I've done so far since I go back and forth between my CD player and computer/Xbox daily. 
	

	
	
		
		

		
		
	


	


_

 


 Hi Dcheming,

 You have got some serious mod over there. Was wondering if for the volume knob, is there any other option besides going for a blue velvet, i figured that i might spoilt the stuff with my half past six skills on the machine.

 Would also like to check with you if u have any idea of this amp also by Xiangsheng, the H-02A.


----------



## dcheming

Yeah I guess the list of mods is getting fairly big.
	

	
	
		
		

		
		
	


	




 The Blue Velvet is a good deal for $18, but you can surely do better and there is enough space to use pretty much anything. The ALPS Black Beauty would probably be better (if you can find one), not sure though as I've never used one. A TKD CP2511 would be nice too. Above these would be the various stepped attenuators, but I would never put something that nice in a cheap amp like the 708B.

 The Xiang Sheng H-02A looks interesting being a 2×30W hybrid amplifier, but I'm not familiar with it though. How much does it cost? Considering the low quality parts and the 110v transformers used in XS equipment, I'd pass personally. I would much rather build something like a Charlize amp kit with quality parts instead.


----------



## drarthurwells

I would love to hear your modded 708B - bet it is terrific.


----------



## phergus_25

You should look for one of the fake ALPS stepped attenuators. One came in my amp, and although I would have much rathered a real one I dont think you can argue much with its ability to preform.


----------



## dcheming

Quote:


  Originally Posted by *drarthurwells* /img/forum/go_quote.gif 
_I would love to hear your modded 708B - bet it is terrific._

 

Well if we ever run into each other at a meet, I'm bringing it for sure.


----------



## zer061zer0

Quote:


  Originally Posted by *dcheming* /img/forum/go_quote.gif 
_Yeah I guess the list of mods is getting fairly big.
	

	
	
		
		

		
		
	


	




 The Blue Velvet is a good deal for $18, but you can surely do better and there is enough space to use pretty much anything. The ALPS Black Beauty would probably be better (if you can find one), not sure though as I've never used one. A TKD CP2511 would be nice too. Above these would be the various stepped attenuators, but I would never put something that nice in a cheap amp like the 708B.

 The Xiang Sheng H-02A looks interesting being a 2×30W hybrid amplifier, but I'm not familiar with it though. How much does it cost? Considering the low quality parts and the 110v transformers used in XS equipment, I'd pass personally. I would much rather build something like a Charlize amp kit with quality parts instead._

 


 Its cost slightly lesser then the 708B, but its sound rather bad. I just gotten mine recently, cos a frd brought it in for me. its almost immediate after a few days of listenning i started changing some parts. Yes, the Xiangsheng products are utilising a lot of cheap parts, even the caps looks used. 

 I am again modding my 708B after having successful modded some stuff recently. The below are some of the mods that a frd of mine did for me:
 some caps change to polyproplene ones, and resistor change












 as well as tubes rolling













 Will proceed to some 2nd stage mod myself soon...


----------



## zer061zer0

Some work in progress pictures, ahahhaaaa....
	

	
	
		
		

		
		
	


	









 dcheming, i might need ur help in areas such as recabling, changing the headphone out, on/off swtich and the volume pot. 

 Do have the model no. of the swtichcraft headphone out?

 I would also like to know what is a good value for the powersupply caps(the one with the rectifier) to go up to. 
 The default ones are rated at 330uf/450v, and i am thinking of increasing te value, i did the same with the H-02A and did hear some improvement, I would need to know what a range i can go up to for the XS708B. 

 As for the other ecaps in the other areas, i am thinking of changing them to better ones or maybe higher value ones as well. what do u thinking will be a good and safe value to go up to. Are Rubycon ecaps good enough, cos the high voltage caps are quite difficult to find over here on my side. 

 Lastly can i also request to have a picture of the wiring to the ur volume pot, might be using another pot with 6 poles but i am quite unsure abt the connections. Same goes to the connections to the on/off swtich and headphone jack.

 Many thanks in advance..

 Regards,
 Marcus


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## dcheming

That doesn't surprise me that the H-02A sounds bad. It might have some potential, but I don't know if it would even be worth buying considering the overall low quality of everything inside XS amps. I'm also starting to think that they must utilize used parts. The stock RCA jacks on mine where horrendous, most of the ecaps were dented and scratched, and a lot of the resistors are chipped and scratched.

 Those Vishay caps look nice! They're massive.
	

	
	
		
		

		
		
	


	




 What type of resistors did you use?

 I really want to try some Fairchild Stealth diodes in the PS soon.

 I don't know if I'd change the values of the ecaps. I would be worried about how that would affect the circuit. If anything I'd just upgrade their quality, but use the same uF value. Maybe someone more knowledgeable can comment on this.

 Which Rubycon series are you referring too? They make some excellent ecaps in a variety of series, but they are mainly lower voltage it seems. For the two PS ecaps (the big 330uF and the 33uf) you can use whatever you want as long as it's rated for 450v. Nichicon, Xicon, Vishay/Sprague, United Chemi-Con, and Mallory all offer caps in both these values rated at 450v and are available from places like Mouser.

 Unfortunately the Switchcraft 1/4" jack doesn't have a part number on it, but as long as it will fit through the 8mm front panel you can use whatever. I'll try to find out the PN for it.

 The pin-out for the volume pot will depend on which one you use. The only wiring diagram I have is the one that came with my ALPS RK27. When you finally get one I'll help you figure out how to hook it up properly.

 For the switch I went with a GC #35-024 On-On 3PDT mini toggle. I think the easiest way to explain the wiring procedure would be to draw some pictures so give me some time. It's not hard at all, just hard to describe; a picture is worth a 1000 words right?
	

	
	
		
		

		
		
	


	




 Do you have a PayPal account Marcus? If you'll pay shipping I could send you a small parcel containing the harder to find parts like the ELNA caps, the HV ecaps, and the TRS jack. I could also include a GC switch and the cheap NTE resistors for the PS if you'd like. PM me if you're interested.


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## dcheming

Here are the labeled pics to show how to hook up the input selecting toggle switch.











 I think that this is pretty self-explanatory but I'll write it out anyways:
 L1 is the left channel of input 1.
 G1 is the ground of input 1.
 R1 is the right channel of input 1.

 L2 is the left channel of input 2.
 G2 is the ground of input 2.
 R2 is the right channel of input 2.

 L is the selected left "output" to the volume pot.
 G is the selected ground to the volume pot.
 R is the selected right "output" to the volume pot. 

 Assuming that you'll be mounting it on the front panel, when the handle is to the left this will be input 1 if you wire everything as it's labeled in the two pics. Honestly you could probably just use a 2PDT switch and make the grounds of both inputs common. I wanted to avoid any possibility of ground loops or crosstalk between sources so I opted to switch between the grounds as well.


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## zer061zer0

Hi Dcheming, 

 Thanks for the really informative picture. This really helps a lot. I just saw a similar swtich the other day and I guess i will be getting it to form the 2nd pair of RCA input.

 I also used some cat5e cables, teflon coated i think, but i am thinking of using some 24awg silver plated wires also teflon coated, for the grounding of the rca and and perhaps other ground for other areas as well. I cant remember where i read that silver plated cables actuallly helps in a faster discharge resulting in some improvement which i cannot recall, not sure if u could verify this.

 Will be posting some pics of what i have done so far soon....


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## zer061zer0

Some pictures of the parts removed.

















 Detaching pcb for replacement.





















 Will have to check with Dcheming, the purpose of this wire connecting to the 2 hooks holding onto the wire holding onto the tubes, it seems that they are giving some charge and there are some sparks when i am disconnecting some parts nearby, have the same thing happening when i was tube rolling the last time too. Just a quick one and luckily nothing was shorted.


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## zer061zer0

Hi Dcheming, 

 When changing the power cables, did u just cut of at one end and joint it to the thicker power cables by means of soldering and cover them up with the heat shrink?


----------



## dcheming

Quote:


  Originally Posted by *zer061zer0* /img/forum/go_quote.gif 
_Hi Dcheming, 

 When changing the power cables, did u just cut of at one end and joint it to the thicker power cables by means of soldering and cover them up with the heat shrink?_

 

Yeah that's exactly what I did. Since you have the 220v version your transformer's primaries might be hooked up differently then on mine. Just replicate the way it is done with the stock wire when you upgrade the power wire.


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## zer061zer0

hi dcheming, 

 can i check with you the power supply cable from the transformer to the ps board the yellow ones do they have any -ve and +ve differentiation when connecting to the board.cos the wire was twisted when i desoldeered them and now it seems that my markings are gone..

 do help me out here. thanks.


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## dcheming

The yellow wires, and the blue wires also, to the PS board are AC so they don't have a polarity. The only thing to be careful about is making sure to hook up the right color to the proper ACIN. On my amp (120v version) the yellow twisted pair goes to ACIN1 and the blue twisted pair goes to ACIN2. This is very important since the yellow is 6.3VAC for the heater in the rectifier tube and the blue is 215VAC that the PS itself uses. If you accidentally swapped these it would surely destroy your 6Z4 tube, pop the fuse, and maybe damage the transformer.

 Just for reference,
 Yellow twisted pair - 6.3VAC @ 0.6A for rectifier heater.
 Green twisted pair - 6.3VAC @ 1.5A for audio tube heaters.
 Blue twisted pair - 215VAC @ 0.08A for power supply.

 On that note, the output from the PS is DC so make sure you observe polarity here. I wrote little + and - signs on the PCB to be double sure I hooked it up right when I replace that twisted pair. If you are unsure about the proper polarity here just follow the traces from the DCOUT pins back to the big ecaps. The pin that traces back to the negative solder joint of the 330uF cap will be the negative out. Just use the same procedure for the DCIN by tracing the pins to the 33uF caps to get the proper polarity. I used red and black wire when I changed it just to make it easier to keep track of things.


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## zer061zer0

i have soldered the yellow one down was tracing back some of the old photographes that i have taken. thanks dcheming. 

 i guess i will swtich the DCOUT Wires colours as well. 

 Do you have any idea to the black wires connecting underneath the boards, i posted some pictures earlier and was wondering if we should remove them, i dun understand why the wire holders are wired to the circuit.

 i have also ordered some really long unbranded headphone jack and will be getting sometime next week. i have also seen a alps volume pot similar to yours but its rh027 comes with the half shaft and the grip shaft, however some of them have eight connectors. Can i check with you if the change of volume pot helps to track the volume better, meaning there isn't so much shift with a little turn on the knob. 

 I was old that we can try to add some 100k resistors to th pot to help the tracking, can someone verify this. 

 To date, i have soldered in some new caps and drill holes for the rca, but my workmanship isn't as fine as dcheming, will be posting some pics when i can get the things together, currently i am still waiting for some parts, while modding my H-02A and cdp.


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## dcheming

It sure is handy to have old photos for reference!

 Your boards are a little bit different than the one in my 708. Mine has traces that connect the two outermost wire loops to ground instead of black wires like you have. My best guess as to why the tube retainers are wired to ground would be for noise control possibly. I thought it might be for safety at first, but since the amp itself isn't grounded to earth that wouldn't make sense to me. If you plan on continuing to use them in your amp I would keep them connected to ground until we find out exactly why they are connected so, but if you're not going use them you could remove those little black wires. Just make sure that they are only grounding the wire loops for the retainers and nothing else in the circuit before you take them out; it's hard to tell just from your photos.

 Hopefully the new headphone jack will have better contact quality than the stock one. Whichever shaft you end up ordering only really makes a difference in what knobs you can use with the pot. I'd try to find some volume knobs that you like and that fit in the milled-out pocket first to see what kind of shaft you'd need. The stock pot on my amp had horrible tracking, especially at very low volumes. Out of curiosity I measured the difference in resistance between the two channels at a few different rotations. The differences were around 1700-2500Ω, which explains the crappy balance. The same test on the ALPS RK27 yielded differences of mostly around 200-300Ω with one test point being 700Ω. So as you can see the RK27 tracks about an order of magnitude better than the stock pot. I don't notice any major balance issues anymore.

 I've read mixed views on the benefits of applying a shunt resistor to a potentiometer. You could always try it, but I would pass on this myself.

 Thank you for the nice comment.
	

	
	
		
		

		
		
	


	




 How is the modding of the H-02A going? What are you planning on doing to your CDP? I still want to swap the OP amps and change out some more caps in my NAD.


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## zer061zer0

I did a cap change on my H02a with some increase in value for some caps, will be changing those caps next to the ICs with cerafines, hoping that it will reap better benefits. Tha H02A is a chipamp, utilising some LM38xx, i am actually ordering some LM3886 to build another amp and was wondering if it could go on as a direct replacement on the H02A. I also have another wacky idea which is to remove the preamp stage in the H02A, remove the 708B and recase the 2 unit into one, but i guess that will required a lot of understanding of electronics circuit which i am still lacking of. 

 For my cdp, i am doing some cap change, opamps swap and and some jumpering and bypassing of certain circuit. 

 Will be shopping for some stuff today. Hopefully i can see some nice cables, and useful items.

 As for the black wires, i will try to post more pictures before i do anything. Would it be possible, to show me the ground spot on the flip side of the pcb..?


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## Chops

I just bought a 708B yesterday from another Head-fier after I had read through a little bit of this thread, seeing that it should power my K701's with ease, and that there are plenty of somewhat easy and cheap mods to perform.

 I spent a good portion of today reading through this thread again, and doing research all over the place online as far as a parts list is concerned for the mods.

 The first thing I'm going to do is get rid of that entire "window tube" assembly to free up a little power and space, which will also reduce the heat a little I'm sure. Also, I'm going to put those bleeder resistors on the PS caps for good measure, and use a nice, beefy power cable.

 After that, I'll probably just listen to the amp for a few days/weeks so I can get a "feel" of the stock sound as is. 

 The fun part afterwards will be replacing the input RCA's, the coupling caps, the output caps, the signal wiring, and the volume pot.

 I'm going to try and get a 100k TKD CP2511S (S = stepped verion) attenuator, 2 Mundorf Supreme Silver/Oil 0.22uF coupling caps, and a set of 4 Elna 100uF output caps <-- Probably from you, dcheming, if you still have some and don't mind parting with them. 

 Oh, and I'm going to see about mounting the transformer on some rubber grommets which will raise it a little bit, but mainly for isolation from the chassis. And then I'll play around a little with driver and output tubes. 

 I tell ya, I'm glad I found this thread and that you guys have done your homework to provide great information and excellent pictures of your findings and mods to this little amp.

 Looks like I'll be helping to keep this thread chugging right along for a while. And don't worry, I'm sure I'll have plenty of questions along the way.


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## zer061zer0

Hi Chops,

 have fun with the mod, i guess dcheming is the guy who really understand the mods most.

 Anyway would it be possible for anyone to supply me the values for the items R1 and R2 on the PS board? I just realise that mine went a little too hot and burn the pcb alittle. By the way whats the R2 on the PS board, i am not sure if its a resistors.

 Regards,
 Marcus


----------



## Chops

Quote:


  Originally Posted by *zer061zer0* /img/forum/go_quote.gif 
_By the way whats the R2 on the PS board, i am not sure if its a resistors.

 Regards,
 Marcus_

 

Hey there! Thanks.

 Also, I'm no expert on this, but I'm pretty sure that if there's an "R" in the name, as in R1 and R2 like you mentioned, then it's a *r*esistor. If you look on the PS pcb where the two large 450V - 330uF caps are, you'll notice they have next to them a C1 and C2, the "C" standing for *c*apacitor. If there are any *j*umpers on any of the pcb's, they will have a "J" something next to them, like J1 and J2.

 So yes, to answer your question, it is a resistor. I hope that helps you some.


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## zer061zer0

Quote:


  Originally Posted by *Chops* /img/forum/go_quote.gif 
_Hey there! Thanks.

 Also, I'm no expert on this, but I'm pretty sure that if there's an "R" in the name, as in R1 and R2 like you mentioned, then it's a *r*esistor. If you look on the PS pcb where the two large 450V - 330uF caps are, you'll notice they have next to them a C1 and C2, the "C" standing for *c*apacitor. If there are any *j*umpers on any of the pcb's, they will have a "J" something next to them, like J1 and J2.

 So yes, to answer your question, it is a resistor. I hope that helps you some. 
	

	
	
		
		

		
		
	


	


_

 

I know that R1 is a 2k 2w, but can u see the value for R2, mine is burned..


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## Chops

Quote:


  Originally Posted by *zer061zer0* /img/forum/go_quote.gif 
_I know that R1 is a 2k 2w, but can u see the value for R2, mine is burned.._

 

From what I see, it looks like R2 is the one at the bottom left of this pic, sitting horizontal, NOT the one between the caps. I edited this pic, darkening the rest of the pic and adding the red rectangle around the resistor in question. 

 If those lines are red-black-red with gold, then it is a 2k ohm resistor with a 5% +/- tolerance.


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## dcheming

Chops, glad to have you here on this thread doing mods.
	

	
	
		
		

		
		
	


	




 I'm looking forward to seeing how it will drive your 701's since those are probably going to be my next cans.

 I personally don't really miss the window in mine. Not that I don't like looking at nice glowing tubes, I just don't like the way they did it just for show with a useless tube. I wish they would have arranged the active tubes in a triangle or an arc to be visible through the window instead.

 Man those are some nice parts you're planning on putting in! The ELNA 100uF 250v caps that I've been using are simply the best HV axial ecaps that they have at the electronics store that I shop at in Salem. You could probably find something better, but I'd think they would be much more than $3 each. The last time I picked up four more they only had about 6 left so I should call them to see if they can order some more. They have an entire bin of those Switchcraft thick-panel TRS jacks though. Just PM me if you'd need either.

 The rubber grommet idea is nice. This will definitely help to isolate any vibrations from the chassis and since some tubes are so sensitive to vibration it might be worthwhile to do the same for the audio PCB as well. When replacing all the various little screws I went with SS socket-head fasteners and washers from ACE hardware. Kind of expensive, but still under $12 to do the entire amp in SS. I'm still waiting on my 1/4-40 tap for the input switch. 
	

	
	
		
		

		
		
	


	




 Good pictures can be extremely useful when doing stuff like this. Plus it gives me a good reason to play with my camera.
	

	
	
		
		

		
		
	


	




 I went through the entire amp and made a parts list with all the various values. I'll post this for reference as soon as I double check it all.

 On my PS board, R1 is 1kΩ 2W and R2 is 2kΩ 2W. R2 is the one that was getting very hot on my amp also (not hot enough to burn the PCB though). That's why I used a 3W instead and mounted it with a 1" air gap underneath for better cooling. I'm not sure if R1 was getting hot too, but I replaced it anyway while I was in there. A 3W won't fit which is why I used 4x 1W resistors instead. I'm not exactly sure but I think these resistors are for limiting inrush current when you first turn on the amp.

 Speaking of jumpers, I'm pretty sure the little one they use in the 708 is the same steel wire that the tube retainers are made out of. I threw mine away long ago when I swapped it so I can't check to see, but definitely replace this jumper with the wire that you're using for the rest of the amp since it's directly in the signal path of the right channel.


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## dcheming

zer061zer0, can you post a overhead pic of just your PS board since it is definitely different from the ones in our 120v versions.


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## Chops

Quote:


  Originally Posted by *dcheming* /img/forum/go_quote.gif 
_Chops, glad to have you here on this thread doing mods.
	

	
	
		
		

		
		
	


	




 I'm looking forward to seeing how it will drive your 701's since those are probably going to be my next cans.

 I personally don't really miss the window in mine. Not that I don't like looking at nice glowing tubes, I just don't like the way they did it just for show with a useless tube. I wish they would have arranged the active tubes in a triangle or an arc to be visible through the window instead.

 Man those are some nice parts you're planning on putting in! The ELNA 100uF 250v caps that I've been using are simply the best HV axial ecaps that they have at the electronics store that I shop at in Salem. You could probably find something better, but I'd think they would be much more than $3 each. The last time I picked up four more they only had about 6 left so I should call them to see if they can order some more. They have an entire bin of those Switchcraft thick-panel TRS jacks though. Just PM me if you'd need either.

