# A lo-cost HT-supply true transformer-coupled tube headphone amplifier



## wakibaki

I designed this:-
   
   

   
  Bits came yesterday. This is just the trial fit for the parts.
   
  It says lo-cost in the thread title, but that's comparative. The parts cost is at least twice what you can build a very good solid-state amplifier for, $220 - $250 including chassis.
   
  I haven't finalised the design, although I've got a pretty good idea where I'm going with it. When I do I'll have a more accurate costing.
   
  Here's an early simulation, although I've changed a few things since then...
   
   

   
  That's the nice thing about simulations, you get the opportunity to do revisions and tweak the performance. I've since decided that maybe I'll do without the global negative feedback, I can get a milliwatt into 16 ohms (110dB SPL in my Klipsch Image S3's) with about 0.2% THD (almost all benign 2nd. harmonic) with no GNFB. The amplifier is intended to offer about 300mW and drive > 166mA or +/- 9V, although with increased distortion. Exactly what can be done in terms of power and distortion is yet to be determined. I don't think anybody will be arguing that RMAA results are insufficiently accurate though.
   
  Nominal output impedance is switchable, 4-32 ohms.
   
  Obviously it's single-ended, class A 
   
  As drawn the tubes are Russian 6N6P, $7 the pair. These are said to sound good when you get them over 8mA quiescent, both stages run 10mA. 6SN7's, which are widely held to have low distortion, could be substituted. 
   
  The output transformers are Hammond general-purpose 125SEB, rated 5W and 45mA DC bias, 100Hz - 15kHz, 1dB. They're considerably underrun at the power and DC bias stated, and the frequency response should be considerably better than nominal, they should easily achieve 20kHz and will probably extend down to 20Hz with 1dB flatness.
   
  The PSU uses a silicon bridge rectifier to achieve ~280V with a capacitor-input CLC filter, I originally drew in active regulation, but the choke makes for a much simpler implementation. Tube rectification is an option at the cost of some HT voltage, but for the moment I've got a relay drawn in to delay the appearance of HT.
   
  Unfortunately I incorrectly dimensioned the choke in my haste so a more suitable one won't get here until about Wednesday, along with some other components which I'm still missing.
   
  w


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## estreeter

Subscribed.


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## Oberst Oswald

Thanks for your design and making the DIY forum interesting.  Looking forward to see this being built with your goals in mind and also with "spare no expense" upgrades which hopefully will follow by people who like to improve things.  I would like to build one along with the puppy DAC this winter.  What name are you going to call it?  Challenger, Wiki HP1000 or...


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## colonelkernel8

Fantastic work. I love your design process, objective and incremental improvement via simulation and testing. This is real engineering folks.


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## FrankCooter

Always good to see a new tube amp design, particularly one that attempts performance, simplicity, and low cost all in the same package. Not enough of this type of project around here.
   
  Having built a number of similar projects here's my $.02:
   
  1) I'd change out the 6N6P for a 6EW7. The first section of the 6EW7 is similar to a 6SN7, The second is a power section with a u of about 5 and an rp of about 1k. This will be a much better match with your transformer primary than a 6N6P. There will still be room for a few dB of global feedback, which will probably help your output transformers.
   
  2) Substitute an LED for the cathode resistor/bypass cap on the first stage. Think about a CCS as the plate load. With the IXYS10M90 high voltage current regulators, a CCS becomes trivial.
   
  3) I like the CCS in the cathode of the second stage. Set the CCS to elevate the second stage cathode to the point where you can direct couple the second stage grid to the plate of your first stage, and thus eliminate the coupling cap, which is always a weak point in any amplifier.
   
  4) While your design models well and your physical layout looks nice and tight, my guess is that that your magnetic components will interact with each other and you'll have hum problems. Unfortunately the only cure is often physical separation of components.  Sometimes a choke actually creates the problem. You may want to return to the idea of some sort of simple regulation.
   
  5)  Good luck and keep us posted!


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## wakibaki

OK, thanks for the support and expressions of interest, particularly FrankCooter for the detailed and useful advice, I'm a bit out of touch with tube designs, and I'll take _all_ that aboard. I'll probably proceed with the design as I have mooted it for the moment, but it _is_ my intention to iterate it at least once. I've already had some doubts about the room available with this chassis. I think I'll put the choke on the outside of the end plate at right angles to the field of the toroid. It's not the best aesthetically, but there's the aesthetics of hum to consider. The 6EW7 looks good, do you know of a spice model? I've found a model rated 'very good' for the 6EM7.
   
  I'm still mulling over what to call it.
   
  Parts have been shipped and I should have everything necessary to complete a working amplifier tomorrow or Wednesday.
   
  w


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## wakibaki

After dozens of simulations, this is what we're going with for the first try:-
   

   
  This should live with 2V in without clipping. The transformer model is basic, I get 0.15% THD with 1mW into 16 ohms across the 4 ohm output, ~2.5% THD with 9V across 600 ohms for 2V in using the 32 ohm tap, _with this limited accuracy simulation_.
   
  w


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## elliottstudio

Since you're not using a tube rectifier you can use a very large cap after the SS bridge and lower the value of your choke to around 1H as most of the ripple will be taken care of by the big-ass cap, thereby eliminating any magnetic coupling of hum from the choke to surrounding circuitry.. The choke will do a nice job of filtering out all the HF nasties from the SS bridge and help to sweeten up the sound. The following cap can be a smaller value high quality polypropylene (not exotic, just run of the mill) and you will get very pure DC out of this filter. Then you can use a simple regulator after this filter if you wish.
   
  I can't read your schematic, it won't go big enough so the digits are not pixelated on my small screen so I have no idea what values you're using for your caps and chokes but this is a method that I've used and it works.


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## dsavitsk

frankcooter said:


> 1)
> 2)
> 3)
> 4)




Everything Frank said is absolutely correct. Listen to him.




wakibaki said:


> I get 0.15% THD with 1mW into 16 ohms across the 4 ohm output, ~2.5% THD with 9V across 600 ohms for 2V in using the 32 ohm tap




That's a little high, but the important thing is the shape of the distortion, not the aggregate. So long as harmonics are decreasing, it should sound fine.




elliottstudio said:


> Since you're not using a tube rectifier you can use a very large cap after the SS bridge and lower the value of your choke to around 1H .... The following cap can be a smaller value ...




This would be the recipe for making the power supply ring. PS caps should be in ascending size, not descending.


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## elliottstudio

Yes, but if followed with a regulator or a large bank of caps located right at the supply of each plate you're covered. I was thinking in terms of the power supply feeding into a circuit with adequate bypassing at each plate. I forgot to mention that.


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## vixr

wow...this is awesome. Thank you guys for doing this out in the open.


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## wakibaki

You can find a .jpg generated from a 600 dpi .bmp here:- http://dl.dropbox.com/u/107775480/challenge_choke_1.jpg
   
  It's slightly different from the schematic shown, but I'm always thinking of improvements, get rid of a component here or there...
   
  w


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## nikongod

What does the Relay do? B+ delay? Its completely unnecessary in this circuit. There is no shunt regulator waiting to explode. No truly unlimited current tubes that will get screwed up if B+ is applied before the heaters are hot. Will it send an annoying "pop" through the headphones when it turns on? Save some space, money, and a headache. 
   
  Why do you spend so much on the heater supply when you put so little into B+? 
  So I know the choke costs some serious money in a budget build, and it is a VERY nice addition, but it wont be enough by its self. Put the regulators on B+. Even if they are just series pass transistors with zener references - or the more complicated version in your original schematic from the other thread.
  Use AC heaters biased above ground. Noise levels will be very significantly lower, cost should be close enough to the same once all things are considered.  
   
  (edit - added next morning) 
  AC heaters does not account for hopelessly microphonic Russian tubes. It may be prudent to stay with DC heaters - CRC should be quite adequate though. 
  (end edit)
   
  Nit pickery: the various connection points in the schematic dont all line up. The transformer has AC2 on BOTH sides of the HV secondary - the relay shows a connection to AC1. The heaters for U1 go from one V-reg to the other (LT1 to LT2) - not to ground (LT1 - LT3). 
   
  Quote: 





elliottstudio said:


> Yes, but if followed with a regulator or a large bank of caps located right at the supply of each plate you're covered.


 
   
  If followed by a large bank of caps what you said a few posts up of "choke followed by a smaller cap" is nothing like what you are describing now. 
   
  If followed by a regulator:
  Do regulators work better with cleaner supplies or "dirtier"? Cleaner of course. Why subject the regulator to the nasty HF of a ringing power supply when you could feed it a simple LF ripple with very little HF to speak of for basically the same cost... 
   
