# Low Voltage 6N6p-OTL by H. Ahammer



## FallenAngel

Hey guys,

 So I started building one of these and so far have most of the PSU constructed.

 There is a couple of concept here that I don't seem to grasp:

 1) With the way the tube rectifier is wired, I'd be getting quite a bit of AC on the output - (I measured 76V)! How is this a good idea?

 2) I looked at 5U4G application notes and they show using a 2x 2.5V transformer wired differently from the 5V transformer in the amp schematic. The scheme from the notes look kind of cool considering I'm actually taking the "middle points" of the 2 transformers - wondering if there's any benefit to this versus a single 5V winding.

 Thanks


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## dsavitsk

Quote:


  Originally Posted by *FallenAngel* /img/forum/go_quote.gif 
_1) With the way the tube rectifier is wired, I'd be getting quite a bit of AC on the output - (I measured 76V)! How is this a good idea?_

 

Something's not right -- you should have ~200V DC depending on current draw, and ~2.5mV of ripple.

  Quote:


  Originally Posted by *FallenAngel* /img/forum/go_quote.gif 
_2) I looked at 5U4G application notes ... wondering if there's any benefit to this versus a single 5V winding._

 

In practical terms, not much. If you have a center tapped 5V winding, there is no harm, but don't go out of your way to find one.


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## FallenAngel

PSU isn't "done" yet, just measured output of the tube rectifier with no load. I have 188VDC output (Antek toroid at 2x200V so it's rated 200V unloaded) and 76V AC on the output. I'm sure ripple will go down after the caps and choke, just wandering how THAT much went through.


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## dsavitsk

I wouldn't worry about it. It's a meaningless measurement. Do the same with a 24VAC transformer and some solid state diodes and see what it looks like.


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## Fred_fred2004

I'm proberbly going to be shot down in flames but that circuit looks very different to the one on headwize 

HeadWize - Project: A Low-Voltage Class-A Tube Headphone Amplifier by Helmut Ahammer 

 don't know if you've seen it

 cheers
 FRED


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## FallenAngel

Doug - Thanks, I'll keep building the PSU, should have the rest of the parts needed tomorrow (and of course I forgot to order a 220uF cap. 
	

	
	
		
		

		
			





 Have a couple of Rubycon 100uF/400V for the rest of the amp, but no 220uF).

 Fred - Saw that one, the version I posted is the "newer" version using cheaper and easier findable tubes and an ever so slightly higher B+


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## nikongod

I would bias the heaters above ground. It can help a lot with noise & hum.

 The second one here:


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## TimJo

Quote:


  Originally Posted by *nikongod* /img/forum/go_quote.gif 
_I would bias the heaters above ground. It can help a lot with noise & hum.
_

 

Looks like the max k-/h+ voltage on the e88cc is 60 volts, and the max k/h on the 6n6p is 100 volts.

 Based on experience, how large a bias would you use?

 I guess if cost isn't an issue, the other approach would be to provide the 6n6p it's own heater supply.


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## nikongod

I *assume* the cathodes of the output tubes are about 55-65V above ground. I tried to reverse engineer the exact operating points, but couldn't make it work. I probably forgot to take something into account.

 Anyways, I would probably bias about 40V above ground but the exact value isnt too important. Anything from 20 to 60V should work nicely. 40V gives you a a lot of screw-up room on either side which is nice 
	

	
	
		
		

		
		
	


	




 This keeps the heater voltages much higher than the cathodes of the gain tube, and does not come anywhere near testing the Vh-k limits of any of the tubes. Im not quite as concerned with noise pickup in the output stages: IME cathode followers run quietly with the heaters way below cathode voltage.


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## FallenAngel

How would I do that with this amp design? The way the 6N6p heaters are wired, it's taking output below them. Yes, I'm very confused - solid state guy making his first foray into tubes.


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## nikongod

The bias supply is usually made up with a resistor divider with a cap across the lower resistor. Click the photo above, it links to a page all about tube heaters.

 I would put the bias supply right after the choke. It dosnt need super-clean voltage to work right.

 The schematic you posted dosnt have bleeder resistors shown. The bias supply can be used to bleed the power supply caps.