 The rubber grommet idea is nice. This will definitely help to isolate any vibrations from the chassis and since some tubes are so sensitive to vibration it might be worthwhile to do the same for the audio PCB as well. When replacing all the various little screws I went with SS socket-head fasteners and washers from ACE hardware. Kind of expensive, but still under $12 to do the entire amp in SS. I'm still waiting on my 1/4-40 tap for the input switch. 
	

	
	
		
		

		
		
	


	




 Good pictures can be extremely useful when doing stuff like this. Plus it gives me a good reason to play with my camera.
	

	
	
		
		

		
		
	


	




 I went through the entire amp and made a parts list with all the various values. I'll post this for reference as soon as I double check it all.

 On my PS board, R1 is 1kΩ 2W and R2 is 2kΩ 2W. R2 is the one that was getting very hot on my amp also (not hot enough to burn the PCB though). That's why I used a 3W instead and mounted it with a 1" air gap underneath for better cooling. I'm not sure if R1 was getting hot too, but I replaced it anyway while I was in there. A 3W won't fit which is why I used 4x 1W resistors instead. I'm not exactly sure but I think these resistors are for limiting inrush current when you first turn on the amp.

 Speaking of jumpers, I'm pretty sure the little one they use in the 708 is the same steel wire that the tube retainers are made out of. I threw mine away long ago when I swapped it so I can't check to see, but definitely replace this jumper with the wire that you're using for the rest of the amp since it's directly in the signal path of the right channel._

 

Well hello dcheming!

 Yeah, I won't miss that window either. In fact, I plan to put it to good use if I can. If it's sturdy enough, I'm going to mount a total of 4 small low intensity LEDs and 2 SPST mini toggles switches on it. 

 In the top center of the window is where I'll have a single purple LED to indicate that the unit is powered up. Below that to the left, I'll have one mini toggle with a red and orange LED to indicate input 1 or 2 (I'll be changing the preamp output into a second input). The mini toggle on the right side will have a single blue LED to indicate the activation of a crossfeed circuit.

 Here's a cheesy drawing I just whipped up in Photoshop to show what I'm talking about... Don't laugh! LOL 






 About the ELNA caps, didn't you say somewhere else in this thread that this total value can be even more than 200uF per channel, say 300 or 400uF? If so, I believe you said this would give slightly better bass extension without effecting the top end? I was just looking through PC and saw the Mundorf M-Lytic HV electrolytic capacitors (70421 or 70426) and (70427) which are 100+100 and 150+150 respectfully. Could these particular caps be used in this area since they are axials as well, or are they for a totally different application altogether?

 As for the rubber grommets, I was thinking about adding them to all the pcb's as well, like you suggest. Great minds think alike I suppose. hehe

 I'll be taking plenty of pictures as well. Some people say I go overboard with pictures, but I always say you can never have too many, especially on a project like this. 
	

	
	
		
		

		
		
	


	




 I'm definately going to do something similar with the R1/R2 resistors on my unit as well. I'd much rather play it safe than sorry. I think I might go with the 4 - 1w resistors in both locations to be on the safe side. I'll be using this amp a lot and it will be on for hours on end and don't want things getting hot.

 I'll also be sure to replace that steel jumper you mentioned. I don't want that junk in my music. Heck, I don't even listen to heavy metal music anyway, so why do I want it in my signal path?! LOL 
	

	
	
		
		

		
		
	


	




 Thanks for all the fine tips so far. I can't wait to get this amp and get working on it. BTW, this will be my first venture into modding electronics, especailly a tube amp of all things! I've designed and built custom passive crossover networks before, so it shouldn't be that much of a difference, except for the high voltages. Don't worry though, I have my father close by that can give me a hand with this stuff if need be. And he's been dealing with tubes and high voltages ever since he was 8 years old (now 66)!!


----------



## zer061zer0

Quote:


  Originally Posted by *dcheming* /img/forum/go_quote.gif 
_zer061zer0, can you post a overhead pic of just your PS board since it is definitely different from the ones in our 120v versions._

 

Here are some pictures which i can find straight off my old collection, 

 This is my R2





 and the R1 is the one in red over here.


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## dcheming

So I'm thinking that R2 is the 1kΩ in your amp (240V version). Just to be sure I'd measure it when you remove it to be certain of it's value. It looks like around a 3W resistor and I'm kind of surprised that it got hot enough to burn the PCB.


----------



## dcheming

Chops, I'd only have one reservation about using the transparent window to mount the switches and LED's, mainly that you'd be able to clearly see the entire mess of wiring that would be involved. Unless that's the look you're going for some smoked or other dark colored plastic would neatly hide all of that. If you matched this plastic sheet to the same color as the volume knob you end up going with I think it would look better than the stock transparent plastic sheet IMO. Also if you used something thick like 1/4" plastic you could drill and tap the mounting holes for the switches to make them much cleaner looking. I think most mini-toggle switches use 1/4-40 threads.


----------



## Chops

Quote:


  Originally Posted by *dcheming* /img/forum/go_quote.gif 
_Chops, I'd only have one reservation about using the transparent window to mount the switches and LED's, mainly that you'd be able to clearly see the entire mess of wiring that would be involved. Unless that's the look you're going for some smoked or other dark colored plastic would neatly hide all of that. If you matched this plastic sheet to the same color as the volume knob you end up going with I think it would look better than the stock transparent plastic sheet IMO. Also if you used something thick like 1/4" plastic you could drill and tap the mounting holes for the switches to make them much cleaner looking. I think most mini-toggle switches use 1/4-40 threads._

 

Funny you should mention that. I was actually thinking about making a print of a picture of raw metal, like brushed aluminum or stainless steel, and putting it behind the window, then mount the LEDs and switches on top of it (through the print and window). That way, it would look like another piece of metal there instead of a plastic window.


----------



## dcheming

Quote:


  Originally Posted by *Chops* /img/forum/go_quote.gif 
_Funny you should mention that. I was actually thinking about making a print of a picture of raw metal, like brushed aluminum or stainless steel, and putting it behind the window, then mount the LEDs and switches on top of it (through the print and window). That way, it would look like another piece of metal there instead of a plastic window._

 

If that's the case then IMO it would be better to just pop the plastic window out and trace it's shape onto a piece of 3/16" or 1/4" brushed aluminum. You could orient the brushed grain to match that of the 708's faceplate. If you don't want to see the mounting nuts for the switches on the outside, aluminum would hold threads much better than plastic. IMO real metal would look way better than using a print behind the plastic window. I'd be worried about the cheap plastic window getting all scratched up over the years. You could use some epoxy to mount the plate or if you just want to try it out some hot glue would make it removable.


----------



## Chops

Well I'm getting ahead of myself with the whole window thing. that little project will come later down the road. 

 The first thing I want to do is replace all the vital and/or crutial parts in the signal chain before doing the dual input and crossfeed mod.

 I was looking around on PC again and can not find attenuators anywhere on there, just those rotary switches.

 Do you know of anyone in the states who carries either TKD or ALPS? Or possibly anyone overseas that's reasonably priced?


----------



## dcheming

It's still good to plan things out way ahead of time though since this gives you plenty of time to come up with new ideas before you start cutting things.

http://www.percyaudio.com/ has them, but I don't know how their prices compare to others on these.


----------



## Chops

Quote:


  Originally Posted by *dcheming* /img/forum/go_quote.gif 
_It's still good to plan things out way ahead of time though since this gives you plenty of time to come up with new ideas before you start cutting things.

http://www.percyaudio.com/ has them, but I don't know how their prices compare to others on these._

 

That's true. Best to think things out completely before going through and cutting, then it not working out as planned. Good point!

 Thanks for the link! Looks like I'll be going with the Alps Black Beauty since I don't want to spend $60+ for the TKD, at least not on this particular amp.

 Also, I just found the answer to another question I had earlier! right below the attenuators in the catalog, I see the toggles that I will eventually need, and with the chrome paddles as well.

 Thanks again!


----------



## zer061zer0

My mods as of yesterday night, and everything is working, all thanks to dcheming, I am getting back my glow.....
	

	
	
		
		

		
		
	


	




 Will be getting the socket today and the whole thing will be put together for some music...
	

	
	
		
		

		
		
	


	


































 on a seperate note, i do have a few diy crossfeed unit, can someone suggest how i can place the Crossfeed into the chasis....


----------



## dcheming

Yeah I also think an Alps Black Beauty is about as fancy as I'd ever go in the 708B. I'd save the really nice stuff for DIY amps.

 I wanted to use those TOCOS switches too, but didn't want to wait for shipping and just went with a generic GC instead. Let me know how they work out when you get them.

 I totally passed over your Mundorf ecap question.
	

	
	
		
		

		
		
	


	




 The better bass response would mostly apply to low impedance cans, of more concern for me is their sound quality. I went with 200uF since I didn't want to downsize to a single 100uF. Again my choice was simply due to what they had in stock. I'm really growing to love the Mundorf line in general and it seems like these caps you listed would be great to use here. I think the 450v axial 100+100 would be the way to go since the snap-in type would not fit directly on the PCB and would require short wires to connect them remotely. With the axial you could mount them they same way I did. You'll still need a short wire, but only on one side. These caps are big, but looking inside my amp you could make them fit. Might be a good idea to get some 0.22uF or so FF caps to experiment with bypassing these for improved sonics. I've been wanting to try the M-Cap ZN for bypass duty.


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## dcheming

zer061zer0, I'm really wondering how it'll sound with those nice big ecaps you put in. Please let us know if it helps tighten up the bass or anything. I really want to redesign the PS to use a 6X4 rectifier combined with Fairchild Stealth diodes and would love to use some bigger PS caps too.

 You shouldn't have any more problems with hot resistors in you PS!
	

	
	
		
		

		
		
	


	




 Let us know your impressions of how the sonics changed whenever you get 'er up and running.

 Don't forget about the little jumper. You could just use some Cat5 or even some excess copper lead wire from a resistor. Can someone please verify if the stock jumper is made of steel or copper?


----------



## Chops

Quote:


  Originally Posted by *dcheming* /img/forum/go_quote.gif 
_Yeah I also think an Alps Black Beauty is about as fancy as I'd ever go in the 708B. I'd save the really nice stuff for DIY amps.

 I wanted to use those TOCOS switches too, but didn't want to wait for shipping and just went with a generic GC instead. Let me know how they work out when you get them.

 I totally passed over your Mundorf ecap question.
	

	
	
		
		

		
		
	


	




 The better bass response would mostly apply to low impedance cans, of more concern for me is their sound quality. I went with 200uF since I didn't want to downsize to a single 100uF. Again my choice was simply due to what they had in stock. I'm really growing to love the Mundorf line in general and it seems like these caps you listed would be great to use here. I think the 450v axial 100+100 would be the way to go since the snap-in type would not fit directly on the PCB and would require short wires to connect them remotely. With the axial you could mount them they same way I did. You'll still need a short wire, but only on one side. These caps are big, but looking inside my amp you could make them fit. Might be a good idea to get some 0.22uF or so FF caps to experiment with bypassing these for improved sonics. I've been wanting to try the M-Cap ZN for bypass duty._

 

Now help me out with something here, please...

 The Mundorf axial caps that I want to use are said to be 100+100uF or 150+150uF. Does this mean that each cap is actually equal to 200uF and 300uF, and that they only have one lead per side like any normal cap? 

 Or are they like the one on the bottom left corner of this panel, with one lead on one side and two leads on the other side? If so, how would these be wired up and what would the total uF rating be for each one? Sorry for all the questions.


----------



## dcheming

They're basically two caps in a single can so yeah the 100+100uF would be equal to 200uF if you tied the leads together, basically paralleling them. 

 I'm imagining that they would indeed be just like the lower left cap. Some fine stuff in that pic by the way...


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## Chops

Cool. I guess I'll be going with two of them then, one for each channel. You recommend the 100+100uF?

 BTW, remember these Transparent Audio MusicLink interconnects? Back in the mid-90's, they sold for something like $600 for a 1 meter pair. The place I worked at back then, I was given a pair of these for a job I did.






 Yeah well, to make a long story short, I'm going to be hacking these up (since I never bought into the whole "circuit in the cable" thing, just like with those M.I.T. cables) and just use the cable itself for rewiring the amp. 

 It's very nice OFC cable, it's flexible, it's shielded, and it should make for a nice and neat upgrade compared to what's already in the unit. 
	

	
	
		
		

		
		
	


	




 In fact, I have a pair of the cheaper Transparent cables as well *without* that stupid circuit and they sound much better, so I think this will be a good choice for in the amp.


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## dcheming

I just think the axial version will be the easiest to work with, hence the 100+100. I've seen as low as 15uF up to 450uF used as output caps. Finding the ideal value depends on the rest of the circuit and your ears. I know 200uF works fine in this amp since that's what I'm using now, but feel free to try the bigger ones if you think their fairly significant extra cost is worth it.

 Those Transparent's should make some nice signal wiring. 
	

	
	
		
		

		
		
	


	




 You might have to drill out the holes in the PCB with a 1/16" drill bit to fit the center conductor through depending on how thick it is.

 I've always wondered about the whole "circuit in the cable" thing too...
	

	
	
		
		

		
		
	


	




 You could probably sell the circuit boxes on Ebay, but I'd keep the RCA plugs to reuse on other projects.


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## zer061zer0

hi dcheming, i would like to check with you the possibility of me replacing the 47n/1kv ceramics caps, with some ployester cap that i have spare at 47n/400v will that suffice...


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## zer061zer0

The headphone socket have arrived, and i am also attaching some pictures of my crossfeed, would be interested to know how it can incorporated into the headamp...


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## zer061zer0

Derek, from the picture above can i say that the right output from the audio board should go to the right connectors on the jack(the one in bronze colour)


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## dcheming

I did some searching, but I'm not sure. Is there a schematic for it on the website that you ordered it from? 

 If not, then the easy way to find out would be to use jumper wires to temp connect it to the circuit. Only plug in the right channel into the input RCA. When you hear audio coming from the right side of your cans then you have it hooked up with the proper polarity. Sorry I couldn't be more helpful.


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## zer061zer0

This where i got mine from...

http://uk.farnell.com/jsp/endecaSear...sp?sku=8041474
 no specs given


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## dcheming

It's going to be tough to fit those two crossfeed boards in there. The only area with a bunch room is right up front, if you remove the tube reflector assembly. It would be much easier to build this into it's own dedicated case.

 Not sure about the polyester caps. I'd probably try to find some with a higher voltage rating for safety or better yet get the type specifically designed to be used in PS circuits (class X capacitors).


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## zer061zer0

i wil only need one of those crossfeed, i created them from some of the spares material from other project.

 i have tried them before on XS via wires, meaning from cdp to xfeed to the XS, i actually had them incorporated onto the interconnects and it works.

 so going in this manner, can we let the xfeed comes into the circuit after the RCA input (with a bypass swtich to bypass xfeed) then from the crossfeed to theinput selector swtich to the volume pot then out to the audio board. in this way the high voltage will be avoided right?

 I initailly thought that the xfeed is merely a relay to meddle with the signal inputs. I am not sure about this..


----------



## dcheming

I misunderstood, I thought you separated the CF circuit into two parts...
	

	
	
		
		

		
		
	


	




 Since this will only be on the input side high voltages won't be involved yet. If you want the crossfeed to be available for both inputs then it would have to be placed after the input selector toggle. So you'd have the two pairs of input RCA's going to the input selector toggle going to the CF circuit/bypass switch going to the vol pot going to the audio board.


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## zer061zer0

True true, hmm.. i will do up a much smaller crossfeed this might be alttle too bulky for the XS.

 For those who are interested here is a link to do up one:

http://www.rock-grotto.co.uk/x-feed.htm


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## Chops

Quote:


  Originally Posted by *zer061zer0* /img/forum/go_quote.gif 
_True true, hmm.. i will do up a much smaller crossfeed this might be alttle too bulky for the XS.

 For those who are interested here is a link to do up one:

http://www.rock-grotto.co.uk/x-feed.htm_

 

Ahh... Another link to add to the ole' Favorites! 
	

	
	
		
		

		
		
	


	




 BTW, the amp is headed to my town tonight. I'm hoping that FedEx might deliver it tomorrow sometime instead of waiting till Monday. That would really make my weekend! Let's hope, let's hope, let's hope.


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## dcheming

Chops, so I take it the amp didn't show up today?


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## Chops

Quote:


  Originally Posted by *dcheming* /img/forum/go_quote.gif 
_Chops, so I take it the amp didn't show up today? 
	

	
	
		
		

		
		
	


	


_

 


 Yeah, naturally. 
	

	
	
		
		

		
		
	


	




 Oh well, it will be here Monday. I'm off Tuesday, so I'll have plenty of time to play around and listen to it! 

 And don't worry, you guys will be the first to know when I get it. There will be plenty of pics to follow shortly after its arrival.


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## zer061zer0

Derek, 

 not sure if u experience this but my amp, sound softer now. I am not sure if its because of the different wires i use in the power section and signal lines. 

 Or could it be the 82k resistor soldered to the ecaps?


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## dcheming

Definitely no softening here, if anything the sound has tightened up. What wires did you use for signal wire? This amp draws very little power so the power section wire won't cause anything like this nor would the bleeder resistors. Might be one of the caps that were swapped. What input resistors are you using now?


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## zer061zer0

Quote:


  Originally Posted by *dcheming* /img/forum/go_quote.gif 
_Definitely no softening here, if anything the sound has tightened up. What wires did you use for signal wire? This amp draws very little power so the power section wire won't cause anything like this nor would the bleeder resistors. Might be one of the caps that were swapped. What input resistors are you using now?_

 

I didn't change the input resistors. It is the same as the first time a friend modded for me at 1komh/1w/1%.

 I used the same Cat 5e wires for signal wires and for ground, I used a unbranded silver-plated cable which I had also used for my other projects.


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## dcheming

Do you have a different 6N3 to try?


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## zer061zer0

Yes I have. But it didn't light up therefore I do not know if it is faulty.


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## dcheming

I was just thinking that maybe either the 6N3 or one/both of the 6922's got damaged somehow when it was miswired.


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## zer061zer0

I have 2 replacement 6N22s. What I did was I played it once, and then, I replaced both 6N22s with another pair, and played the same track again. The sound is still softer.

 Do you think it is due to the 6N3 then? Could it also be due to the wires? Cause I used different kinds of wires in the power supply section and also for the AC in and DC out.


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## dcheming

It might be the 6N3. This shouldn't be due to any of the wires. What do you mean by softer? Can you elaborate.


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## zer061zer0

What I mean is that in the past, I could not even turn the volume pot to 9o'clock as it was too loud. However now after the mod, even after I have turned the volume knob to the max, it is still rather soft, and I can hear the background noise at home.


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## dcheming

Have you double checked all the values of the resistors you replaced? You might of accidentally put in a wrong value somewhere important like a bias resistor or something.


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## zer061zer0

the only resistors that i have changed are those on the PS board and 2 pairs on the audio board.

 The PS board being the R1 and R2. The audio board, i only chnage the R204 & R104, R7 & R8. The only different comes in on R7/R8 as i use some 51 ohms resistors as i couldnt get any 50 ohms.

 Another pair of 82k was added underneath the PScaps.


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## zer061zer0

http://www.head-fi.org/forums/showpo...&postcount=203

 Some photos for reference.

 Can I also check with you if it is ok for me to use Cat5e for those connection under the audio board, and a 1uf cap on the input stage as I have some spares.


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## dcheming

In my amp,

 R7,8 are 100R 1W
 R104, 204 are 200R 1/2W

 Do you still have all the old resistors? Can you verify what value the old ones were. Your amp is a bit different than mine, but it's possible this might be your problem.

 Which connections under the audio board are you referring to?

 I was wondering if a bigger uF would be better for an input cap too. It should have a lower corner frequency as well as less phase distortion in the bass, but I don't know what's considered too big here. Try it out and as long as the bass doesn't get muddy or anything it should be okay. I'm trying to find some 1uF caps locally to try this out also.


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## zer061zer0

Oh no.. Looks like I made a silly mistake when purchasing, and I ended up buying ones with smaller values.