  The big cap straight off of the rectifier is still not a great idea with SS diodes. Its always nice to keep the peak currents low. 20uf is more than enough for all but the scary-largest of tube amps, but nobody believes it. Since this is a budget build I think it will be a nice place to save some money. If it were a "money is no object" amp I would use the same size part, just better quality.


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## elliottstudio

Quote 





> If followed by a large bank of caps what you said a few posts up of "choke followed by a smaller cap" is nothing like what you are describing now.


 
  Yes, you're right. I need to say what I mean and mean what I say.


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## wakibaki

Quote: 





nikongod said:


> What does the Relay do? B+ delay? Its completely unnecessary in this circuit. There is no shunt regulator waiting to explode. No truly unlimited current tubes that will get screwed up if B+ is applied before the heaters are hot. Will it send an annoying "pop" through the headphones when it turns on? Save some space, money, and a headache.
> 
> (edit - added next morning)
> AC heaters does not account for hopelessly microphonic Russian tubes. It may be prudent to stay with DC heaters - CRC should be quite adequate though.
> ...


 
   
  OK, you sure about that with the relay? I got the impression from the Morgan Jones book that a B+ delay was a good idea with a silicon PSU due to cathode stripping, but I can do with the simplification. It was easier not to take the risk. I wasn't joking when I said I haven't done this for 40 years.
   
  The tubes are what I had. I'll look at different tubes when I see some progress. I need to lose 3+V though, 1.5A means a 10W resistor I haven't got lying around.
   
  As regards the nit-pickery, I've redrawn this circuit a lot, the wire labels keep deleting themselves when I redraw the wires. I'm not dependent on the schematic, I'm not making a PCB. I wouldn't want anybody else trying to follow it though, so I'll post alterations soon.
   
  I thought single-ended had poor PSRR, so I've got a lot of capacitance in there, I wish I'd known, these caps are huge and it's crowded already.
   
  I haven't done a lot on it. I've picked up a chest infection. I had surgery and chemo for lung cancer a couple years back and I had radio iodine for hyperthyroidism about 6 weeks ago, I don't throw things off like I used to and I feel like sh1t so I've taken some prescription codein and settled down to wait it out.
   
  Thanks for the help nikongod.
   
   

   

   

   

   
  w


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## dsavitsk

wakibaki said:


> OK, you sure about that with the relay? I got the impression from the Morgan Jones book that a B+ delay was a good idea with a silicon PSU due to cathode stripping,




Only an issue if you have rare expensive tubes, and a very high voltage supply.



wakibaki said:


> I thought single-ended had poor PSRR, so I've got a lot of capacitance in there, I wish I'd known, these caps are huge and it's crowded already.




Terrible. The best things for keeping a SE tube amp quiet are space between magnetic parts, and a regulator. Try using a MOSFET as a pass transistor, and put an RC circuit on the gate to provide a slow start if you are concerned about the above.


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## elliottstudio

Waki,
  I hope you feel better soon.


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## wakibaki

Thanks, I'm already pretty much recovered.
   
  A question for all you tube enthusiasts. I see quite a few designs out there like the Millet variations, most of which are inverting, e.g. a common cathode stage (inverting), buffered by a mosfet source follower (non-inverting), which adds up to an inverting amplifier. Nobody seems to care. I searched one of the Millet threads for the word 'inverting' and it wasn't mentioned once. Phase inversion is a big issue for many audiophiles though, and it is increasingly considered a no-no. It's easy to avoid in solid state amps, particularly opamp-based ones and discrete amps mostly have the option of applying positive feedback to the inverting input of a LTP.
   
  I have the option of building this amp as inverting or non-inverting depending on how I connect the transformer, it's just a question of which wire to connect to B+ and which to connect to the OP tube plate, but it makes a big difference when considering any global NFB. I can apply NFB from an overall non-inverting amplifier to the cathode in the input stage, but I think this is a bit ugly, but it means that the amplifier overall  can be built non-inverting.
   
  What do you think? Obviously I can switch it around later, but I'm close to finishing the first iteration and it affects the neatness of the wiring. Once cut, I'll have to patch in a bit of wire if I want to change it later.
   
  w


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## elliottstudio

You have two stages - that's non-inverting unless I'm missing something...
	

	
	
		
		

		
			





 I say build it as you have it drawn, as non-inverting and deal with the NFB, if you apply it, as you must.


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## qusp

I think its a red herring, ask the engineer if he used an inverting or non-inverting signal path when he produced the album …


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## wakibaki

Here's the sim:-
   
   

   
  Green is Vout, blue is Vin and red is the grid of U3, the output stage. Now look at the phase dots on the OPT and how I've got it connected. I've had to take B+ to the bottom of the transformer to get it to invert, it's naturally a non-inverting stage, which with the first stage being inverting would make the whole amplifier invert.
   
  I first noticed this when I connected up the feedback and the gain went UP.
   
  Or perhaps I've got something completely bassackwards?
   
  w


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## wakibaki

Quote: 





qusp said:


> I think its a red herring


 
   
  I'm inclined to agree qusp, but I note from Doug Self's Small Signal design book that it's the practise in professional equipment such as mixing desks to preserve polarity. I always make the effort to do so now in my solid state designs, it rarely costs anything other than paying attention, but now that I _have_ started paying attention to it, it feels a bit odd to deliberately go against the grain, which is what I'm tempted to do here, because I'd rather feed back an inverted output into the high impedance non-inverting input instead of taking a non-inverted output back into the comparatively low impedance at the cathode, to say nothing of the potential complications of providing a second DC path (and magnetising current) to ground via the transformer unless it's completely AC coupled.
   
  w


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## elliottstudio

It is common practice in power amps to take the feedback from the output winding to the cathode of the input stage (or appropriate stage) through a divider, assuming you intend to take the feedback from the secondary of the OPT. Your headphone amp is essentially a power amplifier, though very low power. As for voltage at the cathode across the secondary causing a magnetizing current; the secondary winding is so small it's DC resistance will effectively shunt it to ground. The feedback divider at the cathode will isolate it from this shunting resistance and greatly limit the current that could actually shunt through the secondary so it should have little to no effect on the bias setting.


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## wakibaki

So do I take the B+ to the blue as in the simulation, or to the brown as in the schematic to yield an inverting stage, and hence a non-inverting amplifier overall?
   
  I've searched all over the place, but I can't find a definitive answer as to whether a transformer-coupled common cathode amplifier is normally inverting, although I suppose a sensible answer to the question would be, 'it depends which way you connect it up'. I would expect that if the phase dots are at the same ends of the transformer (brown connected to B+), that as the voltage at the grid rises, the current in the primary would rise and this would result in a rising current in the transformer secondary (non-inverting stage with black connected to ground) but as per the simulation, which I'm inclined to trust, the phase dots must be at different ends of the transformer (blue connected to B+) to get an inverting stage.  It's when the stage is resistively loaded that the rising current results in a greater voltage drop across the load (falling output voltage at the anode-resistor junction) and the stage is inverting.
   

   
  w


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## elliottstudio

There is no phase reversal across a transformer if hooked up in phase. Your amp is two stage so it is non inverting and will still be so on the secondary. If you want to apply NFB you can go to the cathode of the first tube from the secondary winding, as you know. Since you have to have one side of the secondary grounded for the feedback to work you can't reverse the phase on the primary or you will be in 'buck' mode and lose signal (this being an SE amp). If the output was not providing NFB you could float the output and hook either the primary or the secondary with a phase reversal.
   
  At the power level you're going for global NFB is probably not necessary, and taking it from the transformer secondary introduces phase shifts and other nasties. You may consider looping from the plate of the second stage to the cathode of the first stage. Yes, you would need AC coupling but the series resistor at the cathode of the first tube would necessarily be large enough that the coupling cap could be a small value film cap. Or you could run with no loop feedback and adjust your gain structure accordingly; you may want to reduce the drive voltage from the first stage plate to second stage grid with a divider for best linearity.
   
  The simulation is confusing. Leave the primary leads long enough so you can build it either way and prove it out with the secondary floating. I _hope_ I'm answering your questions.. I'm sure others will chime in.


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## wakibaki

Quote: 





wakibaki said:


> I've got a lot of capacitance in there, I wish I'd known, these caps are huge and it's crowded already.


 
  Quote: 





dsavitsk said:


> Terrible. The best things for keeping a SE tube amp quiet are space between magnetic parts, and a regulator.