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## FallenAngel

Sounds good.

 I'll add a 100R/5W bleeder after the 200uF cap in the HT supply.

 For the heater bias supply, that's a low voltage cap right? Also, what values should the resistors be?


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## nikongod

Quote:


  Originally Posted by *FallenAngel* /img/forum/go_quote.gif 
_I'll add a 100R/5W bleeder after the 200uF cap in the HT supply._

 

You only want the bleeder to pull a few mA.
  Quote:


 For the heater bias supply, that's a low voltage cap right? Also, what values should the resistors be? 
 

100V has a nice voltage overhead. Fairly low capacitance is adequate: 10-33uf.

Voltage divider calculator


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## FallenAngel

100K bleeder should pass 1.6mA off 160V supply. Sounds reasonable. 
	

	
	
		
		

		
		
	


	




 For the bias supply, really not sure of resistor values - only hint in there is not to go above 100K for R2. The VD calculator doesn't help me much as I'm not sure what to look for.


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## luvdunhill

Quote:


  Originally Posted by *FallenAngel* /img/forum/go_quote.gif 
_100K bleeder should pass 1.6mA off 160V supply. Sounds reasonable. 
	

	
	
		
		

		
		
	


	




 For the bias supply, really not sure of resistor values - only hint in there is not to go above 100K for R2. The VD calculator doesn't help me much as I'm not sure what to look for. 
	

	
	
		
		

		
		
	


	


_

 

It's just a voltage divider that sets the bias voltage. 

 Probably the best introduction to this topic that I've seen, is here:

The Valve Wizard


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## FallenAngel

Well, as I'll likely have a single 100uF/400V cap left over after I use 3 in this amp, I'll stick that into the bias supply. 
	

	
	
		
		

		
			





 As for resistors to set bias voltage, what should the bias voltage be?

 luvdunhill - I read that article a couple of times (it was actually posted by nikongod above 
	

	
	
		
		

		
		
	


	




), still not sure what it should be. 
	

	
	
		
		

		
		
	


	




 AHA! 20V-90V. As I'm expecting should have ~160V after the choke, R1 of 100K and R2 of 30K should give me 37V bias. Sounds reasonable?


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## luvdunhill

Quote:


  Originally Posted by *FallenAngel* /img/forum/go_quote.gif 
_AHA! 20V-90V. As I'm expecting should have ~160V after the choke, R1 of 100K and R2 of 30K should give me 37V bias. Sounds reasonable?_

 

... okay, now compare this to the Vhk (max heater to cathode voltage)of the tube in question.


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## nikongod

I guessed at where I thought the cathodes of the output tubes were in my above post. I spent some more time with the calculator reverse engineering the schematic. I misread the value of the cathode resistors as 30ohms earlier, they are in fact 38ohms. Everything makes much more sense now. 

 I got the following operating points.YMMV, etc.
 The idle current of EACH section in the output stage is about 25mA, BOTH sections in parallel at 50mA.

 The Cathodes of the output tubes are about 52V above ground at idle.

 PS:
 How do you make a "not equal to" symbol on these forums? I wanted to write 30 (not equal to) 38 in that above line for dramatic effect, but could not.


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## pabbi1

!= ORR <>


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## FallenAngel

OK, I'm royally confused. Teach me oh mighty tube gods!


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## nikongod

Something to look out for with tubes is the Heater-cathode voltage limit. It is basically the maximum voltage the tube can safely operate with between the heater and cathode. Depending on the tube it may be different positive and negative, so that should be noted. If the voltages are exceed you run the risk of the insulation (the heater is electrically insulated from the cathode) breaking down and the tube failing.

 The issue is particularly problematic in DC coupled amps because the cathodes of the tubes may be at such significantly different voltages. The 2 common ways of solving the problem are avoidance and using several heater supplies. 

 It is not as difficult of a problem to solve in most cap coupled amps although they are not immune to this issue at all, so it is always worth checking that the voltages don't exceed the limits.

 There is always a bit more to it... i would get a copy of "Valve Amplifiers" by Morgan Jones at the very least. You'l pick it up really quick.