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## zer061zer0

I have re-wired my audio board and replaced all the resisters with the old ones, yet the volume is still soft.

 More surprising, is that I tried both my 6N3s, and both work!


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## zer061zer0

Hi Derek and all,

 I have tried quite a few things from redoing up the audio board to changing all the wires out from the transformer to a standard kind. 

 Replace all the signal cables with cat5e with the grounding wires change to the cat5e as well (previously were some silver plated wires). 

 Check all connections, replace the old 100 ohm resistors back on to the audio board, i measure wrongly the last time i went to buy my parts, too many porjects at the same time.


 Remove all the connections to the addiontional swtich and re route all connection to one connection only. 

 Swap tubes and headphones, still the volume is low. What do you guys think would be the cause?

 Remove all the connections to the addiontional swtich 

 My prime suspects now are my new ecaps, at the PS Board, power bypass and the headphone output. 
 The 82k resistors, the 472/1kv ceramics under the Big caps. 
 The headphone socket.
*10k head phone bleeder resistors.*
 Do give me some advice.. damm tired...

 I realise my mistake only until i type this......hahahhahahahahaa


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## dcheming

So are you really using some 10kΩ resistors for the headphone out like you wrote above? These should be only 10Ω and if you did use 10kΩ here it would certainly mess with the sound quite a bit. You might of accidentally ordered 10k instead of 10R.


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## zer061zer0

Yah, silly me. 

 I have been making some really silly mistake ard, and its so embarrassing. Hahahaha
	

	
	
		
		

		
		
	


	




 Can i check if i can get replace them with some 51ohm, i dun have any 10ohm with me now...


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## dcheming

I guess now would be a good time to post this. Keep in mind that this is for the 120v version and at this point I'm not sure if the 220v version uses the same numbers or even the same parts/layout.

Capacitors

 C1,2 330uF 450V electrolytic - Main filter.
 C6,7 150uF 200V electrolytic - Headphone output.
 C8,9 33uF 450V electrolytic - Main filter bypass.
 C101,201 0.1uF 50V film - Input.
 C104,204 0.22uF 400V film - Interstage coupling.
 C105,205 0.22uF 400V film - Preamp output.


Resistors

 R101,201 1M 1/2W
 R102,202 33K 2W - Anode load for 6N3.
 R103,203 1K 1W - Cathode bias for 6N3.
 R104,204 200R 1/2W
 R105,205 220R 1W - Cathode bias for 6922.
 R106,206 1M 1/2W
 R107,207 1k 1w - Input.
 R1 1K 2W
 R2 2K 2W - This should be upgraded to a 3W or even 5W rating.
 R3,4 100R 1W
 R7,8 100R 1W
 R9,10 330K 1/4W - The value was hard to read so I'm not 100% positive on this one.
 R1,2 10R 1W - Headphone output.


Misc
 Vol Pot - 100K stereo
 D1,2 1N4007 1A 1000V diodes
 Screws - M3 x 0.5mm


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## dcheming

I don't think you'll have any problems using 51Ω for the time being. You could parallel them with something else to get them even lower if you have some other suitable resistors.

 Making mistakes is part of the risk of DIY, but now you know the value of double checking everything before soldering. Especially when dealing with resistors.


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## zer061zer0

Hahahaa, true true.. 

 I dunno what gotten into me that day i got a pack of resistors that are of wrong value...


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## Chops

I just got word that my amp showed up at the house. The only problem is that I won't be home for another 3 hours. 
	

	
	
		
		

		
		
	


	




 At least I'll have all night tonight and all day tomorrow to listen to it.


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## Chops

Well I finally have my amp.

 It sounds pretty decent, but hate to admit that it doesn't sound much different than my LDM+. Bass, midrange, treble, they all sound pretty much the same between both amps! I'm hoping that doing these mods will help out a lot.

 Actually, that's not _totally_ true... The midrange and treble are a tad bit smoother sounding than that from the LDM+, which is to be expected from tubes. And that's not my imagination either since I directly compared between both amps with the same track at the same output levels. And the bass _does_ seem just a little bit stronger, but not necessarily deeper.

 Soundstage and imaging isn't really any better with the 708B either. It's all still between my ears for the most part, and not extending at all, if just a little beyond the headphones. Honestly, at this point, my brother's LD II++ seems to sound a heck of a lot better.

 I have to say though, this amp is dead silent. There's absolutely NO hiss, NO buzz, NO hum of any kind at any volume. That's a good thing. It's like it's not even powered up!

 I've had it playing since 6pm last night non-stop. On almost all the CDs so far, I can only get the volume up just past the 8 o'clock position. It's loud with musical peaks, but not super loud to the point of hurting one's ears. It's at about the same output level as I ever had my LDM+, which that's totally fine with me.


 Now for the only real negative thing about this amp, more than anything else...

 If I want to, I can easily push the LDM+ a little harder with no problems whatsoever. However, with the 708B, if I gently bump the volume to the 9 o'clock position, I get distortion. Not all the time mind you, but with heavy bass or musical peaks.

 My question is this, A) Is this distortion a result of the K701's not being able to handle the power of the amp, or B) Is the 708B not delivering enough power to properly drive the K701's?

 The reason I ask is because I know what distortion from a driver sounds like, at least from a loudspeaker, and the distortion I'm hearing sounds more like amplifier distortion, almost like clipping. 

 Any ideas?

 Also, in performing the mods to this amp, will any of them increase the headroom of this amp to cleanly play musical peaks without stress or distorsion? And will these mods also help in extending the bass output a little more as well as extending the soundstage and imaging?

 I know these cans are much more capable than what I'm getting out of either one of my amps right now since I have heard them do better with my brother's LD II++.

 At any rate, here's a pic of the volume knob at the loudest, distortion free level setting with my K701's. Is this normal?


----------



## tbonner1

Chops
 Your impressions are very similar to mine.

 1-it doesn't sound much different than my LDM+

 I agree with the stock tubes on the Xiang

 2-my brother's LD II++ seems to sound a heck of a lot better.

 I agree, a stock LD II, II+ and II++ has better detail and is more dynamic with hard to drive headphones or hard to drive IEM's like the Altec 716's.

 3-this amp is dead silent.

 Silent and runs much more cool than any of the tube LD's

 4-with the 708B, if I gently bump the volume to the 9 o'clock position, I get distortion. Not all the time mind you, but with heavy bass or musical peaks.

 It was very easy to get the two Xiang's I had to distort even with the Altec IM716's with the stock tubes.

 5-B) Is the 708B not delivering enough power to properly drive the K701's?

 The AKG 701's sound superb with my Bada PH12. That said you need to change the tubes on the Xiang per drarthurwells detailed and experienced instructions in his original Xiang post. You may like the amp much more after experimenting with his tube rolling ideas. My feeling is this amp is a blank canvas for the sound of the tubes and the stock tubes are not that great.

 Only if you like the amp with the upgraded tubes would I go about the extensive mods outlined.

 On the other hand, the stock LD's sound very good as is. They are dynamic with good bass and can drive a variety of loads, but have limited choices for tube rolling. 

 6-Also, in performing the mods to this amp, will any of them increase the headroom of this amp to cleanly play musical peaks without stress or distorsion?

 I have not experiemented with mods on the Xiang but you are lucky to have dcheming in this thread as the expert. I have done extensive mods on other amps and the mods have an aggregate effect that add up. I have had (relatively big) improvements with upgrading the volume control and caps. I would say the biggest improvement would be to upgrade the tubes first to see if you prefer the amp over the LD II++. Then I would get a nice volume pot.


 7-And will these mods also help in extending the bass output a little more as well as extending the soundstage and imaging?

 I think the tubes will have a big effect but will defer to dcheming as to what will produce the biggest bang for the buck. It depends on if you can do the mods yourself(for free) or pay somone $50 an hour to tinker with the unit. If you cannot do the mods yourself you may consider merely buying a more expensive amp that will cost you less than the Xiang plus mods plus tubes.


----------



## Chops

Quote:


  Originally Posted by *tbonner1* /img/forum/go_quote.gif 
_Chops
 Your impressions are very similar to mine.

 1-it doesn't sound much different than my LDM+

 I agree with the stock tubes on the Xiang

 2-my brother's LD II++ seems to sound a heck of a lot better.

 I agree, a stock LD II, II+ and II++ has better detail and is more dynamic with hard to drive headphones or hard to drive IEM's like the Altec 716's.

 3-this amp is dead silent.

 Silent and runs much more cool than any of the tube LD's

 4-with the 708B, if I gently bump the volume to the 9 o'clock position, I get distortion. Not all the time mind you, but with heavy bass or musical peaks.

 It was very easy to get the two Xiang's I had to distort even with the Altec IM716's with the stock tubes.

 5-B) Is the 708B not delivering enough power to properly drive the K701's?

 The AKG 701's sound superb with my Bada PH12. That said you need to change the tubes on the Xiang per drarthurwells detailed and experienced instructions in his original Xiang post. You may like the amp much more after experimenting with his tube rolling ideas. My feeling is this amp is a blank canvas for the sound of the tubes and the stock tubes are not that great.

 Only if you like the amp with the upgraded tubes would I go about the extensive mods outlined.

 On the other hand, the stock LD's sound very good as is. They are dynamic with good bass and can drive a variety of loads, but have limited choices for tube rolling. 

 6-Also, in performing the mods to this amp, will any of them increase the headroom of this amp to cleanly play musical peaks without stress or distorsion?

 I have not experiemented with mods on the Xiang but you are lucky to have dcheming in this thread as the expert. I have done extensive mods on other amps and the mods have an aggregate effect that add up. I have had (relatively big) improvements with upgrading the volume control and caps. I would say the biggest improvement would be to upgrade the tubes first to see if you prefer the amp over the LD II++. Then I would get a nice volume pot.


 7-And will these mods also help in extending the bass output a little more as well as extending the soundstage and imaging?

 I think the tubes will have a big effect but will defer to dcheming as to what will produce the biggest bang for the buck. It depends on if you can do the mods yourself(for free) or pay somone $50 an hour to tinker with the unit. If you cannot do the mods yourself you may consider merely buying a more expensive amp that will cost you less than the Xiang plus mods plus tubes._

 

I'm glad to see that I'm not alone in feeling this way. At least I know it's not just my unit or cans. However, I already ordered the key parts from Parts ConneXion...

 Attenuator: Alps Blue Velvet (1) SKU ALPS- 64605 Stereo Conductive Plastic Potentiometer 100k Ohm ... $17.95


 Mundorf M-Lytic HV Cap: (2) SKU MUNDORF- 70421 100+100uF 450V ... $20.77 ea - Output Caps


 Hovland Musicap Coupling Cap: (2) SKU HOVLAND- 51694 0.22uF 600V ... $13.10 ea - Coupling Caps


 Caddock MP-930 Resistor: (2) SKU 68426 82K Ohms 2.25-25W ... $6.75 ea - Power Supply Cap Bleeder Resistors


 And now I'm off to write *drarthurwells* to see if he still has any of those RCA 6DJ8 output tubes for sale.


----------



## Chops

Well that was quick and painless!

 Thanks to *drarthurwells*, I now have 2 RCA 6DJ8 output tubes and 1 6N3P EB Russian military driver tube coming my way. And since we both live in Florida, I'm sure I'll probably have them before the weekend. Fun times!


----------



## dcheming

I'm glad you finally got your amp. I have those same RCA tubes from drarthurwells in mine and I prefer them to the stock tubes too.

 The only amp I've directly compared the 708B to was a Millet hybrid with Diamond buffers and they had a very similar sonic signature. The Millet had tighter bass but the 708 had better midrange and was a tad more "fun" to listen to IMO. This was with most most of the mods done to the 708B. I'll be able to compare it to a SOHA next time.

 In a few weeks I'm going to be giving a fellow head-fier's 708B the "treatment". Before I do all the mods I'm going to compare his bone stock amp with mine to see exactly what the mods have done to the sound of this amp. I'll be posting detailed impressions whenever this happens.

 I don't have any distortion or clipping problems while using HD580s. I can't make any such claims for K701s though since I've never tried them.

 You've got the four most crucial parts on the way. Concerning soundstage and imaging, the ALPS and Auricaps helped a lot in this department in my 708B. You are using much better ecaps than I used too so that should help also. I'm pretty sure the bass has tightened up a bit, but this is going from memory. Man I can't wait to do the A/B comparison later! I'm going to build an A/B switch-box for this test. 

 Did you also get some signal wiring and RCA jacks from PC? Is there any reason why you went with such nice resistors for the bleeders? Spending $14 here is a bit of a waste IMO and could buy two pairs of RCA jacks instead. Just some generic 2W metal films ought to be fine, unless you just want to be fancy.
	

	
	
		
		

		
		
	


	




 I'm looking forward to seeing how your modded and re-tubed 708 will compare to the LDII++


----------



## Chops

Quote:


  Originally Posted by *dcheming* /img/forum/go_quote.gif 
_I'm glad you finally got your amp. I have those same RCA tubes from drarthurwells in mine and I prefer them to the stock tubes too.

 The only amp I've directly compared the 708B to was a Millet hybrid with Diamond buffers and they had a very similar sonic signature. The Millet had tighter bass but the 708 had better midrange and was a tad more "fun" to listen to IMO. This was with most most of the mods done to the 708B. I'll be able to compare it to a SOHA next time.

 In a few weeks I'm going to be giving a fellow head-fier's 708B the "treatment". Before I do all the mods I'm going to compare his bone stock amp with mine to see exactly what the mods have done to the sound of this amp. I'll be posting detailed impressions whenever this happens.

 I don't have any distortion or clipping problems while using HD580s. I can't make any such claims for K701s though since I've never tried them.

 You've got the four most crucial parts on the way. Concerning soundstage and imaging, the ALPS and Auricaps helped a lot in this department in my 708B. You are using much better ecaps than I used too so that should help also. I'm pretty sure the bass has tightened up a bit, but this is going from memory. Man I can't wait to do the A/B comparison later! I'm going to build an A/B switch-box for this test. 

 Did you also get some signal wiring and RCA jacks from PC? Is there any reason why you went with such nice resistors for the bleeders? Spending $14 here is a bit of a waste IMO and could buy two pairs of RCA jacks instead. Just some generic 2W metal films ought to be fine, unless you just want to be fancy.
	

	
	
		
		

		
		
	


	




 I'm looking forward to seeing how your modded and re-tubed 708 will compare to the LDII++_

 

Hey, hi, yo! LOL

_"You've got the four most crucial parts on the way."_

 That's exactly what I was going after. I figured if there's any major improvements to be had, I better do the ones that had the biggest/best improvements that you found first. See, I do my homework. 
	

	
	
		
		

		
		
	


	




_"Is there any reason why you went with such nice resistors for the bleeders?"_

 I went with those resistors because I thought I needed a fairly high wattage one there considering they're connecting directly to the 450V powere supply caps, that and I don't know any better. LOL! What can I say, it's my first time doing something like this. Umm... Apparently I didn't do my homework too well here. Oh well. 
	

	
	
		
		

		
		
	


	




_"Did you also get some signal wiring and RCA jacks from PC?"_

 No, I didn't. Like I mentioned a couple days ago, I have those Transparent Audio cables that I'm going to hack up and use for the internal wiring, and from what I can tell, the RCA jacks seem to be fairly decent for now, yet low on the priority list of things to replace. I felt that money could be better used elsewhere on this project, but that's just me.


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## Chops

At a closer look, I'm not totally sure that these are the stock RCAs and wires. The reason I say this is because they look an awful lot like the ones you installed, and the wire is a twisted pair instead of being in a single sleeve. 

 What do you think?







 And just for fun, you've gotta love the soft glow of tubes.

 Ahh...


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## dcheming

Man I totally forgot about those Transparent Audio cables. It's been a long day. 
	

	
	
		
		

		
		
	


	




 I think it's still the stock wire by the looks of it. Your RCA jacks are much nicer than the ones that came in mine, but still cheap looking and it looks like they used the wrong size ground-rings since they're way too big. Here's an old pic of my stock signal wire for ref:


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## zer061zer0

Quote:


  Originally Posted by *Chops* /img/forum/go_quote.gif 
_Well I finally have my amp.

 It sounds pretty decent, but hate to admit that it doesn't sound much different than my LDM+. Bass, midrange, treble, they all sound pretty much the same between both amps! I'm hoping that doing these mods will help out a lot.

 Actually, that's not totally true... The midrange and treble are a tad bit smoother sounding than that from the LDM+, which is to be expected from tubes. And that's not my imagination either since I directly compared between both amps with the same track at the same output levels. And the bass does seem just a little bit stronger, but not necessarily deeper.

 Soundstage and imaging isn't really any better with the 708B either. It's all still between my ears for the most part, and not extending at all, if just a little beyond the headphones. Honestly, at this point, my brother's LD II++ seems to sound a heck of a lot better.

 I have to say though, this amp is dead silent. There's absolutely NO hiss, NO buzz, NO hum of any kind at any volume. That's a good thing. It's like it's not even powered up!

 I've had it playing since 6pm last night non-stop. On almost all the CDs so far, I can only get the volume up just past the 8 o'clock position. It's loud with musical peaks, but not super loud to the point of hurting one's ears. It's at about the same output level as I ever had my LDM+, which that's totally fine with me.


 Now for the only real negative thing about this amp, more than anything else...

 If I want to, I can easily push the LDM+ a little harder with no problems whatsoever. However, with the 708B, if I gently bump the volume to the 9 o'clock position, I get distortion. Not all the time mind you, but with heavy bass or musical peaks.

 My question is this, A) Is this distortion a result of the K701's not being able to handle the power of the amp, or B) Is the 708B not delivering enough power to properly drive the K701's?

 The reason I ask is because I know what distortion from a driver sounds like, at least from a loudspeaker, and the distortion I'm hearing sounds more like amplifier distortion, almost like clipping. 

 Any ideas?

 Also, in performing the mods to this amp, will any of them increase the headroom of this amp to cleanly play musical peaks without stress or distorsion? And will these mods also help in extending the bass output a little more as well as extending the soundstage and imaging?

 I know these cans are much more capable than what I'm getting out of either one of my amps right now since I have heard them do better with my brother's LD II++.

 At any rate, here's a pic of the volume knob at the loudest, distortion free level setting with my K701's. Is this normal?




_

 


 Glad to know that you have finally gotten ur amp. Do listen to it for a while more and have a go at the mods. They really bring about a huge amount of improvement.

 Just a short listne to my modded amp last night and it sounded very gooood.. Background ground were darker and yes Derek, the bass was taut and deep and note that i am still using the dafult tubes, not the ones that i have gotten to replace. 

 I have managed to find a alps but then it doesnt seems to fit properly and i figured that it might be a hassle for me to fit it as i do not have much of the tools. 

 Chops, for your proble, i suggest you try a 100KA pot and use bleeders of higher value maybe ard 50R might be good. This was discovered accidentally with my 10k bleeders used instead of the 10R. I am now now using some 14R and will be trying some 18R and 51R kiwame resistors. While i considered btw recasing teh whole amp with a bypassable xfeed or to remain at the current casing.

 Regards,
 Marcus


----------



## dcheming

I didn't have the proper size drill bit for the ALPS so I just used an X-acto knife and a round file to slightly enlarge the hole.
	

	
	
		
		

		
		
	


	




 I just took my time and it turned out fine. Definitely worth the effort though.

 I wonder how much of the improvement to bass tautness is from those bigger ecaps you put in? I would also like to play around with some different headphone output resistors. Some RIKEN carbon film and/or Audio Note tantalum film would be interesting to try here.


----------



## zer061zer0

i think much of the iprovement should be attributed to the better caps use, as my value did not differe too much from the original. But i would think that the most remarkable improvement comes from the quieter background.


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## Chops

dcheming, when you get the chance, I'd like you to help explain to me the whole idea behind these "bypass caps" you keep referring to from time to time. Did you say that these might still help eventhough I'll be using Mundorf's in the outputs?

 And if you could include a couple of detailed pics in that area, possibly even a shot on the bottom side of the PCB to show the connections. 

 BTW, those Hovland 0.22uF caps I ordered, PC was out of the 600V ones, so I went with the 400V ones instead. That shouldn't matter much, should it?