 
   
  Yes, but too small a cap before the choke results in a reduced HT supply. This is a common ploy for reducing B+ without wasting power across a resistor. The old 'small capacitor ploy'. I like a nice small capacitor ploy, but not when it results in a reduced Pink Panther, nicht wahr?
   
  w


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## elliottstudio

Quote: 





wakibaki said:


> Yes, but too small a cap before the choke results in a reduced HT supply. This is a common ploy for reducing B+ without wasting power across a resistor. The old 'small capacitor ploy'. I like a nice small capacitor ploy, but not when it results in a reduced Pink Panther, nicht wahr?
> 
> w


 

 I too have used the small capacitor ploy in the past but it never reduced my Pink Panther 
	

	
	
		
		

		
		
	


	




- Now I just have custom transformers wound so I end up with the correct B+ in the first place! And in the case of my preamps regulators are used.


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## wakibaki

Had to order some TO220 insulator kits, I'm waiting for them...
   
  w


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## elliottstudio




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## wakibaki

Sorry, I went downhill again a bit with the chest infection, but should be finished tonight~tomorrow. Had a routine follow-up chest x-ray a couple of days ago tho', still in the clear with the big C. 
	

	
	
		
		

		
		
	


	



   
  w


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## Avro_Arrow

Glad to hear your feeling better...
   
  Quote: 





wakibaki said:


> Sorry, I went downhill again a bit with the chest infection, but should be finished tonight~tomorrow. Had a routine follow-up chest x-ray a couple of days ago tho', still in the clear with the big C.
> 
> 
> 
> ...


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## estreeter

Its interesting that this effort has silenced many of the naysayers - way to go, waki. Keep the faith.


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## wakibaki

OK, I finished building it. Now I have to work up the nerve to turn it on.
   
  It's 1.30 A.M. I'm going to leave it now until I'm fresh.
   
  w


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## tomb

Quote: 





estreeter said:


> Its interesting that this effort has silenced many of the naysayers - way to go, waki. Keep the faith.


 
  As long as he foregoes with the challenges, dares, and bets on how to run a business and determine costs and price, and instead - actually build something, then yes.  This is a DIY forum and we're going to watch, help if necessary, and see if he's really come up with something.
   
  Not sure I understand why those differences are hard to comprehend.


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## estreeter

Quote: 





tomb said:


> As long as he foregoes with the challenges, dares, and bets on how to run a business and determine costs and price, and instead - actually build something, then yes.  This is a DIY forum and we're going to watch, help if necessary, and see if he's really come up with something.
> 
> Not sure I understand why those differences are hard to comprehend.


 
   
  Point taken, but there was a point where *anything* he typed was seen as a challenge - I'll leave it there.


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## wakibaki

Avro_Arrow, thanks for the good wishes.
   
  estreeter, thanks for your continued support and positive attitude.
   
  Just sorting out some final issues with the safety ground. Trying to find a suitable star washer to bite into the alu. chassis in my junk. 
   
  w


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## wakibaki

Only one tube lit. Swapped them. LT is OK so it's the tube. Have to dig out another one.
   
  w


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## Misterrogers




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## estreeter

Quote: 





wakibaki said:


> Only one tube lit. Swapped them. LT is OK so it's the tube. Have to dig out another one.
> 
> w


 
   
  Fantastic stuff - I would consider any such effort where I didnt electrocute myself to be a win, but clearly you've done this sort of thing before.


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## elliottstudio

Hope you feel better soon, be careful not to breath those solder fumes!


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## KimLaroux

Interesting discussions. Sad to hear you're going trough those health problems, and wishing you feel better soon. 
   
  I'll stick around to follow your progress, as those 3 first pages are promising. I think I'll learn a bunch of stuff here.


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## Armaegis

Quote: 





estreeter said:


> Fantastic stuff - I would consider any such effort where I didnt electrocute myself to be a win, but clearly you've done this sort of thing before.


 
   
  Aw but if you do it enough times, you can do the lightbulb trick like Uncle Fester


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## wakibaki

I remembered why I like solid state so much, I'm terrified of this bloody thing. It's a bit of deja vu, the last time I felt like this was learning to hover a large model heli. You start out thinking 'a nice harmless little hobby' and you end up thinking 'why am I standing 10 feet from a barely-controlled flying chainsaw?' Tube amplifier design - the original extreme sport.
   
  Thanks, Kim, elliottstudio.
   
  Armaegis, are you clairvoyant? How did you know that I look like Uncle Fester?
   
  Everybody, please bear with me, I have to wait a month between blood tests and they won't adjust my thyroxine dose until they see the levels, right at the moment I feel too exhausted to do very much at one go, just a couple of weeks ago I was hyper. Still, at least it's not life threatening, unlike this amplifier.
   
  -------------------------------------------------------------------------
   
  I noticed that the first time I swapped the tubes, the lit one seemed to move. I know though, that tubes don't always show much light, although these are of identical manufacture. I checked all the tubes with an independent LV lab. supply with 1A out. They all showed a visible glow. 'Course this isn't a trick that's open to everybody, but when you're debugging something you have to use any little clue that you can get. It only occurred to me that I could do this while walking the dogs, it's best in these circumstances though not to rush at things and give yourself a chance for ideas to come to you.
   
  Originally I drew this:-
   

   
  ...but I actually built this...
   
   

   
  ...which seemed a reasonable way to save 2 diodes and a slight simplification. Now I'm not so sure. I don't see any theoretical reason why it should be a problem, but I'm fairly sure that only one random LT supply at a time is working properly, so I'm going to revert to the first arrangement. Since it's all built point-to-point, it's a bit of a fiddle though.
   
  Here's what I had to do to get enough room with this little chassis.
   

   
  I'd have liked to keep the choke on the inside, but it simply wasn't possible given the other parts that needed to be fitted, and apart from slightly spoiling the simple appearance, it's actually better in electrical terms, it's further away from the OPTs and particularly the PT.
   

   
  At least I didn't have to drill any more holes, I was able to use the existing ones.
   

   
  The grounding system is dual star. The noisy star is located at the junction of the 220u caps. This was prewired. Wherever possible soldered joints are shrink-wrapped. Power grounds such as the CCSs are returned to the noisy star. I'm not too keen on earth-follows-signal.
   
  Components are cable-tied to self-adhesive tie-mounts, or glued with a hot glue gun. The HT bridge rectifier is bolted to the chassis, as are the LT regulators. The 10k anode load resistors are 10W, they only dissipate 1W, but a close watch will be kept on them during the first few hours of running. The classic style of turret-board + right-angle dressing simply wasn't achievable in this space.
   
  The quiet star is located at the output jack. Signal grounds are returned to the quiet star. A single wire joins the stars.
   
  w


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## nikongod

Please tell me I dont see hot glue. 
   
  Are you getting the correct voltages at the output of the heater regulators?


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## FrankCooter

Why not use a single regulator for both tubes? Or better yet, get rid of the regulation entirely. It's really not needed here.


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## nikongod

So, how does it sound?


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## wakibaki

Sorry I had to take a break from working on this, I had some problems with the LT supply. There have been some other issues with access to tools and things 'cause of other work going on, I just stopped enjoying what I was doing since I felt it was kind of like work, but I wasn't getting paid for it. This _is_ supposed to be recreational for me.
   
  I'm taking a few days to update my website, which is another of my creative outlets, and which I have neglected of late.
   
  I thought you guys were supposed to be looking over my shoulder.
   
  In order to get ~8.3Vpk from a 6.3V winding, that's 2 Shottky drops taken into consideration, cause the peak would otherwise be ~8.9V, you need to think about the ripple. In order to get the ripple down under a volt you need a 22,000uF cap, not a 2,200uF. So I had it assembled with a 2200 cap embedded in some glue and in my impatience to get it out, I damaged one of the other caps, and you know how it goes from there, you can just get caught up in a cascade of increasing problems and damaged insulation. Stop.
   
  Then I decided to build a low-dropout regulator to pass 1.5A out of components I had in hand, wait for the 2200uF cap to arrive and try and do something else constructive.
   
  This seemed to simulate all right:-
   
   

   
  The cap arrived, so I made a PCB, but there was a problem with it and I got bogged down when it wouldn't regulate off the LAB supply...
   
  So a little vacation from this one is required, and I'll get back to you when I've got myself back into the right head-space.
   
  w


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## nikongod

Glad to hear you are doing well.
   
  Why not just use a simple CRC heater supply?


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## elliottstudio

In my experience with regulators for high current draw such as tube filaments, you need more than a 2V difference between V-in and V-out. The current draw will drop a few volts through the filter and secondary resistance even without the regulator. It sounds like your running out of available voltage and your regulators are dropping out. Been there, done that!
   
  If you have another 6.3V winding you could series them for an unloaded voltage of around 15V and do a 12V filament circuit (series the two filaments). Half the current draw, less voltage drop, less ripple and less heat dissipated in your regulator circuit.
   