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## luvdunhill

Some people will claim Vhk numbers are merely guidelines and can be exceeded. You find this very often in commercial gear, by vendors of good report (RSA for example) and those of not-so-go report (Singlepower) 
	

	
	
		
		

		
		
	


	




 I suspect in both cases (unfortunately), these people don't really understand the basics of what you're trying to understand, so kudos there.


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## FallenAngel

Ok, next part of PSU is done - choke installed along with the 3.3uF and 100uF cap. After the those two, I get 188V DC and my auto-randing multimeter goes haywire trying to find AC. When I select range in 100V range, it shows about 3VAC, the small cheap DMM shows 605VAC


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## adamus

some cheap multimeters will struggle to read the ac on the high DC.


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## FallenAngel

Thanks, that's probably it, it's a DVM850BL, bought off some surplus store for under $20.

 Did a little casework - the 4 small 9-pin tube sockets are installed and I've planned out most of the layout of components. Still need to drill a hole for the 5U4G, and it's larger than any stepper bit I've got, will likely have to take a wood bit to it. 
	

	
	
		
		

		
		
	


	




 Either that or make the largest hole I can, then take some time with a hand knife. Still surprised me how steel cuts aluminum.


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## nikongod

For large holes Greenlee punch are unbeatable.


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## FallenAngel

Ok, so I have most of the parts - a couple of things i'm not clear on in the PSU portion.

 1) The 390R resistor between the 100uF and 200uF (250uF in my case) capacitors. What rating should this resistor have? If I'm expecting roughly 180mA (based on 15mA Ia per 6922 tube and and 30mA Ia per 6N6P - I'm assuming Ia is measured by section, not for whole tube) going through it; following V=I*R to get voltage drop, that's almost 70V I'm dropping across that resistor! First of all, that's absolutely nuts as I only have 188V before it and would probably want the B+ to be closer to 150V (I have 160V BlackGate NH output caps I want to use). Second of all, I can't imagine a 70V voltage drop without burning a resistor.

 2) The 4K 1W resistor following the 200uF cap. With my layout, it's actually easier to have a pair of these, one for each channel. Is there anything wrong with doing this and as the current through each should be halved, can I stick with a 1/2 watt resistor?

 Thanks in advance.


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## regal

Quote:


  Originally Posted by *FallenAngel* /img/forum/go_quote.gif 
_Ok, so I have most of the parts - a couple of things i'm not clear on in the PSU portion.

 1) The 390R resistor between the 100uF and 200uF (250uF in my case) capacitors. What rating should this resistor have? If I'm expecting roughly 180mA (based on 15mA Ia per 6922 tube and and 30mA Ia per 6N6P - I'm assuming Ia is measured by section, not for whole tube) going through it; following V=I*R to get voltage drop, that's almost 70V I'm dropping across that resistor! First of all, that's absolutely nuts as I only have 188V before it and would probably want the B+ to be closer to 150V (I have 160V BlackGate NH output caps I want to use). Second of all, I can't imagine a 70V voltage drop without burning a resistor.



 Thanks in advance. 
	

	
	
		
		

		
		
	


	


_

 


 I'm guessing he is using a separate PS for each channel, this would give you the roughly 35V drop. Also suspect this by judging the size of the transformer. Its common to do with unregulated tube power supplies to keep crosstalk out of the equation. Download PSUD from Duncan Amps it really helps when setting up a tube power supply.


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## FallenAngel

I downloaded PSUD2, put the circuit in and got 70mA across R1. At about 190V that's should be a 13.3 Watt resistor 
	

	
	
		
		

		
		
	


	




 Guess I'm going with a huge metal Ni Power resistor rated for 50W when bolted to the chassis.

 As my PSU is shared, it's probably a bad idea to keep the 160R (100mA) choke as it would be running twice that current and replace it with a 90R (200mA) choke.

 Judging by transformer is tough - 200mA 2x so it easily pulls 180mA total.


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## nikongod

First off, work with the best numbers you have. If you have 2 or 3 solid numbers check them against each other to see if they make sense. 