 Oh yeah, I just got my tubes in today as well. Boy, they made a bigger difference than I had expected! They've only been running about an hour or so though, but I imagine they'll get a little better sounding as time goes by. 

 I can't wait for those parts to get here. I want to see what changes they are going to make to the sound.


----------



## dcheming

Chops, concerning bypassing ecaps with much smaller film caps, this is done to try to improve the inherent low quality sound of most ecaps. Just to get started I would probably pass on bypassing the Mundorf M-Lytic HV that you are going to use. Give it a few weeks to let all the mods burn in and then see if you think it's worth the extra $30 or so to get that last little bit of improvement from bypassing. Mundorf makes some nice stuff and not all ecaps like to be bypassed anyways. Here's a relevant link on this:

http://www.head-fi.org/forums/showthread.php?t=218492

 I'll PM you the labeled pic that I have of the audio board for reference.

 The stock WIMAs are 400v as are the Auricaps that I used, so you'll be fine here.

 Glad to hear you like the new tubes!
	

	
	
		
		

		
			





 I was really anxious for my parts to come as well and luckily they came in on a Friday so I had the entire weekend to play around.


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## Chops

Gee... Ask you a couple of questions and you really deliver! Thanks for the pictures also!

 I plan on letting things work in for a while before I do anything else. This time will also be used to allow my wallet to recover from the recent damage to it. LOL 
	

	
	
		
		

		
		
	


	




 After these parts are installed, I'll just be listening and doing plenty of research. I think the next thing I'm going to look into is resistor upgrades. 

 Speaking of resistors, what's with the ones connected directly to the headphone output jack? I noticed that you changed/upgraded yours. What does this do to the sound? Is it in someway matching the output impedance of the amplifier to the particular headphones used? 

 I think those are the only ones I'm going to worry about anytime soon, if any at all.

 Once again, thanks for everything!


----------



## zer061zer0

Hi guys, 

 This are what currently i have gotten into my XS.






































 Took out the 2nd input pair , but will put them back again after i plan out a decent casing, probably something where i can look inside, Acrylic..


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## dcheming

Quote:


  Originally Posted by *Chops* /img/forum/go_quote.gif 
_Gee... Ask you a couple of questions and you really deliver! Thanks for the pictures also!_

 

Just trying to help a fellow modder out!
	

	
	
		
		

		
		
	


	




 Honestly you might just want to do the resistor upgrade along with the caps since there are only two resistors in the direct signal path. A pair of 10R and a pair of 1K are the values required. You'd end up paying more for shipping than what they would cost if you ordered them separately so I'd just add these to your PC if you still can.

 From what I understand the 10R resistors that are inline with the headphone jack are there to protect the tubes from the momentary short that happens when you plug/unplug the headphones. They could also serve another purpose like impedance modification, but I'll have to check more into this to be sure. These combined with the 1K input resistors are the two crucial signal resistors.

 zer061zer0, so how does it sound with those new signal caps and wire? I'm really curious how stepping up to 0.47uF coupling and 1.0uF input caps affects the sound. Do let us know once you get a handle on the new sound of your amp. BTW, Solen makes some pretty large-value metallized caps that are available as big as 220uF and you might be able to get them cheap where you live. It would be very interesting to have all film caps in the signal path.


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## zer061zer0

Derek, my default amp came with the .47 film cap and the input was recently change to a 1uf after hearing ur suggestion on how the 1uf could possibly lessen the phase distortion. 

 I am trying to spend a little more itme on it first before i could post any impression of it. If u remember i have also recently modded my cdp and I am worried that some of the improvement might be from the cdp instead. 

 But i am quite certain that i am getting much more definition, fuller music, coupled with a darker background giving space to more details. 

 Can i check how do u know if u are getting some microphonics from the tubes. Do they sound like some lighting tingling sound on the glass surface of the tubes? I suspect that i am having some microphonics from the tubes but then again, its always at the same section of a particular track playing some cello.


----------



## dcheming

Chops, well I did end up putting back in the RIKEN input resistors today so here's the new close up pic of how I connected the paralleled HP output caps:








 I had to use a 1/16" drill bit to be able to fit both leads through the PCB and you'll probably have to do this as well on one side with you Mundorf ecaps.


----------



## zer061zer0

Derek,

 just a quick question, i have gotten a pair of input caps solen ones rated 1uf/630v for the input caps, i have also gotten some .1uf/630v for bypassing the e-caps, which in your opinion should i bypass? The headphone output caps or the power filter caps on the audio board?

 The solen at big values are not cheap over here. however the smaller ones are relatively cheap here.



  Quote:


  Originally Posted by *dcheming* /img/forum/go_quote.gif 
_Chops, concerning bypassing ecaps with much smaller film caps, this is done to try to improve the inherent low quality sound of most ecaps. Just to get started I would probably pass on bypassing the Mundorf M-Lytic HV that you are going to use. Give it a few weeks to let all the mods burn in and then see if you think it's worth the extra $30 or so to get that last little bit of improvement from bypassing. Mundorf makes some nice stuff and not all ecaps like to be bypassed anyways. Here's a relevant link on this:

http://www.head-fi.org/forums/showthread.php?t=218492

 I'll PM you the labeled pic that I have of the audio board for reference.

 The stock WIMAs are 400v as are the Auricaps that I used, so you'll be fine here.

 Glad to hear you like the new tubes!
	

	
	
		
		

		
		
	


	




 I was really anxious for my parts to come as well and luckily they came in on a Friday so I had the entire weekend to play around._


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## dcheming

Bypassing the headphone output caps would have more of an audible effect. If done with the incorrect value, bypassing the power filter caps could lead to ringing of the PS.


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## zer061zer0

what would be a correct value for the bypassing..??

 My power bypass are rated at 47uf/400v and the hp output at 470uf/200v.
 Any chances of me bypassing the ecaps with the 0.1uf/630v mkp.

 If anyone is interested i would be able to get some parts for you guys over here much cheaper..?


----------



## dcheming

Quote:


  Originally Posted by *zer061zer0* /img/forum/go_quote.gif 
_what would be a correct value for the bypassing..??_

 

I'm not sure, but I'll look to see if I can find some formulas for this. It sure would be nice if my oscilloscope wasn't in storage in Vancouver. 
	

	
	
		
		

		
		
	


	




 So what brand of HP output caps did you use? 470uF should be great if you ever use some lower impedance cans. If you made some short jumper wires you could experiment between bypassing both the HP ecaps as well as the power filter bypass ecaps to see which one yields an improvement. Have you considered upgrading your diodes to some HV soft-recovery types?


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## zer061zer0

Derek, i am always open to mods and in fact the latest addition to the mod are the solen input caps at 1uf. If u can give me the farnell code to the diode i am more then willing to experiment it. I am not very sure about the soft recovery. 

 The ecaps are all rubycon ones on the audio board. U mean i could just use jumper wires????


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## Chops

Hey guys, where in the signal path is C101 abd C201? It's those two little blue caps right along side the "IN-2" from the volume pot. Are these the 1uF input caps you (zer061zer0) just replaced?

 If so, whouldn't these be a vital upgrade since the signal is going through these first?

 Thanks!


----------



## Chops

Oh, and what voltage should they be? Is 200-400V enough?


----------



## zer061zer0

Quote:


  Originally Posted by *Chops* /img/forum/go_quote.gif 
_Hey guys, where in the signal path is C101 abd C201? It's those two little blue caps right along side the "IN-2" from the volume pot. Are these the 1uF input caps you (zer061zer0) just replaced?

 If so, whouldn't these be a vital upgrade since the signal is going through these first?

 Thanks!_

 


 Yes those are the ones where i use 1uf caps on 200v-400v should be ok..

 I am currently using solen 400v previously some 1uf/250v-400v by AV.

 Regards,
 Marcus


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## Chops

Thanks Marcus!

 What kind of change did it make to the sound, or have you given it a listen yet?

 -Charles


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## zer061zer0

Hi Chops, 

 I can't attribute the sonic changes to just the change of the input caps as you can see that I had done my mods in quick succession. All this is due to the lack of parts available. 

 However, I will say that the most significant change is the quieter background with more details. My girlfriend, who is very sensitive to the hiss and background noise, also noted the change. 

 Soundstage wise, there is a slight widening, but I am still trying to tune the sound. It should also be noted that my CDP was also modded last month. 

 All these attributes to my not being able to pinpoint the improvements in sound. Derek did mention that the input caps at 1uf could cause a lower phase distortion. 

 I am currently looking at recasing the headamp to include space for a crossfeed. And also, waiting for Derek to come up with a possible solution to play around with the power supply. 

 Regards
 Marcus


----------



## Chops

Quote:


  Originally Posted by *zer061zer0* /img/forum/go_quote.gif 
_Hi Chops, 

 I can't attribute the sonic changes to just the change of the input caps as you can see that I had done my mods in quick succession. All this is due to the lack of parts available. 

 However, I will say that the most significant change is the quieter background with more details. My girlfriend, who is very sensitive to the hiss and background noise, also noted the change. 

 Soundstage wise, there is a slight widening, but I am still trying to tune the sound. It should also be noted that my CDP was also modded last month. 

 All these attributes to my not being able to pinpoint the improvements in sound. Derek did mention that the input caps at 1uf could cause a lower phase distortion. 

 I am currently looking at recasing the headamp to include space for a crossfeed. And also, waiting for Derek to come up with a possible solution to play around with the power supply. 

 Regards
 Marcus_

 

Hi again,

 Yeah, I forgot about you doing a bunch of mods all at one time. I'll be doing somewhat the same thing whenever my parts come in.

 After I install the 82k resistors and move to the Alps, coulping caps and output caps, I'll listen for a while.

 My next purchase from PC will be the 1uF input caps, the 10R and 1k resistors, and possibly the RCA jacks.

 BTW, those 82k resistors I'm getting are 30 watts. Are those physically larger than the 2-3 watts ones you and Derek used? If so, are they not going to fit under the PS pcb? If not, I'm going to have to run some wires to the back of the chassis and place them there somehow I guess.


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## zer061zer0

i think i use a 2-3w resistors, 30 watts is really over kill in my opinion. infact anything about 5 watts are gigantic in my opinion. 

 But i think that the bleeder resistors are impt as i have experienced some sparks when i remove the tube or soldering some parts on prior to installing the 82k. It really makes the amp a lot safer to work on. If its not going to be too costly and heavy, I can mail you some small parts like resistors and caps, guess they are much cheaper over here, but i will hav to check. Pm me if you are interested.


----------



## Chops

Quote:


  Originally Posted by *zer061zer0* /img/forum/go_quote.gif 
_i think i use a 2-3w resistors, 30 watts is really over kill in my opinion. infact anything about 5 watts are gigantic in my opinion. 

 But i think that the bleeder resistors are impt as i have experienced some sparks when i remove the tube or soldering some parts on prior to installing the 82k. It really makes the amp a lot safer to work on. If its not going to be too costly and heavy, I can mail you some small parts like resistors and caps, guess they are much cheaper over here, but i will hav to check. Pm me if you are interested._

 


 Yeah, but since I already have them coming, I might as well go ahead and use them. I'm just wondering if they're going to have enough clearance under the pcb, hence why I may have to relocate them.

 When you say "over here", where is here? LOL! How cheap can you get stuff where you are?

 Besides, speaking of overkill, just look at the parts I'm putting in this amp, Mundorf and Hovland caps and an Alps Blue. 

 And when I go to upgrade the input caps, resistors and RCAs, I'm going be going with more Hovland caps, Audio Note Tantalum film resistors and DH Labs RCAs.

 Overkill? Sure! As long as it's not breaking the bank and makes improvements, then why not?!


----------



## Chops

BTW, I just went to Caddock's site and found those 82k resistors I'm getting. I think PC is wrong though. They have the resistors I'm getting as the MP930, but according to Caddock's site as my father pointed out, they are the MP925.

 Supposedly, the MP930 only goes from 0.02 ohms to 4.99k ohms. The MP925 goes from 5.00k ohms to 100k ohms.

 If you guys could, could you please double check me on this at their site? It would be much appreciated. Unless PC has something that's discontinued from Caddock or something. I guess I'll find out for sure once they get here.

 Anyway, here's the page on their site with all the specs...

Caddock MP900 & MP9000 Series

 The nice thing is, the particular design of these resistors will allow me to mount them to the rear wall of the chassis next to the a/c connector. And since these things are a massive 25-30W each, they should probably stay cool to the touch at all times.

 And yes, after going over the math with my father, I now realise that at 450 volts and 82k ohms, it only requires almost 2.5 watts, which is exactly why the two of you used a 3 watt resistor. Well, at least I shouldn't have to worry about extra heat! Me and my darn overkill BS again! LOL


----------



## Chops

Oh, I almost forgot... 

 Do either one of you know what the B+ and/or plate voltage is in this amp? My father was just wondering. And I think I have talked him into doing the pcb soldering for me, since I've never done it before and don't want to screw things up.

 I told him I'd buy him some new soldering iron tips!


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## zer061zer0

I paid like a couple of dollars for most of the resistors over here in Singapore. But again, i am not sure if you might be interested in them as I can see that you are investing alot with the use of these expensive caps. 

 Let me know what u are interested in and i will check out the price for you. I thought it could help you guys save some money.


----------



## Chops

Quote:


  Originally Posted by *zer061zer0* /img/forum/go_quote.gif 
_I paid like a couple of dollars for most of the resistors over here in Singapore. But again, i am not sure if you might be interested in them as I can see that you are investing alot with the use of these expensive caps. 

 Let me know what u are interested in and i will check out the price for you. I thought it could help you guys save some money._

 

I'm just wondering, even if you can get some of the components cheaper, I'm sure the price of shipping with bring the total cost back up. I've heard that shipping to the states isn't cheap these days. I could be (and probably am) wrong though.


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## dcheming

The premise behind using soft-recovery diodes is that they cause less ringing when used in rectification. This usually leads to better sound and/or a quieter background.

 C101 and C201 are indeed the input caps and were 0.1uF in my amp. Since they are only passing line level signals a 50v rating is more than adequate here. I took these out in my amp, but will probably put them back in later.

 The PS idea that I'm playing around with is doing a mod to be able to use a 6X4 rectifier instead of the 6Z4 since the former has many NOS as well as new production replacements available. It turns out that the two pins that would need to be swapped to do this mod are in the perfect place on the PCB to do so. However, I'm hesitant to show how to do this one due to liability concerns: if done wrong this could have drastic consequences. Besides, I would much rather build a better PS from scratch anyways since the stock one used is very basic. I'll keep you guys up to date on this one and I might get some PCBs made if I decide this mod would be worth it.

 Dude, 30 watt resistors are huge!
	

	
	
		
		

		
		
	


	







 At the ~200V that is on the power filter ecaps, the 82K resistors will draw about 1/2 watt each so 2w should be more than fine. Even using 3W resistors here would require the PS PCB to be raised up a bit which might make the 6Z4 hit the case top.

 I'm not sure what the B+ is supposed to be, but I measure around 200V. Problem is that the transformer is rated for 110V input so it's going to have a higher than normal output when used with our 120V. My heaters are running at ~7.5V instead of 6.3V as a consequence of this. This will also affect things like bias points, etc.

 I'd love to design an entirely new PS and get a new transformer, but would would it be worth the effort? The 708B is a simple design with cheap parts. Upgrading a few parts for better sonics is one thing, but I don't see the sense in basically replacing everything. I'd much rather just build something better like an Optimized Morgan Jones amp from scratch with nice parts. It's hard to resist the temptation of a complete redesign though..


----------



## Chops

_"C101 and C201 are indeed the input caps and were 0.1uF in my amp. Since they are only passing line level signals a 50v rating is more than adequate here."_

 So running 1uF caps instead of 0.1uF caps here is a good thing? Didn't you also say elsewhere that running high voltage caps usually have lower ESR's, so running 1uF - 200/400V Hovland's here should be fine?


_"Dude, 30 watt resistors are huge!"_

 I've been hearing this a lot lately. Even from my father! LOL


_"I'm not sure what the B+ is supposed to be, but I measure around 200V. Problem is that the transformer is rated for 110V input so it's going to have a higher than normal output when used with our 120V. My heaters are running at ~7.5V instead of 6.3V as a consequence of this. This will also affect things like bias points, etc."_

 If the heaters are running over a volt higher than they should be, this will shorten the life of the tubes some, correct, or is that the plate voltage I'm thinking of?

 Maybe I should leave that tube in the window there. Maybe it's drawing enough power to drop the heater voltage back down where it should be, yes/no?


----------



## dcheming

"_So running 1uF caps instead of 0.1uF caps here is a good thing? Didn't you also say elsewhere that running high voltage caps usually have lower ESR's, so running 1uF - 200/400V Hovland's here should be fine?_"

 Running the bigger input caps should be a good thing due to less phase problems, but I don't know if it will have any negative effects elsewhere. I'm having trouble finding info on this. Maybe someone knowledgeable can comment on this.


 "_If the heaters are running over a volt higher than they should be, this will shorten the life of the tubes some, correct, or is that the plate voltage I'm thinking of?_"

 Yes, this will shorten tube life unfortunately. It sure would be nice to get it back down to 6.3V. Now _this_ would be an excellent use for some 30W resistors!
	

	
	
		
		

		
		
	


	





 "_Maybe I should leave that tube in the window there. Maybe it's drawing enough power to drop the heater voltage back down where it should be, yes/no?_"

 That tube is on the same heater winding as the rectifier tube so it won't affect the audio tubes either way. The three audio tubes have there own winding so we would have to do something else to lower their heater voltage.


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## Chops

Say Derek,

 Didn't you say you were drawing up a schematic for this amp? You wouldn't happen to have it finished yet and/or be able to possibly shoot me over a copy of it, would you?

 That way, my father can look through it as well and see what this amp is made of. Not that he can't do it himself by just looking at it, but he really doesn't have the time right now.

 Anyway, let me know if this is do-able.

 Also, as always, thanks for the answers in the above post!


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## grawk

I'm confused as to why this isn't an MOT thread.


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## zer061zer0

I emailed the manufacturer and this is what i have gotten from them, hope this serve to help everyone who is interested in modding the XS.






 I dont really understand it but i do see some .47uf cas ard.. Derek, do u have any .47uf caps ard..?


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## dcheming

Thanks for getting this for us zer061zer0!

 Well there's definitely a few differences from my amp. Many of the component numbers are different. This 220V version uses 0.47uF signal caps instead of the 0.22uF ones that the 120V version uses, as zer061zer0 already verified for us. The HP output caps are bigger in this schematic and they are also shown being bypassed. 

 The most interesting difference is that the two resistors that set the operating points of the output stage (G2a,b) are significantly different. I downloaded the trial version of PSpice last night with the intention of trying to simulate this amp. The Headwize article on optimizing the very similar Morgan Jones Amp using PSpice is fascinating and I wanted to follow the same path for the 708B to see if it could benefit from some fine tuning as well. Unfortunately the 708 is too big for the trial version to allow. 
	

	
	
		
		

		
		
	


	




 There's a slight chance that I can get access to the full version of PSpice through my school so there's still a possibility.

 zer061zer0, could you verify the values of R7 and R8 in your amp for me please? Thanks again for obtaining this for us to use.


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## dcheming

.


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## zer061zer0

Quote:


  Originally Posted by *dcheming* /img/forum/go_quote.gif 
_Thanks for getting this for us zer061zer0!

 Well there's definitely a few differences from my amp. Many of the component numbers are different. This 220V version uses 0.47uF signal caps instead of the 0.22uF ones that the 120V version uses, as zer061zer0 already verified for us. The HP output caps are bigger in this schematic and they are also shown being bypassed. 

 The most interesting difference is that the two resistors that set the operating points of the output stage (G2a,b) are significantly different. I downloaded the trial version of PSPICE last night with the intention of trying to simulate this amp. The Headwize article on optimizing the very similar Morgan Jones Amp using PSPICE is fascinating and I wanted to follow the same path for the 708B to see if it could benefit from some fine tuning as well. Unfortunately the 708 is too big for the trial version to allow. 
	