  OR just hook the filaments up AC. Chances are your gain structure is such that you won't notice any hum if you run the (tightly twisted) wires carefully, and either a center tap on the 6.3V winding to ground or a virtual ground node via a resistor divider across the winding. I built a mic preamp with AC heaters, a high-gain circuit to be sure, and didn't have problems with hum. Microphonic 6922's yes, but no hum.
   
  Good Luck!


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## nikongod

Quote: 





elliottstudio said:


> I built a mic preamp with AC heaters, a high-gain circuit to be sure, and didn't have problems with hum. Microphonic 6922's yes, but no hum.


 
   
  This was my fear with AC heaters - I have also had AC heaters run dead quiet with good tubes but "set off" microphonic tubes. I would personally say its a good reason to put the tube in a coffee can and crush it up with a brick, but in a commercial product the customer is always right and you built something that hums with the best tube (lolerskates) he owns...
   
  Since this amp uses Russian tubes which I have found to be a bit more microphonic than their American/European equivalents I would be careful here. 
   
  As a different thought - would running the heater regulators in CCS mode instead of V-reg mode get them to light consistently? I still think this is crazy overkill considering to the lack of B+ regulation, but it may be worth a shot.


----------



## elliottstudio

I would think that using a current regulator instead of a voltage regulator would work, I've seen a few articles on this before somewhere but never tried it. It might need more initial voltage than 8 point whatever volts to get it to work.
   
  When I was first experimenting with this stuff years ago I had quite a frustrating time trying to design a DC filament supply that was simple, low ripple and at the correct voltage. It's not easy given the current draw, so I understand the use of a regulator for filaments. But a 6.3VAC winding won't get you 6.3VDC at the currents were discussing.
   
  Waki, what are your plans now?


----------



## dsavitsk

http://www.mouser.com/ProductDetail/NJR/NJM2396F63/?qs=sGAEpiMZZMsGz1a6aV8DcKRceEqesA2aN2QaT9RrkVo%3d


----------



## elliottstudio

Who need to worry when we have NJR to the rescue! I wasn't aware of this, it may work for him.


----------



## nikongod

Its good to know you can rely on dsavitsk to come up with the perfect part for the job.


----------



## tomb

Quote: 





nikongod said:


> Its good to know you can rely on dsavitsk to come up with the perfect part for the job.


 
  Yep.  We've been using that regulator in the Torpedo.


----------



## nikongod

Any news on this build?


----------



## estreeter

Quote: 





nikongod said:


> Any news on this build?


 
   
  +1
   
  Its quiet - too quiet .....


----------



## elliottstudio

Yes, Wakibaki~
  We can't help you if you're struggling when you're out of sight. I hope everything is OK with you..


----------



## wdahm519

I'm very interested in this and will be following it. I always love me a good tube headphone amp. I've never designed an amp before, so reading all this is very educational!


----------



## estreeter

elliottstudio is right, of course - the OP's health woes are of more concern than his progress on the amp.


----------



## elliottstudio

This is a really long movie with no plot...  I hope you're OK Waki.


----------



## estreeter

I think its pretty clear that he isnt OK. Over 3 weeks and nothing.


----------



## wakibaki

Sorry for the long delay folks, but I have not felt very well for the past few weeks. Immediately following treatment with radioactive iodine I was put on a high dose of beta-blocker. My thyroxine levels then fell very low and the combined effect was that I started to feel very tired all the time, having barely enugh energy to walk the dogs once a day. I recently had a consultation following blood tests and I was recommended to ramp up the amount of thyroxine I am taking from 25 micrograms to 125 micrograms, a five-fold increase, so you can see that the amount I was taking was way off. Half-way through this process I already feel much better. I hope to be taken off the beta-blocker soon too, which hopefully will cause me to return to my usual waspish energetic self.
   
  Seriously though, the overactive thyroid which was the initial cause of these problems is known to result in sufferers becoming very short-tempered, so if I have been rude to anybody in the recent past, I do apologise. I should be a bit less inclined to jump down people's throats in future.
   
  ******************************************************************************** 
   
  The good news is that the amp has a clean, satisfactory sound with no hum or perceptible hiss with plenty of gain when turned up full. Perhaps a little light on bass, but I haven't had a chance to evaluate it fully.
   
  There is, unfortunately a fault, a regular ticking on both channels, which is very noticeable with no program material. I will have to get to the bottom of what the cause of this is. It's about at 1Hz, or perhaps a bit quicker, not very loud, it sounds like the right frequency for a relaxation oscillator, something charging to a potential and undergoing discharge or something like that. It's completely masked when the volume is turned up full, but there is a bit too much volume with the phones (cheap) and source I chose for the first trial to listen continuously at full volume.
   
  The amp does work though, and not that differently from what I predicted.
   
  I have taken Frank Cooter's advice regarding DC coupling, and I have worked up a simulation with an active (CCS) anode load. This should perform slightly better than the amp in it's current configuration. You can see the circuit and results here. Treble is somewhat constrained, but bass is only a couple of db down @ 20 Hz, although these simulations are done with a fairly primitive transformer model, and while useful for development purposes, have to be taken with a pinch of salt.
   

   

   

  This shows the amp just below clipping, all harmonics above the second disappear to all intents and purposes when the drive level is reduced.
   
  All the best to everybody...
   
  w


----------



## FrankCooter

Great to hear you're back and that your health is improving! We were beginning to wonder what happened. Hopefully your improvement is permanent and there'll be more amps in the future. There are very few of us DIY tube builders here and last thing we want is to lose one!
   
  It's been a pleasure watching the evolution of your design and I'm happy to have played a small part in the process. Happy to hear it works.
   
  The "clicking" you're hearing sounds almost like a decoupling problem, although this shouldn't be an issue with a two stage design. Can't hurt to try something like a 10uf electrolytic in the C9 position. I'd also be looking for a bad solder joint in the power supply or grounds.


----------



## elliottstudio

Hey, there you are! It's great to see you back in action.
   
  I once had a 'tick, tick, tick' in a circuit that turned out to be a hairline crack in a power trace on a PCB that kept arcing across. Discovered it with the lights out! You may have an arc in you HT circuit somewhere, turn off the lights and have a look; it won't cost a dime.
   
  What was your solution on the filament supply?


----------



## tme110

This is pretty slick.
   
  Hope you feel better soon


----------



## estreeter

Good to see you back on deck, Waki.


----------



## wakibaki

Hi, people, thanks for the concern.
   
  To answer the question about the LT, first I tried wiring it up to AC. This proved a pain to achieve physically, given that I had to contrive a resistive splitter to connect the ground to and get the soldering iron in through the later wiring to get at the valve bases. So I went back to the DC I had originally intended. I changed the cap in the filament supply to 22,000 uF. This improved the ripple by a factor of 10 to under a volt, where it should have been in the first place. The peak voltage was ~8.3V after rectification. ~0.7V ripple took this down to 7.6V. I designed and built a discrete low-dropout regulator, but I had some issues getting this to work, by this time I was pretty groggy due to the medication. Anyway, I started looking at the LM317 again, the dropout voltage is well below 2V at most temperatures, so I wired up 2 more LM317's with 1k2 and 5k1 resistors, which adds up to 6.3V and decided to let the dropout look out for itself.
   
  Now I knew this part of the circuit was going to work OK, unlike the discrete regulator, so I wired it up on some perfboard and wired it in.
   
  Then I left it for nearly 3 days until I got the nerve to turn it on. I knew I had put some dents in one of the HT caps when I took out the original cap for the LT supply, but I couldn't face stripping the heatshrink and substituting it, even though I had a spare. So eventually I turned it on and watched the tubes for any signs of serious problems.
   
  When both heaters fired up, with no signs of distress on the plates, I measured the voltage on both the input and output jacks. When it turned out there was nothing much I decided to connect up some old 'phones and a source. The source is a Sony NW-E003 flash Walkman, known to be a bit quiet, and a pair of Koss K/20's which are quite robust.
   
  Warm it up, turn on the Walkman, crank the volume, and there it is. Loud. But good. Although with an annoying tick, tick when turned down.
   
  Here is the actual circuit as built:-
   

   

   
  The big mistake I made with this build was the chassis.
   
  In order to keep the cost down I looked at the first chassis on the site I was buying from, the chassis was 8 * 4  inches and 2 inches deep. I saw that the prices increased down the page, I though that the 8 * 4 would probably be acceptable and I went with it.
   
  I ended up with a really tight build that was very hard to modify and fix when I had a few minor problems. Not really very smart considering it was a prototype.
   
  It's only on going back to the site where I bought the chassis that I realize that the general trend on the page was rising prices going down the page, but it was only superficially true. I paid 11.80 euros but further down the page was a 10 * 6 inch chassis, 2 inches deep, for 11.20 euros. Not only cheaper, but with 60 square inches of floorspace as opposed to 32 square inches.
   