 I think your estimate of current is too high. With such high currents the voltages around the PSU dont add up. 160Volts is given at the plates of the gain tube and the voltage at the first cap can be gotten from the data sheet for the rectifier. 160V+70V(voltage across the resistor)+28V(voltage across the choke@180mA) = 258V>>190V (5u4gb datasheet says 190V with 200vrms to each diode, cap input, 180ma.)

 Since nobody has come up with a better number, Im sticking to my previous one that the output tubes idle at 25mA/section, 50mA/tube. Both channels together for 100mA.

 My reverse engineering of the gain tubes puts them at 6mA/section, 12mA/bottle. Both together for 24mA.

 All together for 124mA. At this current the single 390 ohm resistor dissipates about 5W, which requires a little planning to make work but is not impossible. Id buy some safety factor, but wire wound resistors are very easy to find in 10W or even larger power ratings.

 160V+(390ohm*.124A)+(160ohm*.124A)=227V (5u4gb datasheet says 210V under conditions above BUT @124ma) Its not exactly what the 5u4gb data sheet says, but within a much smaller margin of error.


 Regarding Question 2:
 I would MAKE SURE to use 2 separate 4Kohm resistors each with their own cap on the gain side regardless of what you do for the power stage.


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## FallenAngel

#2 Thanks, I planned to.

 #1 Yeah, I was (and still am) confused as all hell about those numbers. Sticking to your numbers, and a wide safety margin, I'll just go with this a 390R Ohmite 10W resistor from Mouser.

 Will order it soon and get it over here.


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## regal

Quote:


  Originally Posted by *nikongod* /img/forum/go_quote.gif 
_Since nobody has come up with a better number, Im sticking to my previous one that the output tubes idle at 25mA/section, 50mA/tube. Both channels together for 100mA.
 ._

 


 Yea, it wouldn't make sense for the 6n6p to be biased at 50mA per section, thats 25% of max dissapation.


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## FallenAngel

I think I'll drop that voltage dropping resistor from 390R to 300R to get about 150V unloaded. The transformer I'm using is an Antek unit so it's 200V measured unloaded unlike traditional transformers which measure it loaded. I'm getting 188V unloaded before the resistor and a 300R will drop 39V (assuming all tubes are pulling 130mA combined).

 On Doug's recommendation, I'm not going to use the 160V BlackGate output caps and will go with something higher rated.

 It also means I'll have a pair of BGs to use for yet another project...

 Those 4K 1/2W resistors I got are also getting replaced with 3.9K 2W to be safe.


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## stixx

Quote:


 Since nobody has come up with a better number, Im sticking to my previous one that the output tubes idle at 25mA/section, 50mA/tube. Both channels together for 100mA.

 My reverse engineering of the gain tubes puts them at 6mA/section, 12mA/bottle. Both together for 24mA.

 All together for 124mA. At this current the single 390 ohm resistor dissipates about 5W, which requires a little planning to make work but is not impossible. Id buy some safety factor, but wire wound resistors are very easy to find in 10W or even larger power ratings. 
 

Since I am the originator of the first schematic posted by FallenAngel I might as well throw in some of my thoughts... as stated the schematic is based on the Ahammer one and uses less expensive/ easier to find tubes. 
 The original article on Headwize goes through some of the calculations, but as Nikongod researched correctly I planned on running the tubes some hotter... especially the output 6N6P's. His numbers are quite correct actually...

 It should be easy to find aluminum cased WW's in 25W at your favourite sources... I got most of my stuff at RS Components in Germany.

 I haven't built the schematic myself since I got "distracted" by a 2 month business trip to Tokyo and am planning a 6N6P-Pushpull amp using toroids as output transformers for my 300ohm cans...

  Quote:


 Those 4K 1/2W resistors I got are also getting replaced with 3.9K 2W to be safe. 
 

The 4K resistors are specced as 1W which should be okay but 3.9K 2W is perfectly acceptable.

 Good luck with the project!


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## FallenAngel

Got this baby build and only scared the crap out of myself once - when I shorted the big power cap to the case. 
	