	
	
		
		

		
			





 There's a slight chance that I can get access to the full version of PSPICE through my school so there's still a possibility.

 zer061zer0, could you verify the values of R7 and R8 in your amp for me please? Thanks again for obtaining this for us to use._

 


 No problem on th schematics.. 
 I cannot really understand the symbols anyway, i will check out the resistors value at R7 and R8, i suspect its the ones that i reaplce wrongly the last time.

 I will check out and let u guys know asap..


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## zer061zer0

looking at my past post 
http://www.head-fi.org/forums/showpo...&postcount=203

 i suspect that they are the same as yours, at 100r, as i replace them wrongly the last time time with some 51r


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## zer061zer0

The R7 and R8 are the same as u guys in fatc almost all of the resistors are identical.

 I am not sure if you would like to try out stepping up ur .22 interstage coupling caps to .47uf and see what u get...


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## dcheming

Here's the same schematic cleaned up, but with the part numbers and values for the 120V version. 







 The values in red are where they differ between the two versions. However, zer061zer0 has the 220V version, but has the same parts numbers as the 120V version. Maybe that schematic they emailed you is old or from the 708A version?

 Marcus, due to the different part numbers in the original schematic, the two resistors that I was asking you to please check for me on your amp would be R104 and R105 instead of R7 and R8. Thanks.


----------



## Chops

I got my parts in today. I made a stupid mistake though and ordered the wrong darn resistors. Not only were they the wrong ones for being rated at 30 watts, but I also misread the value and order the 82 ohm ones instead of 82k ohms. 
	

	
	
		
		

		
		
	


	




 Also, those Mundorf caps are HUGE! They're even larger than the power supply's 330uF-400V caps!!! 
	

	
	
		
		

		
		
	


	




 I think I'm going to have to pull the 0.22uF preamp output caps out to position one of the Mundorf's there, then position the other one towards the rear of the chassis, next to the RCA jacks. I'm never going to use this amp as a preamp, and I already have plans to make the "pre-out" jacks into another pair of inputs, so no great loss there.

 The 0.22uF Hovlands and ALPS shouldn't be much of a problem, however.

 Man, haven't I gotten myself into a heap of trouble this time! LOL 
	

	
	
		
		

		
		
	


	





















 And the stupid mistake (2 mistakes in one, actually!)...


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## dcheming

"_I made a stupid mistake though and ordered the wrong darn resistors._"

 In the mean-time while you're going to be working in your amp just use something around 25KΩ or so with some jumper wires to drain the big filter caps.


 "_those Mundorf caps are HUGE!_"

 Dam, they _are_ a lot bigger than what I had visualized!
	

	
	
		
		

		
		
	


	




 Well, do you plan on taking out the front tube/reflector assembly? If so then you could mount the Mundorfs side by side in that area easily and just run the wire from the audio board to them and then on to the nearby jack. I'm not sure if this is an ideal way to hook up output caps, but you're going to have to use some wire either way and at least mounting them up front would make it equal lengths of wire as well as being laid out cleaner. Just think, you could show off your nice pair of Mundorfs in that window! You could use some blue LEDs to light up the front of them.
	

	
	
		
		

		
		
	


	





 "_Man, haven't I gotten myself into a heap of trouble this time!_"

 Nah, just take your time. If you can hold yourself back, I would suggest doing this in steps to see how each individual part affects the sound. It's great that your parts came in just in time for the weekend!
	

	
	
		
		

		
		
	


	




 I'll be home this whole weekend so if you got any questions are could use a reference photo just ask. Have fun!


----------



## Chops

Yeah, I'm not sure how I'm going to approach this yet. 

 Hate to say it, but I kinda like that tube up front in the window. I guess it's the cheesiness in me, I don't know. *BUT* I also _don't_ like it due to the fact that it's taking up valuable room and power, and producing unwanted heat.

 Maybe I'll take it out, put the Mundorfs up there like you suggested, and leave the orange LEDs along them to light them up. 

 With all the radios my father has to work on right now, it's going to be tough getting my amp in for him to work on. If he's willing and able to get to it this weekend (and I really hope he is), then I'm just going to have him do everything at once. There's no telling the next time he'll be able to work on it, so I better not push my luck.

 And thanks for your offer. If I've got any questions, I'll be sure to shoot you a message this weekend.

 Thanks again!


----------



## Chops

Well, she's out of the shop and back up and running! 

 My father noticed that those 82k resistors that you (Derek) used were wired in parallel, so the total coming to 41k for both caps. He decided to put a single 68k-2W resistor in there instead (since the Caddocks I got are useless, thanks to me), tucked back behing the PS. 

 The Hovlands went in without any problems, as did the ALPS pot, minus the small enlargement of the "shaft" hole and new small hole for that little tab thingie to keep the pot from rotating.

 The huge Mundorfs actually were fairly easy to install as well. My father decided to stack them up against the rear of the chassis, and use some cloth covered, solid core wire for the connections, and industrial strength double-sided tape.

 As for changes in sound, the soundstage is much more vivid and wide. The top end is still extended, but the quality of it is much better, smoother and cleaner. The midrange seems a bit fuller and warmer, with a bit more depth. The bass is still on the thin-ish side, but is getting better and deeper the more I listen. In fact, just in the past 40 minutes of playing, the bass is getting a little more pronounced. It's still very tight and controlled.

 One kind of weird thing that I've noticed is that I now have to turn the volume control up to around the 11:30 position to get to the same output level as the original amp set at about 9 o'clock. I don't know if that has to solely do with the ALPS, or because of the new caps, or all three. 

 The B+ is measured at 247v. With that 68k resistor in place, the voltage drops from 247v to 50v in about 20 seconds or so after shut down, then drops from 50v to 0v in about another minute or so. The resistor barely gets warm to the touch, which is a good thing. It's an oldie, but goodie!

 And now, for some juicy pictures! LOL I'll take some better pics once the camera batteries are charged back up.


----------



## dcheming

"_Well, she's out of the shop and back up and running!_"

 Nice! The way you guys mounted the Mundorf ecaps seems like the way to go. They fit perfect in that spot and the double-sided tape will provide a little bit of vibration isolation.


 "_My father noticed that those 82k resistors that you (Derek) used were wired in parallel, so the total coming to 41k for both caps._"

 Yeah, he's right with the bleeder resistors being in parallel and thus equaling 41K. At 250V they will dissipate ~0.76W each so I will still be okay. 


 "_The Hovlands went in without any problems, as did the ALPS pot, minus the small enlargement of the "shaft" hole and new small hole for that little tab thingie to keep the pot from rotating._"

 Enlarging the shaft hole and drilling that little hole for the tab is probably the hardest single mod to do in this amp. As long as you have the right tools it shouldn't be too bad though.


 "_As for changes in sound, the soundstage is much more vivid and wide. The top end is still extended, but the quality of it is much better, smoother and cleaner. The midrange seems a bit fuller and warmer, with a bit more depth. The bass is still on the thin-ish side, but is getting better and deeper the more I listen. In fact, just in the past 40 minutes of playing, the bass is getting a little more pronounced. It's still very tight and controlled._"

 Glad to hear that the soundstage has improved. Since you mention that the top end is still extended and cleaner you probably won't have to worry about bypassing the Mundorfs. As you said, give 'em time to burn in before judging the bass.


 "_One kind of weird thing that I've noticed is that I now have to turn the volume control up to around the 11:30 position to get to the same output level as the original amp set at about 9 o'clock._"

 That is weird. There was no difference in the volume when I swapped in the ALPS. It was the same volume at the same rotation compared to the stock pot, but of course much better channel balance. The caps wouldn't cause this either. I had to run a ground to the case to stop the hum when I put in the ALPS. Do you have any hum now?

 Do you have any of the lead wire left from cutting the leads shorter on the Hovland or Mundorf caps? If so this would make a nice little jumper to replace that stock steel one.

 Nice pics BTW.


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## Chops

Hey Derek,

 Yeah, I like the way he decided to mount the Mundorfs, too. At least that way the leads aren't strung all the way to the front, and the caps are sort of out of the way. 

 Funny how you said that about drilling the volume pot hole larger. My father said it was going to be a PITA and would take a little while. I went in the kitchen to get a drink for the both of us, and when I came back out, he had already drilled both holes and had the ALPS mounted! It couldn't had taken him any more than a minute or two tops! LOL

 The bass does seem to be getting a little louder and deeper, since I've had it running non-stop since yesterday around 5pm. I've tried both my Sony DVD/SACD player with the Signal Cable Analog Two's and an older Sony CDP-397 CD player through Cardas Crosslink 1's. The bass output and quality is the same, so at least I know it's not getting limited by the source.

 Yeah, I'm not sure what the deal is with the lower volume output, but it does sound better none the less, and I'm still getting ZERO hum or hiss. 

 And thanks for the "pics" complement!


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## Chops

And now for some fun...

 I have my brother's Little Dot II++ amp connected for a straight comparison between it and my modded 708B. My Sony DVD/SACD player has dual 2-channel analog outputs, so I have both amps connected via a total of 4 Cardas Crosslink 1 interconnects. Also in the plans, I have my Little Dot Micro+ charging at the moment and will connect it to the front L and R outputs of the 5.1 analog outputs, which I have set for 2-channel output only. This will give me the ability to conduct an A/B/C comparison between all three amps!

 I can say this much for right now, when I first got the 708B, I compared it to my brother's LD II++. The LD was a lot more open with a real wide soundstage, and the bass was fairly deep and powerfull. 

 NOW the tables have turned! With the modded 708B, the LD II++ seems to be a little bit on the congested, soft side. It's soundstage isn't as wide as the 708's now, and the bass isn't as pronounced, punchy and defined as the 708's. In fact, the overall sound of the 708B is more open, lively, detailed and extended in both directions.

 Anyway, that's enough for now. I'll go into further detail sometime tomorrow after more listening. But for now, here's a pic of the dual amp rig. I moved the 708B up on top of the player and put the LD II++ on the side by itself. I just don't like the idea of having that little space heater sitting on top of the gear!


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## dcheming

"_At least that way the leads aren't strung all the way to the front_"

 If the caps were mounted up front the output wire from their negative terminal would have been run directly to the HP jack, not back to the audio board. So basically it's the same amount of wire either way. 

 Later on when you upgrade the Hp jack and the signal wire I'd run the new wire directly from the negative terminal of the Mundorf to the HP jack. This will eliminate one solder joint and a trace from the signal path of each channel. Looking back, I should have done this with the ELNAs, but I'll do it when I upgrade these caps.


 Man, those "up close and personal" pics are super close! You can see every little spec of dust! I can't wait to get the macro lens for my camera. Yeah that _is_ a beautiful solder job on the Mundorfs.
	

	
	
		
		

		
		
	


	




 What kind of wire is inside that cloth BTW?


 "_NOW the tables have turned! With the modded 708B, the LD II++ seems to be a little bit on the congested, soft side._"

 That is very interesting! Even though I've never heard what the LD is capable of, I would not have expected just those three mods to make that much of an improvement. Now I'm very curious to see how things will progress as your amp breaks in and as you do more mods later. 

 AFAIK, this puts you in a unique position since you are the only person with both a modded 708B and a LD2++. Since these are both fairly popular amps here I'm sure there will be quite a few people interested in your comparison of them. Might even be worth starting a new thread later on just for the "Modded 708B".


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## Chops

_"If the caps were mounted up front the output wire from their negative terminal would have been run directly to the HP jack, not back to the audio board. So basically it's the same amount of wire either way. 

 Later on when you upgrade the Hp jack and the signal wire I'd run the new wire directly from the negative terminal of the Mundorf to the HP jack. This will eliminate one solder joint and a trace from the signal path of each channel. Looking back, I should have done this with the ELNAs, but I'll do it when I upgrade these caps."_

 Yeah, I was getting this little mod mixed up with the coupling caps. I don't know why, but I was sure you were referring to running the coupling caps directly to the volume pot for some odd reason. My father kept saying that I must be nuts and not thinking right because it wasn't making any sense to him, since he could clearly see where the coupling caps were in the schematic, and by just looking at the pcb. He's definately no dummie! LOL


_"Yeah that is a beautiful solder job on the Mundorfs. What kind of wire is inside that cloth BTW?"_

 That's what nearly 60 years of electronics will get you right there! The cloth wire is 20awg tinned coper. He gets all of his cloth wire from Radio Daze. It's nothing overlly special, but I'm sure it's every bit as good if not better than the Cat6 wire you used. plus it comes in 8 different colors, 25, 50 or 250' spools for cheap, and in either 18 or 20awg, solid or stranded. Go check them out!


_"AFAIK, this puts you in a unique position since you are the only person with both a modded 708B and a LD2++. Since these are both fairly popular amps here I'm sure there will be quite a few people interested in your comparison of them. Might even be worth starting a new thread later on just for the "Modded 708B."_

 Good point. I was actually planning on starting a new thread when I do a little review tomorrow on both of these amps and the LDM+. I already have one other member interested in the results and wants me to keep him up to date on my findings.


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## dcheming

"_I was sure you were referring to running the coupling caps directly to the volume pot for some odd reason._"

 I was talking about doing this with the input caps going directly from the vol pot to the PCB.
	

	
	
		
		

		
		
	


	





 "_The cloth wire is 20awg tinned copper._"

 This would be nice to use for the rest of the wiring as well and it would give you that authentic tube amp look! Since it's solid core it will stay nicely where you mold it.


 "_I was actually planning on starting a new thread when I do a little review tomorrow on both of these amps and the LDM+._"

 Nice. When I receive the 708B that I'm soon going to be doing extensive mods to I'll be posting impressions of bone stock 708B vs my modded 708B. Later on I'll also compare both fully modded amps since they will be using a few different parts, which should be interesting.


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## Chops

_"I was talking about doing this with the input caps going directly from the vol pot to the PCB."_

 See, I still got it wrong! LOL


 That wire is rather nice to work with, and as you said, it gives it an authentic tube amp look. 

 Here another pic with the LDM+ in action. The cans also make an appearance!


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## Chops

And here's one more with my bro's Grado 325i cans. I wanted to try his cans out since they are only 32 ohms, just to see if they pull anything else out of these amps at the lower impedience.


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## zer061zer0

Very nice pictures you have there..


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## Chops

Thanks! I'll be writing a small review tonight for all 3 amps. Obviously I didn't get around to it last night.


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## Chops

Well, I've been busy today! 

 First thing I did was learn how to solder and desolder on a PCB (out of an scanner or something). After about an hour of desoldering and pulling components off of this board, and making sure to not lift any of the pads, I felt I was ready for the next task at hand...

 Which in this case was doing some tweaking on my amp. This time around, _I_ wanted to be the one doing the mods/tweaks instead of bothering my father to do them. So here's the list of things I did today.

 1) Tested my DVD/SACD player's RCA outputs for any DC. None found!
 2) Replaced the 0.1uF input capacitors with tinned 20awg OFC wire
 3) Replaced the stock 24awg steel jumper with tinned 20awg OFC wire
 4) Completely removed the garish and useless tube/reflector/LED crap
 5) Placed the PS transformer on the nylon stand-offs from the window LEDs
 6) Moved the Mundorf M-Lytic HV output capacitors up front in window
 7) Removed the wiring for the preamp output RCAs
 8) Removed the 0.22uF preamp output capacitors
 9) Took plenty of pictures throughout the entire process... Of course! 
	

	
	
		
		

		
		
	


	



















































 The PS transformer with its newly aquired nylon stand-offs.


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## dcheming

Nice work there Charles! I'm glad to see you're teaching yourself how to solder/work on your amp yourself. That's where the real fun of DIY is IMO.
	

	
	
		
		

		
		
	


	




 "_Completely removed the garish and useless tube/reflector/LED crap_"
 "_Moved the Mundorf M-Lytic HV output capacitors up front in window_"

 So you changed your mind after all huh?
	

	
	
		
		

		
		
	


	





 I like your homemade PCB holder too; I just use the crappy helping hands that I got from Harbor Freight. Man, your "workspace" is cramped indeed, but it beats working on the floor like I had to when I first moved here in summer. I was lucky enough to find a massive wood desk a while back for $5 at the surplus store on campus so now I have a legitimate workbench.

 Looking at your pics I only have one suggestion and that would be to run the output (- side) of the Mundorfs directly to the HP jack. Running the black wires all the way back to the PCB is kind of a waste since after 1" of trace they connect up to the stock wire and then come all the way _back_ to the front to go to the HP jack. It should be pretty straight forward to solder the black wires to the little PCB that the HP jack is on. As for polarity just look at the little rectangular symbol that surrounds the holes on the PCB. Out of the three holes, the outer one that has a square around it is the Left channel, the middle one is ground, and the last one is of course the Right channel. If you do decide to do this small wiring change I'd also run a new ground wire using the same cloth wire from the the output of the PCB to the HP jack. It's always hard to explain these kinds of things with words so if your not sure what I mean just ask.


 So did you notice anything from taking out the input caps?


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## Chops

Quote:


  Originally Posted by *dcheming* /img/forum/go_quote.gif 
_Nice work there Charles! I'm glad to see you're teaching yourself how to solder/work on your amp yourself. That's where the real fun of DIY is IMO.
	

	
	
		
		

		
		
	


	




 "Completely removed the garish and useless tube/reflector/LED crap"
 "Moved the Mundorf M-Lytic HV output capacitors up front in window"

 So you changed your mind after all huh?
	

	
	
		
		

		
		
	


	





 I like your homemade PCB holder too; I just use the crappy helping hands that I got from Harbor Freight. Man, your "workspace" is cramped indeed, but it beats working on the floor like I had to when I first moved here in summer. I was lucky enough to find a massive wood desk a while back for $5 at the surplus store on campus so now I have a legitimate workbench.

 Looking at your pics I only have one suggestion and that would be to run the output (- side) of the Mundorfs directly to the HP jack. Running the black wires all the way back to the PCB is kind of a waste since after 1" of trace they connect up to the stock wire and then come all the way back to the front to go to the HP jack. It should be pretty straight forward to solder the black wires to the little PCB that the HP jack is on. As for polarity just look at the little rectangular symbol that surrounds the holes on the PCB. Out of the three holes, the outer one that has a square around it is the Left channel, the middle one is ground, and the last one is of course the Right channel. If you do decide to do this small wiring change I'd also run a new ground wire using the same cloth wire from the the output of the PCB to the HP jack. It's always hard to explain these kinds of things with words so if your not sure what I mean just ask.


 So did you notice anything from taking out the input caps?_

 

Thanks Derek! Yeah, it's definately more fun doing this stuff myself. And I don't have to try and explain why one cap sounds different than another, eventhough my father still doesn't believe me. (_As long as it's the same value, then it's not going to sound different, no matter who made it._) 
	

	
	
		
		

		
		
	


	




 So I changed my mind about the window. hehe 
	

	
	
		
		

		
		
	


	




 What can I say, I was getting tired of having that thing in there, not to mention it was taking up valuable real estate.

 I have to think up something to do in my room for a workbench. Now since I have a hand on soldering on PCBs, I'm sure I'll be doing a lot more mods, and on different equipment, like my Sony CD/DVD/SACD player.

 As for running the Mundorfs directly to the headphone jack, I'll do that when I upgrade the jack and power switch. Man, those stock parts are CHEAP! 
	

	
	
		
		

		
		
	


	




 I haven't had a chance to really listen to the amp yet. Once I finished up reinstalling the Mundorfs last night after dinner, the family and I settled down in the familyroom (my theater room) to watch a movie, K-19 to be exact. Shortly thereafter, I wrote the above post then went to bed. Don't worry though, I'll give it a good listen tonight and most of tomorrow, and report back.


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## That dude

Hi, I am currently looking to get this amplifier, however, I would not be able to change the tubes mainly because I would have no money left after the purchase, and I do no know where i could get them where i live.

 I will be using it with a k501. Also, My current speaker rig is a NAD3020i(as preamp) to a Denon PMA-880R power amp. Should I switch the NAD with the xiangsheng or should I add a "pre" pre-amp to the mix? And if I did not change the tubes... would the sound generally... suck? Please pardon my laziness to read previous posts, but how do they sound compared to the LD2+ or LD2++... 

 Thanks guys.