  Anyway, now I want to rebuild the amp with a slightly different configuration I'm going to order up the larger chassis.
   
  Although this will mean a bit of effort in marking up and drilling and punching another chassis (particularly the hole for the power connector), it will result in a much less cramped build with room for turret boards or tag strips, and it will be much easier to correct any problems or modify the design if I decide on a third iteration.
   
  Here's the circuit as I'm currently thinking I will build it. I couldn't resist the symmetry of the 2 current sources.
   

   
  It'll probably take me a few days to check the BOM, order up the parts and punch and drill the chassis. I'll get back to you when I've got some real progress to show.
   
  w


----------



## estreeter

Thanks Waki - very professional circuit diagrams. I'll leave it to those with a few more clues to comment further !


----------



## jcx

Quote: 





> very professional circuit diagrams.


 
  ...except for the .jpg - never post tech doc, line drawing in jpeg - use gif or png
   
  clik to see 4x zoom of actual bitmap
   
   

   
  just a suggestion


----------



## wakibaki

Turns out I couldn't re-use the CCS bias on the second channel because it makes the current drain assymetric, so it needs to be duplicated at the cost of another 10mA or so...
   
  Good job that the PT and choke are overspecced.
   
  You can see the most recent version of the circuit (arranged for PCB layout) in high detail and save an easy-to-manipulate copy (by right-clicking and selecting 'save image as...') by visiting this link:- http://wakibaki.com/audio.php#t11
   
   

   
   
  w


----------



## elliottstudio

Pretty...


----------



## nikongod

Why is it -3db at ~5khz?


----------



## wakibaki

Quote: 





nikongod said:


> Why is it -3db at ~5khz?


 
   
  If you change the coupling coefficient of the transformer to 1, the -3dB frequency response goes up to > ~100kHz. The transformer model is primitive, and the 0.95 coupling coefficient is arbitrary. I checked the low frequency rolloff by substituting in progressively smaller caps, you can see things going a bit awry when you put in about a tenth of the capacitance, so I have a bit more confidence that the low end simulation is meaningful, but I don't see much point in worrying about the significance of the top end.
   
  Let's wait for the RMAA results for the rebuilt amp. I ordered some transistors for the CCS's, but I haven't ordered the new chassis yet.
   
  w


----------



## elliottstudio

Are you going to build it out on that PCB you designed? That would be a treat.
   
  Looking at your first stage tube circuit, I've tried using a diode (and an LED too) for the cathode bias and measured more THD than with a plain resistor. Does the CCS load on the plate help alleviate some of the distortion?


----------



## wakibaki

Quote: 





elliottstudio said:


> Are you going to build it out on that PCB you designed? That would be a treat.
> 
> Looking at your first stage tube circuit, I've tried using a diode (and an LED too) for the cathode bias and measured more THD than with a plain resistor. Does the CCS load on the plate help alleviate some of the distortion?


 
   
  I'm still looking at the positioning of the PT, OPTs and Choke for the rebuild. The PCB requires planning, or at least thinking about the whole layout before you start. That's one of the reasons I'm attracted to it, because of the discipline that it enforces, but it's more inflexible than point-to-point. I might go for a combination approach, split the PCB into a number of subcircuit assemblies. I liked the layout I had in the first build, it was just too cramped. I'd like to follow the general scheme for the PCB, but there are problems. It might be necessary to have holes in the PCB to allow wires from the top of the chassis to pass through to the component side of the PCB. I would like to stick with a single-sided PCB because I can make it myself (no through hole plating), a doublesided PCB that size would be expensive to have manufactured. I can't get PCB mount dual-gang 4-way switches, so I'd have some PCB mount parts passing through the front panel, but the 4-way switches would have to go to headers. I can't use a 10 * 6 PCB which I would like to because although the chassis I'm looking at is 10 * 6, it has a lip on the bottom, so actually inserting the PCB is problematic.
   
  While the toroidal transformer suits me, it was cheap and simple, in some ways I'd prefer to revert to the back-to-back arrangement I drew in the other thread. I'd like to have 12V and run the heaters in series because the DC would be a lot less fussy with a peak voltage ~17V, there'd be more overhead for the regulator, and you'd only need one, because the current would be halved. You'd be able to use a 110-0, 110-0 primary on the first transformer which would make the build directly accessible to builders in the US, but sourcing transformers which are available in both Europe and the US is a pain. I'd really prefer to have both power transformers PCB mounting which adds to the difficulty if anything. 
   
  It's all a bit of a puzzle. The way I usually deal with these conundrums is to let the problem stew around in my head until my subconscious solves it. Sometimes a new piece of information comes to light which pushes the decision in a particular direction. Maybe in this case it's not possible to get an absolutely optimal solution. I'm just not going to go at it like a bull at a gate like I did the first time round.
   
  The theory says that the CCS plate load results in a horizontal load line. This should result is a reduction in distortion, the spacing between where lines on the characteristic intersect the load line becomes more constant. Whether this pans out in practice, I don't know.
   
  I'll look at replacing the diodes with resistors in the sim. and see what effect it has on the THD. I'm just following the recommendations from Morgan Jones' book and the suggestions I've had here for the moment. Not that I feel that the sim. is to be taken as gospel anyway...
   
  Transistors for the CCSs arrived.
   
  w


----------



## elliottstudio

It will be interesting to see the difference between diode and cathode resistor THD on your sims with a CCS load. My circuit uses a plate resistor load so not as linear to begin with, but the diode bias increased the measured distortion by an order of magnitude if I remember correctly.
   
  I wonder if the measured THD and the simulated THD would be the same in your design for each case (resistor vs diode)?


----------



## wakibaki

Overall distortion (distortion at the transformer output) rose from ~1.425% to ~1.48% when I replaced the diode with a resistor.
   
  This is with the amplifier driven very close to grid current in the second stage, which is the limiting condition governing the power which can be obtained.
   
  Distortion at the output of the first stage is 0.775% with the diode, rising to 0.79% with the resistor.
   
  These are the figures automatically calculated by the program. I trust the figures calculated by the program for SS amps where the FFT doesn't show a downward slope to the right, but I don't trust the tube sims because I don't know why they show this slope which I suspect throws the calculation off, so I treat them as valid only in comparative terms. I had been led to understand that the sloping noise floor was associated with a DC offset, but even taking the output across a large resistor coupled with a large capacitor does not eliminate it.
   
  w


----------



## ostewart

I would be very interested in one of these when the design is perfected, if parts are easily sourced, great work.


----------



## wakibaki

You should have no trouble obtaining the parts, I see you live in Portugal, I am in the UK. I sourced the majority of them from a German supplier, tube-town.net, they deal in Euros, the rest can be obtained from ebay.
   
  w


----------



## ostewart

Cool, I'm in england next week too, but i can get parts easily i think, now to wait for the board to be ready, how much will be the aproximate cost?


----------



## wakibaki

Here's a BOM with costing, major component prices are accurate, small component prices may vary from those shown. You can probably save a few Euros by being careful. I have simply marked generic components 'ebay supplier'
   
   