	
	
		
		

		
		
	


	




 Impressions:
 1) It works and plays music
 2) It has some serious transformer hum
 3) It sounds distorted at the top and bottom end

 Photos coming in a bit


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## nikongod

Quote:


  Originally Posted by *nikongod* /img/forum/go_quote.gif 
_160Volts is given at the plates of the gain tube_

 

I just re-read this, and it dosnt make sense. 160V is given at the plate of the output tube... Sorry about that.


 Good news it is running sort of 
	

	
	
		
		

		
		
	


	




 Post pics and someone will probably be able to guess where the hum comes from.


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## FallenAngel

The layout is "interesting" to say the least! I have wired the "amplifier" part on the top of the chassis but the "power supply" is attached to the bottom. 
	

	
	
		
		

		
		
	


	




 There is a high-voltage wire going from the bottom of the chassis to the top and the heaters are are twisted together and also running to each tube from the a pair of terminal blocks on the bottom.

 Photos would be kind of hard to do, but I'll try. As for hum, I'm not positive where it comes from but I'd guess the layout, and the distortion, I'm clueless about.


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## nikongod

I feel an "uggh" moment coming on. but bring it!

 My guess so far is to say:
 Wire everything on 1 side. you are making life HARD on yourself this way.

 I would look at how the heaters are arranged as another thing.

 Is it star grounded?


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## stixx

Impressions:
 1) It works and plays music
   
  That's good!

 2) It has some serious transformer hum
   
  Maybe coupling from the transformer into the signal when you have things on top of each other. Otherwise it is most likely a grounding issue (know from personal experience 
	

	
	
		
		

		
		
	


	




)

 3) It sounds distorted at the top and bottom end
   
  Have you checked ALL voltages? Like B+ but more important the voltages across the respective cathode resistors? Nikongod did some pretty good reverse engineering ... verify those voltages to make sure everything is biased up properly or you will be running your valves biased in areas they are not supposed to make music.


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## FallenAngel

It's all packed up to be moved back to Canada with me.  I'm flying out in just 2 days!  It's going FedEx across (yes, I know, there will be hefty customs and brokerage fees but it's traceable and USPS won't take boxes this big anyway).  I should be able to look at it again in about 10 days.
   
  B+ is 160V and heaters are a steady 6.2VAC.  I haven't checked bias voltages but it will be first to look at.  Ground looks surprisingly close to the schematic as the "grounds" on the "amp side" are actually all connected to a terminal strip and that terminal strip has a wire going to the star ground.


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## FallenAngel

This project is back on the table!  Troubleshooting discussion very likely to come shortly, as well as photos.


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## wakibaki

Thought I'd try this with 6N6Ps throughout, since I have 4 doing nothing. Gain of only 2.
  

  

  
 Get a bit more gain with some tweaking...
  

  
 Works so much better without the LTP, but of course you get an inversion...
  

  

  
 w


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## FallenAngel

Well, as I've had this on the workbench for some time and could not resolve the buzzing issue, I've completely disassembled the amp and am going to rebuild it.
  
 The PSU will stay as is as I see nothing immediately wrong with it (aside from the choke and 5V transformer having ground wires attached close by instead of at the star ground (the tower looking thing in the middle, just before the blue/green AC heater wires).
  

  
 I'll post build photos soon, and any suggestions for how to lay out this build would be much appreciated. I'm currently thinking of building the amp part P2P on the top side again, but as "clean" as possible. A single star ground on the top and a single star ground on the bottom, connected via 1 wire.
  
 I am a little concerned about running the AC heaters so close to signal wires, but not sure how to avoid that at the moment.
  
 Also I was reading that I should have connected pin 9 of the 6N6p tube to ground as it might create buzzing, so that's also planned.


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## FallenAngel

Looking over the schematic, I'm not completely understanding the purpose of the 100uF / 400V caps on pin 7 of the E88CC to ground between B+ and Ground.
 Do these need to be as close to the tubes as possible to make sure there's enough current available at all times or can these be a couple of inches away, closer to the star ground?


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## FallenAngel

I've been busy today


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## nikongod

Unless I am reading things wrong:
 The cap you mention should go straight to ground with its own wire. 
 The "non-driven" grid of the input tube should also go straight to ground with its own wire.
  