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## zer061zer0

Quote:


  Originally Posted by *That dude* /img/forum/go_quote.gif 
_Hi, I am currently looking to get this amplifier, however, I would not be able to change the tubes mainly because I would have no money left after the purchase, and I do no know where i could get them where i live.

 I will be using it with a k501. Also, My current speaker rig is a NAD3020i(as preamp) to a Denon PMA-880R power amp. Should I switch the NAD with the xiangsheng or should I add a "pre" pre-amp to the mix? And if I did not change the tubes... would the sound generally... suck? Please pardon my laziness to read previous posts, but how do they sound compared to the LD2+ or LD2++... 

 Thanks guys._

 

I think Chops had a seperate thread on his review on Ldm and LD2++ and the XS708B. I have tried using the 708B as a preamp and wouldnt recommend it as the gain is very high slight adjustment results in high volume change. I also noted some channel imbalances with the default pot. 

 In my personal opinion, i would think that you better hear the K501 over the XS first, to verify if u like them.


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## zer061zer0

Quote:


  Originally Posted by *dcheming* /img/forum/go_quote.gif 
_Nice work there Charles! I'm glad to see you're teaching yourself how to solder/work on your amp yourself. That's where the real fun of DIY is IMO.
	

	
	
		
		

		
		
	


	




 "Completely removed the garish and useless tube/reflector/LED crap"
 "Moved the Mundorf M-Lytic HV output capacitors up front in window"

 So you changed your mind after all huh?
	

	
	
		
		

		
		
	


	





 I like your homemade PCB holder too; I just use the crappy helping hands that I got from Harbor Freight. Man, your "workspace" is cramped indeed, but it beats working on the floor like I had to when I first moved here in summer. I was lucky enough to find a massive wood desk a while back for $5 at the surplus store on campus so now I have a legitimate workbench.

 Looking at your pics I only have one suggestion and that would be to run the output (- side) of the Mundorfs directly to the HP jack. Running the black wires all the way back to the PCB is kind of a waste since after 1" of trace they connect up to the stock wire and then come all the way back to the front to go to the HP jack. It should be pretty straight forward to solder the black wires to the little PCB that the HP jack is on. As for polarity just look at the little rectangular symbol that surrounds the holes on the PCB. Out of the three holes, the outer one that has a square around it is the Left channel, the middle one is ground, and the last one is of course the Right channel. If you do decide to do this small wiring change I'd also run a new ground wire using the same cloth wire from the the output of the PCB to the HP jack. It's always hard to explain these kinds of things with words so if your not sure what I mean just ask.


 So did you notice anything from taking out the input caps?_

 

Yeah DIY, thats where the fun and agony begin, nice work chops.

 I am also interested in whta improvements u get from removing the input caps. 

 Derek,
 did u receive my pm? somewhat itching to get the next project moving, 
	

	
	
		
		

		
		
	


	



 Have u started on it. My 708B is into 5 seperate pieces again, decide to redo it with a proper case, want it to be asetheticaly pleasing as well.


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## That dude

Quote:


  Originally Posted by *zer061zer0* /img/forum/go_quote.gif 
_I think Chops had a seperate thread on his review on Ldm and LD2++ and the XS708B. I have tried using the 708B as a preamp and wouldnt recommend it as the gain is very high slight adjustment results in high volume change. I also noted some channel imbalances with the default pot. 

 In my personal opinion, i would think that you better hear the K501 over the XS first, to verify if u like them._

 

Thanks, but unfortunatly, I can't test before I buy...
 Never mind then... The search, Continues...


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## Chops

Quote:


  Originally Posted by *That dude* /img/forum/go_quote.gif 
_Thanks, but unfortunatly, I can't test before I buy...
 Never mind then... The search, Continues..._

 

Don't think that the stock 708B is a bad sounding amp... It's not!

 Even with the stock tubes, it has a decent sound. But with a little money on a set of better tubes (less than $40 shipped), the sound will improve considerably over stock. 

 All of the mods that some of us have been doing, well, that's just us. With all the stuff I've done to mine so far, I still think the tubes were the biggest and best improvement! The tubes were also the cheapest improvement!

 This amp will drive your K501's pretty good with the stock tubes, and even better with the better tubes. It's a very capable amp and IMHO is a better amp than the Little Dot II ++, just with the tube upgrade.

 Give it a try, you just might like it! 
	

	
	
		
		

		
		
	


	





 BTW, check out my review thread for the latest little tweak/mod I did this morning, thanks to Derek. 
	

	
	
		
		

		
		
	


	




Quick REVIEW: Modded Xiang Sheng 708B vs. LDII++ vs. LDM+...


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## That dude

Whoa... Rating the LDM+ as (1) in stock form did not really encourage me with the buy "all that much" 
 I have a LDM+ myself and I felt the sound was thin, and bright, so If the XS708b is as u said... "like the LDM+" I dun think it's the amp for me...

 Thks though. If only i could test is out myself...


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## zer061zer0

Quote:


  Originally Posted by *That dude* /img/forum/go_quote.gif 
_Whoa... Rating the LDM+ as (1) in stock form did not really encourage me with the buy "all that much" 
 I have a LDM+ myself and I felt the sound was thin, and bright, so If the XS708b is as u said... "like the LDM+" I dun think it's the amp for me...

 Thks though. If only i could test is out myself... 
	

	
	
		
		

		
		
	


	


_

 

Its such a pity, my amp is in 5 different parts, otherwise u can come to my place to try them out. I am also curious as to wat the mods have done to drive the akg501 better. My very first purchase was infact pondering btw the akg501 and the ms2i with the XS708B. 

 If u can wait i might be able to have the amp up in a few weeks time and u can come over to my place to hear them with ur 501.


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## Chops

Quote:


  Originally Posted by *That dude* /img/forum/go_quote.gif 
_Whoa... Rating the LDM+ as (1) in stock form did not really encourage me with the buy "all that much" 
 I have a LDM+ myself and I felt the sound was thin, and bright, so If the XS708b is as u said... "like the LDM+" I dun think it's the amp for me...

 Thks though. If only i could test is out myself... 
	

	
	
		
		

		
		
	


	


_

 

Like I said, I have a pair of K701's and the sound from neither one of these amps is "thin and bright". 

 And stock form ment just that, stock! Meaning that everything including the tubes. Just replacing the tubes will put the 708B over the top. 

 If the LDM+ sounds thin and bright on your K501's but sounds perfectly fine on my K701's, then wouldn't you think it's NOT the amp? If your cans have no bass, it doesn't matter what amp you drive them with.


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## Chops

Quote:


  Originally Posted by *Chops* /img/forum/go_quote.gif 
_Like I said, I have a pair of K701's and the sound from neither one of these amps is "thin and bright". 

 And stock form ment just that, stock! Meaning that everything including the tubes. Just replacing the tubes will put the 708B over the top. 

 If the LDM+ sounds thin and bright on your K501's but sounds perfectly fine on my K701's, then wouldn't you think it's NOT the amp? If your cans have no bass, it doesn't matter what amp you drive them with._

 

Man, I just re-read my last post. It sounds like I was pissed or something. I wasn't. Sorry if someone took it that way. I hope not.


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## zer061zer0

Well its ok dude. 

 Lets hope this helps to shift some attention from it ....hahhahaa~~~

 The next upcoming madness for me


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## Chops

Quote:


  Originally Posted by *zer061zer0* /img/forum/go_quote.gif 
_Well its ok dude. 

 Lets hope this helps to shift some attention from it ....hahhahaa~~~

 The next upcoming madness for me














_

 

That's a real nice little case! Rack handles and all, too! 
	

	
	
		
		

		
		
	


	




 I was thinking, if I were to put the 708B guts into a new case, I'd have the power transformer all the way back in the left rear corner, as far as possible from the driver and output tubes, and have the PS board up front. But that's just me.


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## dcheming

That's a pretty sweet little case! You've got plenty of room to play with now.
	

	
	
		
		

		
		
	


	




 I'm with Chops in thinking that I would change the parts layout. It looks like the audio board would just fit sideways in that case. You could mount the audio board right up front and then mount the transformer and power supply in the very back. You could even fabricate a steel wall to isolate these two areas if you wanted to get fancy and if you put the power switch on the back you could keep all the AC-carrying parts/wire in that one area.

 Well my 1/4-40 taps came in the mail today so I'll finally be able to finish mounting the source selector switch. Man, just seeing the 708B guts in zer061zer0's new case makes me want to do a re-case too!
	

	
	
		
		

		
		
	


	



 So much energy, so little money.


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## Chops

Quote:


  Originally Posted by *dcheming* /img/forum/go_quote.gif 
_So much energy, so little money.
	

	
	
		
		

		
		
	


	


_

 

Yeah, I know the feeling all too well. However, I would have also added "so much time" to the list as well, at least in my case. 
	

	
	
		
		

		
		
	


	




 BTW Derek, I need you to get with me sometime at your convenience (of course) about locating and replacing ALL of the vital resistors in the signal chain, including the ones on the headphone jack.

 Would changing the values of any of these be any good, or should they stay as is, just better quality? Depending on how many I'll need and their prices, I might try going with the Audio Notes or Vishays.


----------



## zer061zer0

Quote:


  Originally Posted by *dcheming* /img/forum/go_quote.gif 
_That's a pretty sweet little case! You've got plenty of room to play with now.
	

	
	
		
		

		
		
	


	




 I'm with Chops in thinking that I would change the parts layout. It looks like the audio board would just fit sideways in that case. You could mount the audio board right up front and then mount the transformer and power supply in the very back. You could even fabricate a steel wall to isolate these two areas if you wanted to get fancy and if you put the power switch on the back you could keep all the AC-carrying parts/wire in that one area.

 Well my 1/4-40 taps came in the mail today so I'll finally be able to finish mounting the source selector switch. Man, just seeing the 708B guts in zer061zer0's new case makes me want to do a re-case too!
	

	
	
		
		

		
		
	


	



 So much energy, so little money.
	

	
	
		
		

		
		
	


	


_

 

I wanted to have hat configuration with all the ac stuff at the back but the only worry is the trouble in switching it on and off. I have also gotten a noise filter iec and some ferrite to be mounted along the powersection. 

 Anyone any experience in cutting a hole for the iec.


----------



## Chops

Quote:


  Originally Posted by *zer061zer0* /img/forum/go_quote.gif 
_I wanted to have hat configuration with all the ac stuff at the back but the only worry is the trouble in switching it on and off. I have also gotten a noise filter iec and some ferrite to be mounted along the powersection. 

 Anyone any experience in cutting a hole for the iec._

 

I remember years ago on the old desktop AT computers with their AT power supplies, some of them had the power switch inside the PS itself. There would be a long metal bar (linkage) that ran all the way from the PS in the back of the computer up front to the actual power button on the front panel of the computer case. 

 Being that this amp is pretty close to being silent anyway, I don't think I would worry about having a couple of power wires coming up front to a switch on the front panel. I'd just have the switch all the way to one side and the wires crammed as far into the corner of the case as possible to keep them away from the audio board.


_"Anyone any experience in cutting a hole for the iec?"_

 Maybe very carefully with a dremel? 

 You could draw the opening on the chassis, then take a drill with a small bit and drill along the line, then go through it very very carefully with the dremel, sliding to each drill hole, creating the entire opening for the IEC.

 Of course, I'm sure there's most likely a better, easier way of doing this than what I said above. 
	

	
	
		
		

		
		
	


	




 Either way, good luck!


----------



## dcheming

Well I just finished up doing the switch mounting. It went smooth, but the only thing I'm not happy with is the finish on the chamfer for the tapped hole. The chamfer bit I used was less than ideal for aluminum so it didn't leave a very good finish. I'll have to get a better chamfer tool later and touch it up to give it a mirror finish. I also removed some of the wording on the front panel with alcohol to make it look cleaner. It took a little while to measure everything out to see if it would fit still, but the end result is that with the mounting hole exactly centered from the outer edge of the vol pot chamfer to the edge of the faceplate, the body of the switch itself rests directly on the chassis bottom so it will never rotate loose. Overall I'm happy with how it turned out.


----------



## Chops

Quote:


  Originally Posted by *dcheming* /img/forum/go_quote.gif 
_Well I just finished up doing the switch mounting. It went smooth, but the only thing I'm not happy with is the finish on the chamfer for the tapped hole. The chamfer bit I used was less than ideal for aluminum so it didn't leave a very good finish. I'll have to get a better chamfer tool later and touch it up to give it a mirror finish. I also removed some of the wording on the front panel with alcohol to make it look cleaner. It took a little while to measure everything out to see if it would fit still, but the end result is that with the mounting hole exactly centered from the outer edge of the vol pot chamfer to the edge of the faceplate, the body of the switch itself rests directly on the chassis bottom so it will never rotate loose. Overall I'm happy with how it turned out.






























_

 






 Talk about tight tolerances, and great work! WOW!

 And there's nothing better than a few metal shavings embedded in the carpet. 
	

	
	
		
		

		
		
	


	




 Nice volume knob, too. Does that actually fit in the stock recess? Pretty slick.


----------



## zer061zer0

Impressive stuff if only i have those tools and skills.


----------



## That dude

Quote:


  Originally Posted by *zer061zer0* /img/forum/go_quote.gif 
_Its such a pity, my amp is in 5 different parts, otherwise u can come to my place to try them out. I am also curious as to wat the mods have done to drive the akg501 better. My very first purchase was infact pondering btw the akg501 and the ms2i with the XS708B. 

 If u can wait i might be able to have the amp up in a few weeks time and u can come over to my place to hear them with ur 501._

 

Haha, thks for the offer but no hurry, cause I live in Singapore.
	

	
	
		
		

		
		
	


	




  Quote:


  Originally Posted by *Chops* /img/forum/go_quote.gif 
_Like I said, I have a pair of K701's and the sound from neither one of these amps is "thin and bright". 

 And stock form ment just that, stock! Meaning that everything including the tubes. Just replacing the tubes will put the 708B over the top. 

 If the LDM+ sounds thin and bright on your K501's but sounds perfectly fine on my K701's, then wouldn't you think it's NOT the amp? If your cans have no bass, it doesn't matter what amp you drive them with._

 

Well, the LDM+ is certainly not strong enough to drive the k501 to its fullest. I have tried them with everything I owned and i still felt they sounded "thin and bright" compared to my go-vibes or my desktop power amps. Moreover, "thin and bright" just suggest that the sound is fatiging. The k701 is a totally different animal, so you can't really compare.

 I've tried it with the Diva tube amp (an amp from TS Lim in Singapore), and it made my k501 sing so smoothly and warmly... The X-tra X1 made the k501 into a hardcore rocker (really unexpected). In short, the amp plays a huge part, especially with such a picky headphone which is SOO difficult to drive.

 Hey, thks for the input, I really appreciate it.


----------



## zer061zer0

Yeah that dude i know that u stay in Singapore, wouldn't have offer otherwise. 

 I have heard the x1 powering the k501, dont think its gd enough too. Maybe i am just picky...
 Will pm u again when my amp is up.


----------



## dcheming

Thanks for the kind remarks guys!
	

	
	
		
		

		
		
	


	




 "_Anyone any experience in cutting a hole for the iec._"

 For my CD player I just drilled three 3/8" holes in the corners of the cutout and then used a little flat file to cut the rest. It took quite a while but it turned out pretty good. I wonder if someone makes a sheet metal punch just for doing IEC cutouts?


----------



## zer061zer0

I did a cut out of the iec on my cdp as well but wasnt very nice
 Besides, the iec cut out on the cdp can be cut right from the top unlike the new case where i have to cut in the centre.


----------



## Chops

Speaking of cd players, I just ordered a brand spankin' new Music Hall CD-25.2 from Underwood HiFi at a more than excellent price! Thanks Walter! I'm also going to have him throw in 4 mirror imaged Burr Brown OPA-627 opamps to replace the inexpensive 2604 opamps in the stock unit.

 I think this will be a substantial upgrade over my current cdp.


----------



## dcheming

"_I just ordered a brand spankin' new Music Hall CD-25.2 from Underwood HiFi at a more than excellent price!_"

 Nice!
	

	
	
		
		

		
			





 That should indeed be quite a bit better than your current source and that round display on the front panel kind of matches with the window in the 708B.


 "_I'm also going to have him throw in 4 mirror imaged Burr Brown OPA-627 opamps to replace the inexpensive 2604 opamps in the stock unit._"

 That's going to help a lot. They should be mounted in sockets so you'll be able to do some opamp rolling if you want to change the flavor later on. One nice thing about this player is that there's a lot of space inside so you'll have quite a bit room for things like film caps, clocks, dedicated power supplies, chassis dampening, etc.
	

	
	
		
		

		
		
	


	




 So what's the estimated delivery time?


----------



## Chops

Quote:


  Originally Posted by *dcheming* /img/forum/go_quote.gif 
_"I just ordered a brand spankin' new Music Hall CD-25.2 from Underwood HiFi at a more than excellent price!"

 Nice!
	

	
	
		
		

		
		
	


	




 That should indeed be quite a bit better than your current source and that round display on the front panel kind of matches with the window in the 708B.


 "I'm also going to have him throw in 4 mirror imaged Burr Brown OPA-627 opamps to replace the inexpensive 2604 opamps in the stock unit."

 That's going to help a lot. They should be mounted in sockets so you'll be able to do some opamp rolling if you want to change the flavor later on. One nice thing about this player is that there's a lot of space inside so you'll have quite a bit room for things like film caps, clocks, dedicated power supplies, chassis dampening, etc.
	

	
	
		
		

		
		
	


	




 So what's the estimated delivery time?_

 

Hi Derek,

 Yeah, I'm really looking forward to getting this cdp. And those new opamps are supposed to really help out a lot. 

 And you're right about there being a lot of room in these units. Depending on how difficult it looks to be in there, I might just go ahead and do a lot of those upgrades that Underwood does. Maybe I can talk Walter into writing up a list of the components that he upgrades. He mentions most of them (brands and values) on his site in the "Modifications" section. And guess what, he uses Rikens in his mods, and PC as his source! I can just go off of his mods, but do them a little at a time when funds permit.

 As for its arrival, the player will be shipped out on Monday from Atlanta, GA and the opamps will also be shipped out on Monday, but from Canada.


----------



## dcheming

"_but do them a little at a time when funds permit._"

 This is the way to go since you can just work your way down the list of goodies as your wallet permits. Just stick with the simple stuff first like resistors, caps, diodes, wire, RCA jacks, etc. Of course you'll want to get used to the sound of the player before you start changing things. 


 "_And guess what, he uses Rikens in his mods, and PC as his source!_"

 I also noticed the picture of the RIKENs when I was browsing his site earlier. I wonder how these compare to the Audio Note tants? I would love to put some RIKEN resistors and better opamps in my cd player now that I'm almost done with the 708B mods. It would be nice to install some more M-Caps, but on the inputs to the opamps also. Some Schottky diodes in the PS section would be a nice 
 simple mod too.


 "_As for its arrival, the player will be shipped out on Monday from Atlanta, GA_"

 So then you should have it by next weekend hopefully.


----------



## dcheming

Quote:


  Originally Posted by *Chops* /img/forum/go_quote.gif 
_Nice volume knob, too. Does that actually fit in the stock recess? Pretty slick._

 

This knob is from Rat-Shack. I thought the style matched since it's a tube amp and it _almost_ does fit in the recess. It's for a 1/4" shaft so I had to wrap a single layer of electrical tape on the shaft of the ALPS to get it to be centered properly. I'm going to try to shave a bit of its OD off using my drill press as lathe to get it to actually fit into the recess.


----------



## Chops

Quote:


  Originally Posted by *dcheming* /img/forum/go_quote.gif 
_This knob is from Rat-Shack. I thought the style matched since it's a tube amp and it almost does fit in the recess. It's for a 1/4" shaft so I had to wrap a single layer of electrical tape on the shaft of the ALPS to get it to be centered properly. I'm going to try to shave a bit of its OD off using my drill press as lathe to get it to actually fit into the recess.
	

	
	
		
		

		
		
	


	


_

 

Ah... So you're going to attach the knob to a drill bit, turn the drill press on and hold some sandpaper around the rim of the knob to file it down some? Pretty nifty!