 Category Quantity Reference Value Price (Euro) Total Source Resistors 2 R1,R2 5k1 0.1 0.2 ebay supplier Resistors 2 R3,R7 1k2 0.1 0.2 ebay supplier Resistors 3 R4,R27,R32 500k 0.1 0.3 ebay supplier Resistors 5 R5,R13,R17,R20,R33 1M 0.1 0.5 ebay supplier Resistors 6 R6,R9,R25,R29-R31 10k 2W 0.17 1.02 tube-town.net Resistors 2 R8,R12 33R 0.1 0.2 ebay supplier Resistors 2 R10,R34 2M2 0.1 0.2 ebay supplier Resistors 2 R11,R35 3M6 0.1 0.2 ebay supplier Resistors 4 R14,R18,R21,R26 4k7 0.1 0.4 ebay supplier Resistors 4 R15,R19,R24,R28 56R 0.1 0.4 ebay supplier Resistors 2 R16,R22 1k 0.1 0.2 ebay supplier Resistors 1 R23 100k 0.1 0.1 ebay supplier Capacitors 4 C1-C3,C8 220uF 350V 3.95 15.8 tube-town.net Capacitors 2 C4,C6 100n 0.03 0.06 ebay supplier Capacitors 1 C5 22000uF 16V 0.5 0.5 ebay supplier Capacitors 2 C7,C9 100uF 350V 3.45 6.9 tube-town.net Capacitors 2 C10,C12 3pF 0.03 0.06 ebay supplier Integrated Circuits 2 U1,U2 LM317L 0.5 1 ebay supplier Integrated Circuits 2 U3,U4 6N6P 2.7 5.4 ebay supplier Transistors 2 Q3,Q4 MJE340 1 2 http://www.ebay.co.uk/itm/200590106893?ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1439.l2649 Transistors 2 Q5,Q6 MJE350 2 4 http://www.ebay.co.uk/itm/180643655858?ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1439.l2649 Diodes 8 D3,D4,D6,D7,D18-D21 1N4148 0.1 0.8 ebay supplier Diodes 2 D5,D8 Vzen=5V 0.2 0.4 ebay supplier Diodes 4 D13,D15-D17 Shottky 0.25 1 ebay supplier Diodes 1 D14 RED LED 0.1 0.1 ebay supplier Miscellaneous 1 BR1 400V 1.2 1.2 ebay supplier Miscellaneous 1 FU1 160mA 2 2 ebay supplier Miscellaneous 2 J1,J2 1_4_IN_TRS_JACK 1.5 3 ebay supplier Miscellaneous 1 L1 10H Hammond 158M 16.55 16.55 tube-town.net Miscellaneous 1 RV1 100k log 1.5 1.5 ebay supplier Miscellaneous 1 SW1 SW-DPST-W 1.5 1.5 ebay supplier Miscellaneous 2 SW2,SW3 SW-DP4T 2.17 4.34 http://www.ebay.co.uk/itm/Rotary-Switch-3-Pole-4-Position-way-Shorting-SW195-/150614736838?pt=UK_BOI_Electrical_Components_Supplies_ET&hash=item2311567fc6 Miscellaneous 1 TR1 tube-town.net 30VA toroidal 22.5 22.5 tube-town.net Miscellaneous 2 TR3,TR4 Hammond 125BSE 36.4 72.8 tube-town.net Socket 2 SCK1,SCK2 Noval Ceramic 1.59 3.18 tube-town.net Chassis 1 CHAS1 AL 1444-16 11.6 11.6 tube-town.net Power connector 1 POW1   1.5 1.5 ebay supplier   Euro       183.61     UKP       148.24     USD       238  
   
  I have had a new idea about how to arrange the transformers, 4-way rotary switches and other components.
   
  If I get 10 sets of PCBs made of the size necessary they will be maybe 15 Euros to you including shipping. A doublesided PCB, properly manufactured, gives me a few more options when laying it out. I can have a vertical daughterboard to carry the 4-way switches, have ALL the components PCB mount with header plugs for the transformers.
   
  It's a lot of money for me to lay out though, so the best thing, I think, is maybe to build it point-to-point, get some RMAA results, work up a 3D CAD model and then see if I can get enough takers to make it worth making 10 sets of PCBs.
   
  Give me some time to try drawing the layout in the software. Maybe I can still make the first PCB myself, that would reduce the risk.
   
  w


----------



## wakibaki

Here's the new PCB with routing completed, showing grounding highlighted.
   
   

   
   
  This shows the chassis in outline with transformer positioning and 4-way switch mounting arrangement. Some of the track spacings are a bit tight, and the whole thing needs a bit of fine tuning, but it shows that the routing is possible. Regulators and CCS transistors will be mounted off-board on the chassis for heatsinking.
   





   
  Oh, I'll be able to fabricate this at home, no problem.
   
  w


----------



## wakibaki

I haven't got round to the PCB rebuild, but I did find the problem with the original build. As I thought, the problem was with a 220u cap in the PSU which I dented while wrestling with the heater supply. I was reluctant to go for a repair, I was thinking of a complete rebuild, but after leaving the amp to sit for a few weeks, I figured a way to do the repair that wasn't too ugly. I replaced the cap, fortunately I had a spare on hand, and the tick, tick, ticking sound went away.
   
  I'm not 100% happy with the overall result, there's not much gain, and the sound isn't as clean as I'm used to. I'll give it a bit more listening though, and maybe fiddle with the gain. Now that its built and going, I'm not so keen on cannibalizing it.
   
  I've found some even cheaper OP transformers, 3.5W Fender reverb drivers, they're only about $10 each. Again they only go down to 100Hz at full output, but at about half-a-watt they should be OK. They've got a very high primary impedance. I'm thinking about buying a few and doing some more experiments with them. That woud obviate the necessity to dismantle this one. 'Course I'll need some power TX's, a couple of chassis, some HV caps and maybe some tubes too...
   
  w


----------



## ostewart

Great, can't wait till you get it up and running to your taste!


----------



## wakibaki

A few more sims:
   
  All these are fed from a low impedance source, the amplifier worst-case input impedance is ~25k. It drives 16 ohms with gain, not much gain, but it's a reasonable impedance transformation, and it's sensitivity is in the correct range for typical sources, 1- 2V for full output, depending on load impedance, and it'll drive 600 ohms to 9V peak. It's a bit better than a buffer, about what you might expect for a headphone amp to meet a present-day requirenent. These kind of numbers are what you might reasonably be expected to cater for, but not with a lot to spare, whereas it would be easy to batter the requirement into submission with gain and voltage and current and feedback using transistors. 
   
  You have to remember that the transformer is idealized, there are current sources in the circuit, and these simulations are particularly revealing of the tubes performances, but indicative of the best performance of which each tube is capable. This means that the tube's performance may in practice be masked by nonlinearities in the transformer not modelled in the simulation. There are in addition nonidealities in the current sink implementations not accounted for and other factors which mean that no real-world circuit will achieve the idealized performance shown here.
   
  Keeping the power output to the minimum truly required makes it easier to overspecify the O/P transformer in practice, which at least brings the theoretical performance closer.
   
  This one is DC coupled into 16 ohms, fully driven. 
   

   
  It turns out that capacitor coupled is best. Here's 1mW into 16 ohms.
   

   
  This is Vmaxout into 600 ohms...
   

   
  Take it from me, they are better all round than the DC coupled version.
   
  It's obvious, as well, from a component cost point of view, capacitor coupled makes the most sense, it means that the bypass on the stage 2 current sink can be a much lower voltage part. 
   
  So there'll be a revised circuit along in a minute, with cascoded FET current sources. This makes the circuit versatile, because you can plug in just about any tube you want, any SE transformer you want, and there's a lot of leeway with B+. Although it's drawn for identical tubes, it's hardly difficult to take 2 channels through 2 different tubes, if you're wiring point-to-point.
   
  If anybody thinks that the THD should be better than it looks, I can only say, you wanted that tube sound, didn't you?
   
  Here's what it looks like with 6SN7:
   

   
  Better, but not a helluva lot better.
   
  w


----------



## wakibaki

Just fiddling around, listening to this amplifier as I have built it, I started tapping on one of the tubes, suddenly the volume level jumped up quite a lot, accompanied with a loss of fidelity. Gave me quite a fright.
   
  Debugging these things is so much more difficult than solid state, because you can't just poke around in there with quite the same degree of freedom. I hate dangerous voltages. Tubes get _hot_.
   
  It seems there's some kind of wiring problem in there that I wasn't aware of until now. I'd probably have found it before now if I wasn't constrained to be so cautious. This is quite good news though, because it seems there's quite a lot more gain available than I had previously thought. Because it was an experiment my expectations weren't very disappointed, I just figured that the circuit didn't perform quite as per the sim. No surprise there.
   
  The sound up until that point was quite good, just a bit quiet, I'm optimistic now that it will turn out to be quite successful. 
   
  I've got some more (3.5W reverb) transformers coming + a new chassis and some other components for a chokeless PSU and HV CCS plate load, etc. etc. I'm going to try another assembly that will be a lot less cramped than the first, maybe I can get it to work a bit better this time. I've learned a bit about how to go about the assembly, i.e. wire the bases before screwing them to the chassis. It should actually be a bit cheaper without the choke. Bits should come Friday or Saturday.
   
  Got to get some sleep.
   
  w


----------



## looser101

Chopsticks are your friend when poking around in a tube amp.


----------



## wakibaki




----------



## wakibaki

I did some more sims. I improved the transformer model, I've put in some parasitic resistances, tweaked the coupling (leakage). I need a transformer spare in my hand to measure.
   
  Then I put in some 6SN7's and optimized. 6SN7's have a couple of things going in their favour.
   
  1) They're better at lower currents than the russian tubes. The tubes are best >= 8mA I read, but the CCS's help the linearity, the sim works best at the values shown, and I like the fact that the standing current in the transformers is low.
   
  2) They're in current manufacture.
   
  3) They don't cost a fortune. They're not as cheap as the russian tubes, but they're nowhere prohibitive. 
   
  4) They've got better distortion characteristics.
   
  This is 1V in for 2V out at 1/2 volume pot: All voltages quoted are PEAK.
   