----------



## dcheming

Quote:


  Originally Posted by *Chops* /img/forum/go_quote.gif 
_Ah... So you're going to attach the knob to a drill bit, turn the drill press on *and hold some sandpaper around the rim of the knob to file it down some?* Pretty nifty! 
	

	
	
		
		

		
		
	


	


_

 

One step above that, I used my Ghetto Lathe 
	

	
	
		
		

		
		
	


	




: 







 I took off about 0.008" and the knob does indeed fit into the recess now, but I didn't shorten the ALPS shaft enough to allow the knob to go much further in than it already did before. I really don't want to cut the shaft even shorter just to fit this crappy Rat-Shack knob, so I'm going to look for a better quality knob in this style to work with first.


----------



## Chops

Just put in another order of parts for the little 708B at PC. 
	

	
	
		
		

		
		
	


	





_Input/Output:_

 TANTALM - 62762 10R 2 watt $12.95 x2
 TANTALM - 62712 1K 1/2 watt $4.75 x2



_PS:_

 DIODES - 56739 Vishay-Telefunken SF4007 $0.35 x2



_Anode Resistors:_

 KIWAME- 66220 33K 2 Watt $1.05 x2



_The rest:_

 PRP - 50324 1K0 1 Watt $0.65 x2
 PRP - 64726 1M0 0.5 Watt $0.45 x4
 PRP - 70164 200R 0.5 Watt $0.45 x2
 PRP - 62501 220R 1 Watt $0.65 x2
 PRP - 50094 100R 0.5 Watt $0.45 x4


----------



## Miller-8

I have one of these anyone know how to stop it making horrible noises when you plug the headphones in?

 Also is it possible to have sound just come out the headphones rather than both the headphones and speakers? Just now I have to turn my power amp off when I want to listen to headphones.


----------



## Chops

Quote:


  Originally Posted by *Miller-8* /img/forum/go_quote.gif 
_I have one of these anyone know how to stop it making horrible noises when you plug the headphones in?

 Also is it possible to have sound just come out the headphones rather than both the headphones and speakers? Just now I have to turn my power amp off when I want to listen to headphones._

 

Mine doesn't make any noise when I plug my headphones in or out, so I don't know.

 The preamp output can not be muted, at least not in stock form. So unless you feel like modding the 708B, you'll just have to keep powering down your amp.


----------



## Miller-8

I am asking about mods here.


----------



## dcheming

"_I have one of these anyone know how to stop it making horrible noises when you plug the headphones in?_"

 Are you referring to the loud pops and crackles as you first plug in the headphones? I built a 1/4" TRS plug that has a 300Ω resistor in each channel that goes to ground. I keep it in the HP jack of the amp while it's warming up and remove it when I finally do plug in my headphones. There is only a _very_ slight click heard upon insertion of the HP when using the dummy-load plug this way.


 "_Also is it possible to have sound just come out the headphones rather than both the headphones and speakers?_"

 As Chops said, you'd have to install a little switch to mute the pre output.


----------



## zer061zer0

Miller, I would suggest that u switch off the power amp. I did exactly wat u have done. Unless u woud like to try adding a switch..


----------



## Miller-8

I'm going to do my first mod - the volume pot. I know a lot of people have fitted an Alps Blue, i was wondering if the Alps Black RK40 would fit ok? Or a TKD?


----------



## Chops

Quote:


  Originally Posted by *Miller-8* /img/forum/go_quote.gif 
_I'm going to do my first mod - the volume pot. I know a lot of people have fitted an Alps Blue, i was wondering if the Alps Black RK40 would fit ok? Or a TKD?_

 

Don't know for sure if the Black Beauty would fit or not. Probably so, but awfully tight, and probably overkill for this particular amp.


----------



## drarthurwells

Has anyone replace the Chinese rectifier 6Z4 tube with the USA 6X4?


 You need to switch two wires in the tube socket.

 You also need to drill a hole in the top cover to the top of the 6X4 has clearance - may need to glue a washer in place on the top cover, to surround the hole and protect the tip of the tube.

 Changing my rectifier tube in my preamps, from a Chinese tube to a USA 5Z4 tube, improves the sound considerably.

 Old USA 6X4 tubes are readily available.

 Will some report on which wires to switch in the tube socket? I will send a GE 6X4 to someone wishing to try this for $7 total shipped anywhere, if they report on the result.


----------



## dcheming

"_Has anyone replace the Chinese rectifier 6Z4 tube with the USA 6X4?_"

 I'm planning on doing exactly that pretty soon. I'm just going to build a temporary adapter using a 7-pin socket and some short lengths of 18ga solid core to avoid having to modify the PS PCB at first. If the 6X4 makes a noticeable improvement then it would definitely be worth it to mod the PS board to use it permanently.


 "_You also need to drill a hole in the top cover to the top of the 6X4 has clearance - may need to glue a washer in place on the top cover, to surround the hole and protect the tip of the tube._"

 This wouldn't work since you have to slide the case on from the back. The only way to try to make it fit it is to lower the PS board as low as possible on its mounting screws. I'm looking for a low profile 7-pin socket to replace the stock one with to help get some extra clearance too.


----------



## drarthurwells

Quote:


  Originally Posted by *dcheming* /img/forum/go_quote.gif 
_"Has anyone replace the Chinese rectifier 6Z4 tube with the USA 6X4?"

 I'm planning on doing exactly that pretty soon. I'm just going to build a temporary adapter using a 7-pin socket and some short lengths of 18ga solid core to avoid having to modify the PS PCB at first. If the 6X4 makes a noticeable improvement then it would definitely be worth it to mod the PS board to use it permanently.


 "You also need to drill a hole in the top cover to the top of the 6X4 has clearance - may need to glue a washer in place on the top cover, to surround the hole and protect the tip of the tube."

 This wouldn't work since you have to slide the case on from the back. The only way to try to make it fit it is to lower the PS board as low as possible on its mounting screws. I'm looking for a low profile 7-pin socket to replace the stock one with to help get some extra clearance too._

 

I forgot about sliding on the cover from the back - I took my cover off when I got the amp and have never used it since. Makes changing tubes easy.

 I'LL BUY YOUR TEMP ADAPTER WHEN YOU ARE THROUGH WITH IT.


----------



## dcheming

"_I took my cover off when I got the amp and have never used it since. Makes changing tubes easy._"

 Hah, yeah me too, I haven't had it on since the day I got the amp. I'm thinking about crafting some aluminum side panels and going with an easily removable mesh top for ventilation. The tip of the 6X4 could then stick through if it's too tall.

 "I'LL BUY YOUR TEMP ADAPTER WHEN YOU ARE THROUGH WITH IT."

 Well I could easily make two of them. It would probably add about 3/4" to the height so the tube would stick out quite a bit just to let you know.


----------



## zer061zer0

I would be interested in the side panels and adaptors..

 This weekend gonna be wiring day for the XS, decide to use the black case for my upcoming CK2III


----------



## Miller-8

Side panel and adaptors for me too! The lid is a pain in the arse to get on and off.


----------



## Miller-8

Can anyone suggest a replacement for the 6Z4 rectifier tube? Can you use a 6X4? If not why not? How about a russian 6Ts4P?


----------



## Chops

Quote:


  Originally Posted by *Miller-8* /img/forum/go_quote.gif 
_Can anyone suggest a replacement for the 6Z4 rectifier tube? Can you use a 6X4? If not why not? How about a russian 6Ts4P?_

 

Dcheming just wrote this last night.

6X4 Rectifier Modification


----------



## Miller-8

Yeah i already read that, doesnt explain whether there are any tubes you can just slot straight in as replacements or why the original tube has different pin locations.


----------



## Chops

Quote:


  Originally Posted by *Miller-8* /img/forum/go_quote.gif 
_Yeah i already read that, doesnt explain whether there are any tubes you can just slot straight in as replacements or why the original tube has different pin locations._

 

There are no other tubes that will work in the existing circuit.


----------



## dcheming

Quote:


  Originally Posted by *Miller-8* /img/forum/go_quote.gif 
_Yeah i already read that, doesnt explain whether there are any tubes you can just slot straight in as replacements or why the original tube has different pin locations._

 

It's very hard finding much info on the mini 6Z4 rectifier and the same goes for finding info about its equivalents. All the new 6Z4 tubes that I've seen for sale appear to be the same low quality one I already have. So as far as I'm aware there's no other viable option for a drop-in improvement. 

 The 6X4 on the other hand has plenty of info available and has many nice NOS versions. It's a little tall for this application, but I don't mind that since I always have the top case off anyways. The mod to the socket in the 708B is required because in the 6X4 one of the anodes and the cathode are connected to different pins than in the 6Z4.


----------



## Miller-8

Have you noticed a sound improvement so far?


----------



## Chops

Quote:


  Originally Posted by *Miller-8* /img/forum/go_quote.gif 
_Have you noticed a sound improvement so far?_

 

It's in the other thread that I already mentioned (and linked) to you earlier.


----------



## regal

Would any of these chinese amps be an serious upgrade over my maxed out millett hybrid


----------



## dcheming

Quote:


  Originally Posted by *regal* /img/forum/go_quote.gif 
_Would any of these chinese amps be an serious upgrade over my maxed out millett hybrid_

 

Maybe a bit better, but not a serious upgrade IMO. I feel that a maxed out Millett would be better than a bone stock 708B, but if you were interested in modding it at all it might be a different story. I did a quick comparison between my 708B with Auricaps and ALPS and a Millett with diamond buffers at a friends house a few months back and they were very similar sounding. The Millett had tighter and deeper bass and the 708B had better midrange. 

 However, it was just a quick test and CD player wasn't the greatest. I'd like to be able to borrow his Millett and listen to it in my system for a few days. I've also done a lot more modifications to my 708B since then so I'd like to know how they compare now.


----------



## Miller-8

Can abyone recommend any 6922 tubes to use with this amp? I'm thinking of trying a couple of these Sovteks:

http://www.tube-shop.com/uk/prod_nos...ProcessType=17


----------



## Miller-8

Hello chaps I now have an Alps Blue to fit to my 708B, am I best to remove the little circuit board thing that the original pot it attetched to and solder the wires straight to the Alps? Also you know the little nodule on the Alps do you cut that off or make a wee hole for it to fit into?


----------



## dcheming

Quote:


  Originally Posted by *Miller-8* /img/forum/go_quote.gif 
_Hello chaps I now have an Alps Blue to fit to my 708B, am I best to remove the little circuit board thing that the original pot it attetched to and solder the wires straight to the Alps? Also you know the little nodule on the Alps do you cut that off or make a wee hole for it to fit into?_

 

I don't think the pins of the ALPS will fit in the little PCB of the stock pot so you pretty much have to solder directly to them. I wonder if someone offers a little PCB that works with the 27mm ALPS pots? That would definitely make it much easier.

 The tab keeps the pot itself from rotating if the mounting nut comes loose so it might be worthwhile to drill the little hole, but it's not imperative that you do so. There's some pics of how I did my ALPS in the mod thread about 1/3 of the way down in my first post.


----------



## Miller-8

Need help wiriring up Alps blue pot. The pot is marked 1, 2 and 3 is 1 the ground connections? and is 2 the output and 3 the input?


----------



## dcheming

Quote:


  Originally Posted by *Miller-8* /img/forum/go_quote.gif 
_Need help wiriring up Alps blue pot. The pot is marked 1, 2 and 3 is 1 the ground connections? and is 2 the output and 3 the input?_

 

My RK27 isn't numbered like yours, so assuming that you have pin 1, then 2, then 3, and then a fourth spot with no pin, then going by the pin numbers you said above you are correct.


----------



## Miller-8

ok thanks. I noticed in a picture someone posted that they had the two ground connections linked with a copper wire. Should I do this?


----------



## dcheming

Quote:


  Originally Posted by *Miller-8* /img/forum/go_quote.gif 
_ok thanks. I noticed in a picture someone posted that they had the two ground connections linked with a copper wire. Should I do this?_

 

Yeah, you should do the same also. The ground is shared between the two channels throughout the amp anyways which is why it's okay to link those two pins together like that. Also you'll need to ground the ALPS to the chassis of the amp to prevent induced hum. You can see how Chops did this in post #5 of the mod thread. He just ran a short little jumper wire from the ground link of the ALPS over to a nearby screw holding the face plate on. If you do it the same way be sure to scrape a bit of the paint off to get a good connection.


----------



## Miller-8

Thanks. Havent yet connected the ground to the chassis but i've plugged in in to try it. A few things I've noticed:

 1. There is a low hum that wasnt there before, not very loud but noticable when no music playing. Perhaps this will go when I connect the ground wire to the chassis?

 2. I can now use a lot more range on the volume knob! The original one must have not been a log pot!

 3. Sounds more detailed and crisper.


----------



## dcheming

Quote:


  Originally Posted by *Miller-8* /img/forum/go_quote.gif 
_There is a low hum that wasnt there before, not very loud but noticable when no music playing. Perhaps this will go when I connect the ground wire to the chassis?_

 

Yeah it goes away completely when you ground it.


----------



## Chops

Here you go, just to keep things easy.


----------



## Miller-8

LOL mine isnt as neat as that, mine is a mess. I've only just started soldering. Anyway i tried to undo that chassis screw that you connected the ground to but it wont budge, so I've just slotted the ground wire into the crevice in the corner where the metal folds over.

 Hum is greatly reduced now and volume of the music is louder again, hmm.

 I think next I shall attempt to replace the output and input resistors, and also the input caps. Bit worried about touching things though will doing any of that put me at risk of a shock from the power supply caps? 

 Also the input caps I notice some people put jumpers in their place is that safe to do? How can I tell if my DAC is putting out any DC? What happens if I put in jumpers if there is DC leakage? Would I get a hum noise or something more serious?


----------



## Chops

Quote:


  Originally Posted by *Miller-8* /img/forum/go_quote.gif 
_LOL mine isnt as neat as that, mine is a mess. I've only just started soldering. Anyway i tried to undo that chassis screw that you connected the ground to but it wont budge, so I've just slotted the ground wire into the crevice in the corner where the metal folds over.

 Hum is greatly reduced now and volume of the music is louder again, hmm._

 

Well I can't take all the credit for the solder job on the Alps. My father did all of the connections on it. However, I am the one who soldered the ground wire after I pretty much taught myself how to solder on PCBs. It's as simple as pie now! 
	

	
	
		
		

		
		
	


	




 You just have to make sure to keep a little fresh solder on the tip of the iron so it will transfer the heat to your work efficiently, and also wipe the tip off on a wet sponge or paper towel every once in a while. 

 For that ground connection, try to use a slightly larger screw driver with a larger handle and brace the face plate in the palm of your other hand, push into the screw really good, and try to unscrew it. It's a bit of a pain, but should come loose.

 I'm not sure why the music got a little louder from adding the ground though. 
	

	
	
		
		

		
		
	


	





  Quote:


 I think next I shall attempt to replace the output and input resistors, and also the input caps. Bit worried about touching things though will doing any of that put me at risk of a shock from the power supply caps? 
 

*YES!! You will be in danger of shocking yourself!!* 

 Before you do anymore mods, you should FIRST put a 2 watt bleeder resistor on one of the PS caps. I went with a single 68kΩ resistor on one PS cap, but *dcheming* went with two 42kΩ resistors, one on each PS cap. I think dcheming did his that way just because he likes symmetry. I went with just one because I didn't have any other resistors big enough and because my father said that both of those caps are tied together in parallel anyways, so one is all that's really needed as both caps will get discharged regardless.

 This single 68kΩ resistor bleeds off all stored power within a few minutes time, but I usually let it sit for about 10 minutes or so just to be on the safe side. I'm usually getting the soldering iron and tools ready durring this time.


  Quote:


 Also the input caps I notice some people put jumpers in their place is that safe to do? How can I tell if my DAC is putting out any DC? What happens if I put in jumpers if there is DC leakage? Would I get a hum noise or something more serious? 
 

Dcheming would be best to explain this one. Honestly, I can't even remember if you put the DMM on "mV" or just "V" when testing your source (DAC in your case) for DC output. I lost all my notes in my PMs for this entire subject when dcheming and I were going over it for my amp. I'm not even sure what the effects would be if there was any DC present. It would be straight DC, so I doubt it would be 60 cycle hum. Maybe something that overloads the input and possibly blows your cans? Since it's DC (Direct Current), it would possibly push the headphone's drivers in one direction and leave them there, burning up the voice coils in seconds?? I honestly don't know.


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## dcheming

Quote:


  Originally Posted by *Chops* /img/forum/go_quote.gif 
_ *dcheming* went with two 42kΩ resistors, one on each PS cap. I think dcheming did his that way just because he likes symmetry._

 

I actually used two 82kΩ 2W resistors originally, but being in parallel as you said it's equivalent to 41kΩ. I swapped those out for two 160kΩ 2W back when I did the 6X4 mod, so I now have 80kΩ across the filter caps. The main reason that I went with two 2W resistors is so that I'd have plenty enough power dissipation and still be able to keep them under the PS for a cleaner look, but I do indeed enjoy symmetry also. 
	

	
	
		
		

		
		
	


	





  Quote:


  Originally Posted by *Chops* /img/forum/go_quote.gif 
_I'm not even sure what the effects would be if there was any DC present. It would be straight DC, so I doubt it would be 60 cycle hum. Maybe something that overloads the input and possibly blows your cans? Since it's DC (Direct Current), it would possibly push the headphone's drivers in one direction and leave them there, burning up the voice coils in seconds?? I honestly don't know. 
	

	
	
		
		

		
		
	


	


_

 

You have it pretty much right Chops. Without the input caps, any DC coming from your source will be amplified and passed on to the headphones. This will cause the membrane of the drivers to push out constantly and this can reduce the dynamic range of your headphones. The much more serious consequence of this problem is that the voice coils can burn up if the DC is high enough. Here's an article that talks more about this.


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## Miller-8

ok so how do i attach the bleeder resister safely? Is it safe to take up the power supply board and just solder the resistor onto one of the caps? Do you have to be careful when doing this, too?


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## Chops

Quote:


  Originally Posted by *Miller-8* /img/forum/go_quote.gif 
_ok so how do i attach the bleeder resister safely? Is it safe to take up the power supply board and just solder the resistor onto one of the caps? Do you have to be careful when doing this, too?_

 

I think my father attached a couple of jumper leads to the resistor, then touched them to the PS cap terminals for a few minutes to drain the caps of all that store energy. After they were down to a safer voltage (about 10v or so), then he went ahead and soldered the resistor on.

 Please keep in mind that these large caps will gain back some voltage on their own by just sitting there. Sure, you may bleed them off to about 10v, but in several minutes they can creep back up to 20v or more, so don't wait too long to solder on that resistor after bleeding the caps through the jumper leads.


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## Miller-8

Would a shock from them be enough to kill you? Do you know how big the current is? (amps)


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## dcheming

Quote:


  Originally Posted by *Miller-8* /img/forum/go_quote.gif 
_Would a shock from them be enough to kill you? Do you know how big the current is? (amps)_

 

I don't think these caps are nearly big enough to kill, but they're still pretty dangerous. I'm not sure what the current would be because that depends on the discharge speed, but capacitors like these can discharge their stored energy fairly quickly when shorted and all together there's up to about 17.5 Joules of energy in the four PS caps.


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## troelsjorg

Hi dcheming/all. I have been following this thread from Denmark, and doing some of the mods on my 708B. (I am guessing i have the only 708B in Denmark?) 
 I'd like to ask for some help on selecting caps. 
 I want to change the IC and the HP caps. I use Grado headphones exclusively, and having read all of this thread as well as the mod-thread, i learned that I should consider getting HP-out caps with a value of up to 470uF. I'd like to use Mundorf, but the closest ones i can find are the M-lytic HV 150+150uF 500V. the 200+200uF is too big for the 708B. They are also very expensive. Can you make any suggestions? 
 And for the IC caps, i read your text "I went with Auricaps for these, but I would recommend the Mundorf M-Cap ZN as a better choice for around the same price". I'd like to try those, but the only ones in 0.22 uF are 250V or 630V. The stock cap is 400V. Can i use a 630V cap and what would the difference be from using a 400V? Kind regards Troels


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## dcheming

Hey troelsjorg, welcome aboard. 
	