   
  This is the AC response at 1/2 volume:
   

   
  This is the FFT at 1mW  into 16 ohms:
   

   
  This is Vmaxout into 600 ohms:
   

   
  This is 1V in for 4V out into 38 ohms:
   

   
  This THD of 1.3V in for 5.5V into 38 ohms. No point in trying to gild the lily. This is where you can really see the output suffering
   

   
  6% THD. Not too shabby. It's not anything it pretends to be. It's buffer from 10k to a few ohms, there's a little bit of gain.
   
  Now there's a question of implementation.
   
  The Supertex LR8 regulators will easily supply these currents, 2 as plate loads, 2 as VR's for the txfrmr stage, then we'll have LM317 CCS's for the cathodes.
   
  Low current means the ripple on the supply will be less, and this time we'll have a CRCRC filter.
   
   

   
  Slightly higher voltage available than that shown.
   

   
  And a couple of PCBs
   
   

   
  w


----------



## elliottstudio

Looks like you've been busy! So this is for the second build?


----------



## wakibaki

Yes, I've got a couple of irons in the fire as far as development is concerned.
   
  These things seem to be multiplying...
   

   
  I'm still interested in this design, so I'm experimenting with what can be achieved at the lowest cost (hence the reverb transformers) and with the greatest simplicity, but also seeing what the best performance obtainable is without pushing up the price too much.  
   
  I've got quite a lot of components for this on hand including some Supertex LR8 450V variable linear regulators. You can use them to build CCSs too. They probably don't work quite as well as a cascoded LM334, I've got some DN2540 MOSFETS to try that with though. I also got hold of some true LDO regulators (MIC2941) for the heater supply.
   
  I've done a lot of sims with different tubes including dissimilar twin triodes. I've also done the smaller circuit board which has a low-ripple HT supply, and the CCSs and voltage regulators on it. This will fit in the small chassis and really simplifies the construction, and it's more versatile than the larger PCB I drew up earlier, you can wire it to identical or dissimilar tubes. You can take this board and build it with minor changes to the resistors and the zeners and run the tubes at currents from 1mA to 15mA. It's got a CRCRC filter with smaller components than the choke filter and each tube has an independent regulator or plate CCS. It's a kind of modern shell with the tubes embedded in it, that lets you set the operating conditions more-or-less without reference to a conventional (resistor) bias calculation, and it extracts the best the tubes are capable of in a common-cathode, cathode bias configuration. The only consideration really is the quality (impedance) of the CCSs, and that can be modified.
   
  I'm angling toward the 6SN7s ATM, but I haven't bought any yet. It's a different socket too, and a different diameter. It has a high Ra, ~7k. Triode load for best power is 2*Ra, which would be 14k. The 6N6Ps have an Ra <=2k.
   
  It's also possible to use 12AX7s for the gain stage, I have one somewhere, it has a LOT more gain than the 6N6Ps or the 6SN7s, then you (I) usually end up adding some GNFB (in the sim), which is necessary to avoid _too_ much swing on the grid of the second tube. Even though the 12AX7s are not known for linearity, the THD still works out pretty good with a bit of NFB, which is to the good in terms of output impedance.
   
  The good thing about the 6SN7s and 12AX7s is that they are available new, which I think is a big consideration. The other good thing is the current saving. The amp I've got 'working', although it's behaviour remains a bit problematic, wants 50mA for best performance, where the 6SN7 + 6SN7 or 12AX7 + 6SN7 only wants 14mA. Which makes a big difference to the amount of ripple attenuation I can get for a reasonable resistance drop using RC instead of a choke. With 360 ohms I get -58.8 dB with a voltage drop of 11V, the LC filter delivers -61.9dB and a voltage drop of 0.44V. 3dB difference and we lose 10V. We don't want to be giving away any B+ that we can avoid.
   
  The present amp sounds reasonably quiet. I'm doing my best to isolate the tubes from the power supply, the PSRR should be better than the existing arrangement, with it's resistor anode load being replaced by a CSS. The B+ at the transformer is now regulated. The PSU as built is long on capacitance, but traditional in concept. It way outperforms the RC filtration, but there is the ripple rejection offered by the regulators yet to take into account, plus the fact that the tubes each enjoy a greater degree of isolation, constant current and stable voltage supplies. Anyway the net impact of all that is that I think the amplifier will have somewhat better hum performance than it did in it's previous incarnation, and it is pretty good already.
   
  The changes make the whole thing easier to build, regardless of tube selection.
   
  One reservation I've got is that Morgan Jones suggests >=8mA for best linearity with the 6SN7, but I really like 5mA for best performance in the sim. I will be looking at that again. I can get 0.06% THD for a milliwatt into 16 ohms with the 12AX7 + 6SN7 with a gain of 4. That is better than many solid state amps, and it's all second harmonic, although obviously it's not hard to better in a transistor amp with a bit of effort..
   
  Let's think about what we're doing here. We're gradually improving the power output at a notional transformer primary impedance of 10k and notional output impedances of 4, 8, 16, and 32 ohms, with a load of 16-600 ohms distributed across the transformer taps at the operators discretion, with the main axis of the the performance optimization being output into 16 ohms as simulated, with the simulated output into 32 (and sometimes 38) and 600 ohms being taken into consideration. The circuit delivers a minimum gain of 4 into 16 ohms with the volume fully advanced. Minimum input impedance is 25k.
   
  These are not dissimilar to the performance criteria chosen for the O2, although only in terms of what voltage (9v pk-pk) is required and what impedances catered to. It's what is generally recognized as the majority of the range of loads a headphone amp is likely to be exposed to nowadays. Obviously output impedance is 'notional'.
   
  We're trying to keep the components to off-the-shelf, and minimize the cost and effort. We've got a power transformer, this limits our other choices to a degree, although it's in other ways a good match for our requirements.
   
  Tube amps are more touchy-feely than solid state. You never know really what's going on in there in terms of the tube, primary and secondary impedances, so there's no point in getting overly worked up that things aren't working out exactly.
   
  Although efficiency (power output) falls as a triode anode load is increased, this does not mean that output disappears. One of the consequences is a reduction in distortion. I'm using transformers rated for 45mA, the 6SN7s will run 5mA, so DCR should be good. (I don't know the current rating of the reverb tx's, they say 6W).
   
  After this you have to up the power output by looking for a bigger output tube to underrun, and/or change OPT. There are larger dual triodes out there, but we're at the limit of our heater supply, and we want really to stick in the 10k primary range of transformers.
   
  You _can_ approach this from a different point of view. That's to build an amp that will deliver your required voltage (or a bit more) into a (say) 8 ohm load, and slug it with a resistive shunt of (say) 16 ohms. So when anybody plugs a pair of 16 ohm (or greater) phones in, the amp is happy to deliver 9vrms, or whatever it is you want and the load is a minimum of (16 || whatever you plug in) ohms. Which is 10W.
   
  No. We have to look for some finesse, instead of just beating the problem to death. 
   
  w


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## elliottstudio

Great work W. You've thought things through very well indeed. So is the amp with the smaller transformers the reverb ones? How did that work out sound-wise? I think you've done a good job of proving that 'cheap' iron can be utilized in these amps to good effect. I use 70V 4W speaker transformers in my builds. They have a high impedance ratio and low output impedance. Loaded with 32 ohms they present roughly 10K to the plates from the 1W primary tap. Flat response, too.
   
Here's the frequency response using a surplus Quam 70V speaker transformer tapped at 1W into 150 ohms on secondary - intended use as line preamp.
Here's the distortion characteristics , 1.5VRMS in, .5VRMS out (1.414V p-p) out into 150 ohms on secondary again intended as line preamp.
   
Frequency response into 32 ohm load @ 1.75Vrms (95mw), this is from the 2W tap for higher gain for headphone amp service. You can see that the larger the load the flatter the response, but higher distortion.
   
Distortion characteristics @ 95mw into 32 ohms.
  The circuit is a parallel feed 12au7a common cathode, nothing special (plate and cathode loaded with resistors). Of course, no current thru the primary means smaller transformers are possible; easier to get the response noted above. This isn't too bad a number, of course it's tube dependent - the right channel measured a bit better in fact because of the tube. All distortion numbers at the bottom of the graph are from the 1kHz sample, you can see it gets higher as you go lower in frequency due to core saturation and the interaction between the output coupling cap and the primary inductance of the transformer. The ear is less sensitive to distortion at low frequencies, thank goodness.. otherwise all single ended amps would sound terrible!
   
  To be honest, I haven't tried to make a true transformer loaded single ended headphone amp.. built a low-power guitar amp that way for a buddy once but those are supposed to have 'colored' (high 2nd order distortion) sound... so I wasn't really trying to wring the best performance out of the circuit. You've done a great job with that~


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## Armaegis

Just curious: can you estimate roughly how many hours have gone into the design/prototyping process so far?


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## wakibaki

Not really. The first prototype was built in under 2 weeks IIRC. What I can tell you is that since I stopped treating it like a job I had to finish to a price, I've had some fun.
   
  I don't want this thread to spiral off into another disagreement about costs.
   
  Whatever it's cost me, it wasn't so much I couldn't afford to put it all in the public domain.
   
  I'm not about arguing with people who think good SQ should be expensive, I'm about trying to make it available to those who don't.
   
  I just filled a BOM for the major parts. Tubes, sockets, regulators, caps, chassis, transformers, jack sockets, fuse, IEC socket, pot, switch, knob. U$214, so we're looking at U$250 all in. Add on a bit to make a full kit with a punched chassis.
   
  Here's the board for the second build, nearly finished populating it...
   

   
  This will make the build straightforward, even in quite a small space.
   
  w


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## J Epstein

*Wakibaki*:
   
  I am reading this thread with quite a bit of interest, thank you for sharing your herculean efforts and I wish you all the best overcoming your health issues. *elliottstudio*, thank you for sharing your results with the inexpensive line transformers - interesting indeed to this cheapskate builder.
   
  One thing I'd like to share is that I have had very nice results using a couple of the untested-but-speculated ideas you've mentioned above, I can encourage you enthusiastically there.
   
Back-to-back transformers: exactly as you said - the 12 V approach allows you to get the headroom for a regulated, 12V series filament supply. Also the transformer values you need and the ability to make the circuit 110 or 220, all come with that decision. I was once able to source (surplus) size-matched 110:12 and 240:12 transformers that made this SO easy, cheap, and simple to lay out neatly. Sadly, they sold out and I've used all my pairs. But toroids of those types are certainly available, reasonably-priced and new.
   
Dual triodes: DO IT! You can get a wide variety of TV vertical oscillator dual/dissimilar triodes, that are absolutely perfect for this application. The biggest obstacle is that they often sit on oddball 12-pin compactron sockets and those are probably the hardest part to find reasonably-priced in the whole project. 
   
  Here's one project that used these (13FM7) with some commentary:  http://home.earthlink.net/~jeremyepstein/ironconcertina.html
   
  If 13FM7 has too much gain (mu = 66 IIRC) either revisit NFB, or there are many other types with a selection of different mu on the 1st (voltage amp) section. And they are all very nice low plate impedance on the second section (1K is pretty typical) so they do quite well driving a transformer. This is what they were designed to do to begin with. And - THESE TUBES SOUND GOOD which is the dice-roll in the situation, I can speak from experience on that. I built a cute little stereo power amp that used just two 13FM7's operating (for about 15 hours a day, no re-tubes needed over the course of several years) in a local restaurant/slacker cafe. No one ever wanted to leave, partly (I hope!) because the sound was DAMN good. The amp outlasted the business.
   
  These tubes are cheap, plentiful, and no one cares about them at all. And you can run the _completely neglected_ 13 and 15 V filament types off a lightly loaded 12V secondary with no problem. If you are going to offer kits, I would suggest getting a decent stash of the tubes and especially the sockets, after you prototype and before you sell. Leeds Electric in my hometown of Brooklyn NY was one good supplier for compactrons as I remember - it was several years ago.
   
  I'm surprised you had no fun with direct coupling - I always prefer it when possible. It does require a higher voltage supply, all else equal, but in general I find it easier to live with the power supply caps than the coupling caps.
   
  -j


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## elliottstudio

Epstien,
  I like your circuit, very sexy. Next time I come across some dual secondary interstage trannys I'll give it a try. Takes the whole capacitor voicing issue off the table.
   
  Here's a link to a place that sells new compactron sockets, 9, 10, 12 pin, etc. in CERAMIC! http://www.vacuumtubesinc.com/Products/SocketsAdaptersParts/NewManufactureTubeSockets.aspx


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## nikongod

Quote: 





elliottstudio said:


> Epstien,
> I like your circuit, very sexy. Next time I come across some dual secondary interstage trannys I'll give it a try. Takes the whole capacitor voicing issue off the table.


 
   
  Agreeeeeed, Epstien's circuit looks really neat!
  The interstage transformers are dual primary. (maybe split primary - is it anything but semantics?) 
  Lundahl makes tons of em. 
  If asked nicely I'd bet that any number of places could make you something to experiment with. 
   
  I don't think the cap voicing issue is really *solved* as you now have transformer voicing. BUT the stupid-fast recovery from driving the grid positive is pretty sweet with a truly transformer coupled design.


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## elliottstudio

Yes, of course - dual primary. As for voicing, there are so many variables that can affect the sound one could devote a ton of time to one circuit 'voicing' different parts, wires, sockets, connectors, layouts..


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## wakibaki

J Epstein, thanks for the encouragement. Some interesting stuff on your site.
   
   elliottstudio, thanks for the transformer data. nikongod, thanks for helping keep the thread alive.

   

   
  I managed to get the heater supply board for the second build done. It should be working in the next couple of days.
   
  Unfortunately some new symptoms have meant that the oncologist sent me for a CAT scan, the surgeon sent me for a PET scan and I'm keeping my fingers crossed that it'll be no worse than a second round of surgery. It could still turn out to be nothing significant. Lung function test tomorrow. I don't really _feel_ ill, but it has made it a bit hard to maintain my focus on audio design. Hopefully it's only temporary, I have got plans for at least one more prototype with dissimilar tubes, probably 12AX7 and 12AU7 with feedback. I've got 4 * 4P1Ls coming from Russia too, I don't know if these will go in a headamp or maybe a low-power push-pull to drive some full-range speakers after Morgan Jones' design on diyaudio. I've long had a fancy for building a hybrid push-pull with a SS phase splitter.
   
  w


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## Misterrogers

So sorry to hear about your ongoing heath issues W. I'm just coming off of some major back surgery, but that's nothing compared to what you've been dealing with. Do take care. Rest when you need. You have been and will remain in my thoughts.


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## elliottstudio

Yes, do take care, sir.. eat healthy and be healthy. Let's hope those scans are benign.


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## kramer5150

WOW... great looking build W.  Take things easy / slow and I hope your health returns.
   
  We need more affordable transformer coupled amps.
   
  I just finished an OT box with Nikongod and a bunch of others help.  I use a DPDT (ON-OFF-ON) and can toggle between two different primary coils, depending on which amp I plug the box into.  Right now I am using the 500 ohm and 125 ohm primary coils.  I flip to the OFF position in between to completely disengage the power when I want to swap phones.
   
  I am using a Philmore 70V transformer, similar to the one Elliott measured in his post above, with surprisingly GOOD results.  Thanks Elliott for the analysis.
  fullcompass.com has a large selection, but I got mine at Frys.
   
  Are the $30-40 Hammonds worth the $$ investment?  Taking that discussion a notch further, are the ~$300++ "audiophile grade" transformers a noticeable step up from there?
   
  thanks


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## J Epstein

Quote: 





elliottstudio said:


> Epstien,
> I like your circuit, very sexy. Next time I come across some dual secondary interstage trannys I'll give it a try. Takes the whole capacitor voicing issue off the table.
> 
> Here's a link to a place that sells new compactron sockets, 9, 10, 12 pin, etc. in CERAMIC! http://www.vacuumtubesinc.com/Products/SocketsAdaptersParts/NewManufactureTubeSockets.aspx


 
  Thanks very much for the link, I will probably order from there shortly. I dunno if it's a _sexy_ circuit, exactly, but it does do some unique things. try it!
   
  -j


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## elliottstudio

Wakibaki - What's going on man?


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## wakibaki

Quote: 





elliottstudio said:


> Wakibaki - What's going on man?


 
   
  If you look here... http://www.head-fi.org/t/655798/wakibaki-versus-the-big-c-round-2 you'll see I've had a relapse of the lung cancer that hospitalized me ~3 years ago. I don't feel ill ATM but it's been hard to concentrate on getting anything finished construction-wise. I started showing symptoms in December but the doctors kept me waiting until today to let me know exactly what the plan is, I wouldn't say I'm depressed, but not knowing whether a second round of surgery was possible put me under a degree of stress, I've just been spending a lot of time posting and running sims.
   
  It seems the tumour is small and slow-growing, surgery should be in ~4 weeks.
   
  w


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## ostewart

Good luck man, wish you all the best. Family friend recently donated one of his kidneys to his son, surgery is never nice but I hope all goes well and you bounce back.


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## elliottstudio

Thank you for letting us know what's going on with you. Under the circumstances it's easy to understand your absence. Please take care of yourself and do what is most important for your health. Be well and good luck with the surgery.


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