	
	
		
		

		
			





 I really can't make any other recommendations besides the Mundorf caps that you already suggested, but I'll look around some more to see if there might be a better solution that still fits and is relatively affordable. Even the 150+150uF are going to be a tight fit, as I'm sure you saw when *Chops* first put them in his amp. I know those M-Lytic HV are on the expensive side, but have you priced the same style in Black Gate? 
	

	
	
		
		

		
		
	


	




 Capacitor voltage ratings are simply the highest working voltage that they can safely be used up to. In film caps the various different plastics used have different electrical properties, but all things being equal the thicker the plastic film the higher the voltage rating. So going with the 630V ones will just give you a larger margin of protection in case something ever goes wrong in the circuit. I've also read that some people think the higher voltage version of the same cap sounds better, but I haven't tested that claim myself so I'm not sure.

 Do you have any pics of the mods that you've already done that you'd care to post by chance?


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## Chops

Actually, the M-Lytic HV caps I'm using are the 100+100uF - 450V ones. 

 The 150+150uF caps you're talking about are the "snap-in" ones. 

 Eventually, I plan on moving the 708B's internals into a new all aluminum chassis. When I do this, I'll be getting two more of the 100+100uF - 450V M-Lytic HV caps along with a pair of the 47+47uF - 450V caps, which will give me a total of 494uF per channel. Heck, I might just go with another pair of the 100+100uF caps for a total of 600uF per channel. 

 As for the higher voltage caps, I have also heard that they have less distorsion of some kind as well, IIRC.


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## Stephen_Ri

Is there a way to decrease the gain on the 708B? I'm using it with an Entech 203.2 DAC which has a loud output (2.5V, I think) and Beyer DT-880s. If I turn the volume loud enough for it to "open up" properly both channels, it's louder than I prefer to listen. Lowering through software decreases bit depth, so that's not really an attractive option either. Any suggestions on the 708B gain or otherwise? I've heard of attenuators, but know nothing of implementation (and don't want to spend much money, if possible). Any response welcome, thanks.


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## dcheming

Quote:


  Originally Posted by *Stephen_Ri* /img/forum/go_quote.gif 
_Is there a way to decrease the gain on the 708B? I'm using it with an Entech 203.2 DAC which has a loud output (2.5V, I think) and Beyer DT-880s. If I turn the volume loud enough for it to "open up" properly both channels, it's louder than I prefer to listen. Lowering through software decreases bit depth, so that's not really an attractive option either. Any suggestions on the 708B gain or otherwise? I've heard of attenuators, but know nothing of implementation (and don't want to spend much money, if possible). Any response welcome, thanks._

 

Yeah the gain is pretty ridiculous, my normal listening volume is at only 8 o'clock. The easiest way to reduce the gain of the XS would be to build a simple potential divider at the input using some quality resistors, like I did with my T-Amp. The ideal way would be to find a tube that has similar electrical characteristics as the 6N3 but with lower gain. If such a tube were found I'm not sure if anything else in the circuit would need to be changed to accommodate it though.


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## Agnostic

I managed to get a 708B on Ebay for 120 US including shipping. Man, what a steal, it came in today and it makes my hd595 sing. Much better than the line out of my PC (which isn't bad) or the headphone out of my old Akai solid state. Even better than the portable Microshar uAmp107b that came in the same mail delivery (though I guess that needs some burn-in) 
 Much warmer sound and fuller, tighter bass. It also tames the sometimes slightly grainy mids of the HD595 very nicely. All this straight out of the box. 

 Now I'm waiting for some tubes for tube rolling: 70's NOS 6n3p on the way as well as a pair of SYLVANIA JAN-6DJ8. 

 Anyone found any other nice tubes except the ones already mentioned earlier in this thread?

 And which has more influence on the sound, replacing the 6n3, or the two 6DJ8?


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## dcheming

Nice find! I'm glad it works well with your 595's. Have any mods planned? 
	

	
	
		
		

		
		
	


	




 With my limited tube rolling experience in this amp I'd say the 6N3 tube has more overall influence on the sound.


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## Shinobi

Very nice I may have to see how this will pair up with some beyers.


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## Agnostic

No, not planning on any mods. I have no experience soldering whatsoever and I don't want to burn the house down or electrocute myself. 
	

	
	
		
		

		
		
	


	




 I'll stick with tube rolling for now. From what I read that's what makes the biggest difference in sound anyway, right?

 I have another 6n3 equivalent on the way, GE JAN 5670W.


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## Agnostic

GE JAN 5670W sounds good, a little more airy and open and also has a bit less gain than the chinese original (maybe a one hour difference on the volume dial), it's also more quiet, my 6n3 has a slight buzz that's no longer there with the GE.
 Going to compare those two a bit more extensively tomorrow. Only had a fast listen so far.


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## dcheming

Quote:


  Originally Posted by *Agnostic* /img/forum/go_quote.gif 
_GE JAN 5670W sounds good, a little more airy and open and also has a bit less gain than the chinese original (maybe a one hour difference on the volume dial), it's also more quiet, my 6n3 has a slight buzz that's no longer there with the GE.
 Going to compare those two a bit more extensively tomorrow. Only had a fast listen so far._

 

Nice. I need to get a replacement for my 6N3 too since it is starting to become slightly noisy. Any further impressions of the GE after a few days now?


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## Agnostic

I've done some more listening. Besides the points I mentioned the GE is less boomy than the 6N3, the lows are more controlled which allows more detail on the mids to come through. Highs are very nice, cymbals and hi-hat's sound great. 
 There's better decay on highs as well as on the mids. I was listening to Yasmin Levy, accompanied by classical guitar and you keep hearing the tones of the individual strings decaying clearly, even after a new chord has been played (admittedly it's very well recorded), but I noticed the same on cymbals and hi-hats: decay is sounding quite gorgeous.

 Anyway I found the tubes very cheap, I paid 10 us dollars for two on ebay, including shipping from USA to europe, so I'd say go and try them. 
	

	
	
		
		

		
		
	


	



 Seller: http://myworld.ebay.nl/tubetoons/
 He's not offering any now, but when I ordered he had more than 500 available so maybe you could just send him a message, he replied quite fast when I asked him for a shipping quote.


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## Agnostic

Found another interesting 6N3 replacement tube: Sylvania 5670. I got two and both had a bit of buzz when I got them (used) but cleaning the pins with some brasso took care of that on one, the other didn't stop buzzing after a cleanup; which is a pity because it sounded a bit better in the lower registers than the non-buzzing one. 

 This tube opens up the soundstage quite a bit and improves the definition; mids and highs sound clearer and more detailed. The only problem is that the one that is working without buzz is a just little bit light on lows with HD580, I'm guessing a NOS one should sound better than these used ones (hope I will manage to find one).

 With the HD580 this tube sound very nice for most types of music but for rock they might be a bit to bright and airy for some people, though that depends on taste as well I guess. Listening to Julie London - "You'd Be So Nice To Come Home To" with this tube and the HD580 gave me shivers down my spine though. 
	

	
	
		
		

		
		
	


	




 With the HD595 this tube is definitely better than the GE JAN. Soundstage on HD595 isn't that big and this tube opens it up just enough to clear up the sound and add some air and space without taking away the intimate up front sound of the HD595. Lows seem to be much less affected than on the HD580, I think because HD580 needs a bit more juice and has a wider soundstage.

 Now I'm just waiting for two NOS 6N3P-EV(EB) military stock to arrive from Russia. That will be the last one I try as a substitute for the 6N3 I think. If it's not better than the Sylvania 5670 I will try to find a NOS Sylvania.

 I tried replacing the 6N11 with two Sylvania JAN 6DJ8 but one channel was much louder than the other. Anyway they didn't seem to have a very big effect on the sound signature, not nearly as big as the 6N3 changes anyway (maybe because the used Sylvanias I got were not good or just sound similar to 6N11, anyone have an opinion on that?). Not sure if I will try to find another pair of something or other to try in that position yet. The 6N11's seem to be doing ok there.

 Wow, this became a much longer story than I intended.


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## flu1d

OK, I know nothing about amps but I need some help 
	

	
	
		
		

		
		
	


	




 I bought the 708b nearly a year ago and a few months later I started to get a low hum. I thought it was the tubes, so I replaced all of them and I still get the hum. The hum only comes in when the tubes are warmed up; however as soon as I cut the power off it goes away immediately. Also, it doesn't matter what the volume is set to, the hum is the same. Any ideas? Thanks.


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## sacd lover

Quote:


  Originally Posted by *flu1d* /img/forum/go_quote.gif 
_OK, I know nothing about amps but I need some help 
	

	
	
		
		

		
			





 I bought the 708b nearly a year ago and a few months later I started to get a low hum. I thought it was the tubes, so I replaced all of them and I still get the hum. The hum only comes in when the tubes are warmed up; however as soon as I cut the power off it goes away immediately. Also, it doesn't matter what the volume is set to, the hum is the same. Any ideas? Thanks._

 

Ground loop or RFI picked up from unshielded IC's?


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## Agnostic

Quote:


  Originally Posted by *flu1d* /img/forum/go_quote.gif 
_OK, I know nothing about amps but I need some help 
	

	
	
		
		

		
		
	


	




 I bought the 708b nearly a year ago and a few months later I started to get a low hum. I thought it was the tubes, so I replaced all of them and I still get the hum. The hum only comes in when the tubes are warmed up; however as soon as I cut the power off it goes away immediately. Also, it doesn't matter what the volume is set to, the hum is the same. Any ideas? Thanks._

 

Maybe the replacement tubes have a hum as well, I've had a few that did. Did the sound or volume of the hum change with the tube change? And is it in one channel or both?


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## dcheming

Quote:


  Originally Posted by *flu1d* /img/forum/go_quote.gif 
_OK, I know nothing about amps but I need some help 
	

	
	
		
		

		
		
	


	




 I bought the 708b nearly a year ago and a few months later I started to get a low hum. I thought it was the tubes, so I replaced all of them and I still get the hum. The hum only comes in when the tubes are warmed up; however as soon as I cut the power off it goes away immediately. Also, it doesn't matter what the volume is set to, the hum is the same. Any ideas? Thanks._

 

You might have a faulty ground solder joint on the signal wiring somewhere inside. This would be hard to check for if you don't have a multimeter since a visual inspection probably wouldn't revel the problem area. You could lightly wiggle the signal wiring inside at each place where it's soldered to something though. If the hum goes away momentarily when moving the wire around then you've found the bad joint. 

 Needless to say be careful of the high voltages when poking around inside. Using an insulated pair of needle nose pliers to grab the wire would be a lot safer than using your fingers. If you want I could send you some labeled pics of the specific areas to test. Just PM me if you're interested.


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## flu1d

The hum was the same when i replaced the tubes. The hum also outputs to both channels. (they are not my headphones either) I've tried wiggling the wires around too. I think I'll try replacing the signal wires with some higher quality shielded stuff. Any particular wiring I should use? Thanks for the replies!


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## rivieraranch

I saw a photo of the inside of one of these and there are some goofy wires on top of the tubes inside the unit. What are these? Are these tubes soldered in place?


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## dcheming

Quote:


  Originally Posted by *flu1d* /img/forum/go_quote.gif 
_I think I'll try replacing the signal wires with some higher quality shielded stuff._

 

I don't think that will solve your hum issue though. I run simple twisted pair signal wire in mine and I don't have any hum whatsoever. 


  Quote:


  Originally Posted by *rivieraranch* /img/forum/go_quote.gif 
_I saw a photo of the inside of one of these and there are some goofy wires on top of the tubes inside the unit. What are these? Are these tubes soldered in place?_

 

Those wires serve to retain the tubes in place so they don't fall out of the sockets while the amp is being shipped, which also answers your second question. You might be thinking of the XS model that's below the 708B, forgot the model number, that does have it's tubes soldered in place.


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## nc60865

hi, somebody please help. I got my XS amp 1 month ago, I have to say that it runs well, but I am freaking out by the effect that I didn't notice any difference of the sound with pluging the XS in. I am using A2zs as the source. Then, why do I need to buy it?


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## Agnostic

What headphones are you using it with and what format are your music files (cd, flac, mp3) and if mp3 what bitrate?


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## kpeezy

Ooh ooh. I just saw a picture of this amp for the first time and it would look awesome with my Zhaolu 
	

	
	
		
		

		
		
	


	




 Agnostic -- You said this amp pairs well with the HD580's right?


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## nc60865

Quote:


  Originally Posted by *Agnostic* /img/forum/go_quote.gif 
_What headphones are you using it with and what format are your music files (cd, flac, mp3) and if mp3 what bitrate?_

 

I am using DT990 250ohm version, and listening to ape file, barely notice any difference, so desperate!


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## Agnostic

Quote:


  Originally Posted by *kpeezy* /img/forum/go_quote.gif 
_Ooh ooh. I just saw a picture of this amp for the first time and it would look awesome with my Zhaolu 
	

	
	
		
		

		
		
	


	




 Agnostic -- You said this amp pairs well with the HD580's right?_

 

It does pair well with the HD580 but for the best sound you will have to change the Chinese 6N3 to either a NOS GE JAN 5670W (you can find those quite cheap on ebay) or even better imo a NOS Sylvania 5670.

  Quote:


 I am using DT990 250ohm version, and listening to ape file, barely notice any difference, so desperate! 
 

What were you amping the DT990-250 with before? I don't know the DT990-250 but I heard a clear difference the first time I listened to my HD595 through the Xiang Sheng. I hear an even bigger difference with the HD580, which is logical because the HD595 sounds quite good without an amp while the HD580 needs one to sound good imo. The difference got even bigger when I changed the 6N3 tube which has a nice warm sound but a bit boomy overwhelming bass, the GE JAN 5670w and the Sylvania 5670 both have a lot more air, a bigger soundstage, tighter bass, more detailed highs and really nice decay. To my ears the GE sounds a little bit drier and less engaging than the Sylvania, and the Sylvania has a bit more detail in the highs but a bit less bass.
 As I wrote you can get the GE JAN cheap on ebay, I think I paid ten dollars for two NOS. Maybe tube rolling will change the sound for you though I find it strange you didn't hear much difference to begin with. You say you heard something though, what was the difference that you did hear?


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## nc60865

thanks for your info,Agnostic. I didn't amp my 990-250 before, I just plugged it into the A2ZS directly, the high sounded a bit dry and fatigue. I don't know if that is the fact or not. I actually did roll in a pair of RCA EC88, still can't tell the difference between. To me just sounds the same, I don't know why. Perhaps what you guys indicated that those changes of soundstage, decay or watever are very slight, but it did change indeed?


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## Agnostic

I also tried changing the two 6N11 tubes but changing those didn't have much influence on the sound signature from what I heard.
 Maybe the DT990 is not very responsive to amping? I wouldn't call the changes I heard small exactly, but the change for the HD580 was much bigger than for the HD595.


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## nc60865

Seems that 990s don't have much responsive to this amp, so I don't have much to concern anymore, will play as it is since then on.


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## mapstec

I have one of these as well, won the ebay auction for an unbeatable good price... 
	

	
	
		
		

		
		
	


	




 As yet I have done nothing except change the tubes for NOS Russian ones.

 Apart from the too high gain I have to admit that I like it very much already in this form.

 I will order further tubes to play with soon, also most of the tweaks / modifications discussed here and in the other thread are planned at some stage in the future.

 So far I am really glad I got it and want to thank the Forum again for pointing me towards such nice / inexpensive / good gear.


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## zanash

Well very interesting ....

 I had my 703b for about a year but was not really that taken with the sound so the unit just sat on one of my shelves.....I run the dt990 dt911's...

 early on I swapped the nasty vol pot for a 50k alps blue which sounded great over the mains hum! 

 but the other week I was asked about the 703b ...which then peaked my interest. I found a pair of matched 6922 spares for my opera cyber800's..... so dropped those in ..wow the chinese valves supplied must be rather poor... I then tried to find a 6n3 !

 In the end I bought 10 of fleabay from albainia or somewhere...I was not expecting much but when they arrived in less than a week I dropped one in .....

 I still suffer from ac hum but where that was intruding on the music at anything up 3/4 volume this has been reduced to a small ammount on the right channel.

 So i've now got a headphone amp thats actually nice to listen to !

 With that in mind and reading this thread I had a sort through my spares box and found a 100k stepped attenuator [dale vishay resistors] ....I had to increase the size of the hole again .... but once in and fitted using the original cables for the time being ...the sound has opened up considerably ....

 it always suprises me how much the vol pots actually take away from the music signal. I then fitted a new knob turned alu rather than the nasty plastic one that was fitted from new.

 So rather than wade through ....any quick and simple ac hum fixes ?


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## dcheming

Quote:


  Originally Posted by *zanash* /img/forum/go_quote.gif 
_early on I swapped the nasty vol pot for a 50k alps blue which sounded great over the mains hum! 
 ...
 With that in mind and reading this thread I had a sort through my spares box and found a 100k stepped attenuator [dale vishay resistors] ....I had to increase the size of the hole again .... but once in and fitted using the original cables for the time being ...the sound has opened up considerably ....
 ...
 So rather than wade through ....any quick and simple ac hum fixes ?_

 

The stock volume pot is where the chassis is tied to signal ground. Since you've replaced it you'll need to reestablish that ground connection and doing so should get rid of the hum.


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## zanash

Actually the replacment has dropped the hum from intrusive to only heard between track and only on the right channel... if that makes any sense. But yes will get my test bits out and give it a careful prod .....I'll implament the chasis earth too which seem like a good idea....

 do you recomend floating the signal grounds to the earth ?


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## mojolo

Quote:


  Originally Posted by *nc60865* /img/forum/go_quote.gif 
_I am using DT990 250ohm version, and listening to ape file, barely notice any difference, so desperate!_

 

may be your soundcard. Augidy is really not the greatest as it is resampling everything to 48Khz. the quality of the HW resampling is not very good.

 my XS is being fed by a Bluegears B-enspirer card. i've rolled in Amperex 6DJ8 Orange Globe Halo Getters as well as a late 80's GE Jan 5670W (1960 Sylvania JAN 2c51 on the way).

 i definitely notice a differences compared to my solid state amp. while the XS is not as clean/tight, I find it to be more enjoyable/musical and open. this is my first tube amp though, so it may just be that i like the tube sound.

 with the stock Chinese tubes, i found everything harsh. even after rolling in the new tubes, things were a bit harsh and bass got quite distorted if turned up too loud. After about 20 hrs usage, i find the sound to be smoothing out for the most part.

 all of this is with a pair of Denon D2000's.


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## lordmozilla

hey,

 just got my amp, and am starting to mod it, alps blue velvet is on plus some kiwame resistors replacing the stock ones. However i am trying to get a 0.22uf cap, and I am having trouble finding one that i like. I see you mentioned the mundorf ZN caps, but i can only find them in 250V dc, anywhere i can get them at higher voltage? or would you put two of them together? I know where i can get some auricaps but if you think that ZN caps would be better. What about stock mundorf caps? there are some 450V parts in 0.22uf that seem to fit perfectly.

 ALso is it worth replacing the electrolytic caps? and what would you recommend replacing them with?

 I have the 240V uk version and the plug they ship was damn right cheap, replaced it with an EMI shileded plug and replaced the socket by an anti-emi filter socket :
Audiophonics - Filtre secteur IEC anti-parasites-EMI 230V 10A

 Also i have taken the decorative tube out mostly for heat removal, and i want to rotate it so the glass panel can show a usefull tube.

 Anyways it'd be great if you could get more some capacitor recomendations.

 Cheers,
 brendan


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## Senad

Could somebody tell if it is possible to remount the original knob on alps blue velvet with some tweaking (I am aware that it doesn't fit)?


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## lordmozilla

like i said yes, either drill the knob or get a screwdriver and enlarge it. read my post


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## Radioman1963

Hello everyone Iam a newbie.
 Just Bought a 708B secondhand.
 It has got some hiss coming from the left channel
 i think it is a dodgy Valve
 Looking forward to doing some of the mods
 Starting with the pot and input output resistors
 Best Wishes Martin
 Thanks for putting the mods up on here


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