 It is VERY important that you DO NOT! Share the ground wire between these connections.


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## FallenAngel

Hmm... this thought might explain some noise I was getting in the previous build. This may be a stupid question, but how important is "VERY" important that some of these "grounds" aren't connected before they get to the star ground? I've basically got a "star ground" sitting just beside these caps, and will have a wire going to the PSU ground.
  
 Well, actually, I have the non-driven grid (pin 7) connected to internal shield (pin 9) and then connected to pins 3/8 (cathodes) through a resistor before reaching the top "star ground".
  
 Is this a clear cut case of "star ground means completely separate wires, go back and redo it!" ? I've always been under the impression that it's fine to attach everything to shared wires leading towards a central "ground point".


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## nikongod

Shared wires going back to a central point is called by several names (christmas tree ground, hybrid star ground, maybe others), and it is a valid cost & time saving measure in mass produced amps where you have time to test what can be tied together and what can't without affecting performance. It generally works nicer than amps with a ground bus but not as time or materials intensive (both=money!) as an amp with a true star ground to build. 
  
 In the case of DIY amps you can cut some corners with experience but until you are comfy with it star ground is nice and reliable. It means everything comes back to ground on its own wire. There isn't really much to ground here. Just keep things tidy. 
  
 In the interest of cutting corners - I would ground things that appear to have no current going through them together. No reason to spend extra wire there. The 1Mohm input resistor on the "driven grid" and the "non driven grid" of the input tube can pretty safely share one wire although each channel should have its own ground wire here. The 10Kohm resistors to ground at the output of the amp for both channels can share the same ground wire as the headphones.


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## FallenAngel

Thanks for the clarification, that definitely helps and shines some light on why I was getting tons of noise last time. It also means "back to the drawing board" for the layout of this amp as I'm certain there are things that are grounded together which shouldn't be.


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## FallenAngel

Good news and bad news:
  
 The good news is that after wiring the star ground and fixing a few solder joints that had caused a little trouble, the amp is "complete" and plays music with basically no background noise/hum when no music is playing.
  
 The bad news is that it sounds pretty awful with terrible distortion in the mids/bass. Incredibly muddy and "buzzing bass" when it touches anything above the high-mids.
  
 I'm only testing with a cheap $5 pair of generic brand headphones but they sound a lot better than this when plugged into something else.
  
 I'll try swapping different 6N6P tubes as I have a few, but only 1 pair of 6922 (brand new ElectroHarmonix E88CC).
  
 Aside from that, the only things I can think of is the signal caps.
  
 The coupling caps are Russian Teflon TF-1 0.22uF/400V which should be pretty awesome since I've used them before, but the output caps are a new experiment with Rubycon 1A2304A 410uF/300SV "PhotoFlash" caps bypassed by 1uF/400V Solen.
  
 EDIT: Having done some more research on the Rubycon Photoflash caps, they're used often in the Bijou with good results, so I'm having a hard time figuring out what could make my amp so "muddy"!


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## FallenAngel

A side note on the design - I've never seen this kind of wiring on an input tube before.
  
 It seems to me that you could get away with using a single 6922 tube for both channels here or maybe drive both triodes of each 6922 or put them in parallel? The input signal goes into Grid 1, out of Anode 1, then through a pair of 7K5 resistors where it hits B+ between them, then goes to the output tube, while there's a ground going to Grid 2, and meets that signal as well.  I'm confused to say the least...


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## FallenAngel

Reading a bit more here and understanding a little more http://www.freewebs.com/valvewizard/acltp.html


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## FallenAngel

A little knowledge is a dangerous thing! Sanity check this differential monstrosity please!


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## wakibaki

That's not going to work. You have two out-of-phase signals from the input LTP driving the grids of the second tube in opposition.
  
 That's all she wrote...
  
 w


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## FallenAngel

Good to know.  what I'm not understanding is that the input tube doesn't have a pair of resistors like the 470k that I generally see in ltp implementations.

Little update, the amp is mostly cased up and playing music through my hd650s, but the bass is still really distorted and I'm having noise issues when I move the amp or touch the tubes, so I'm thinking solder joins may be troublesome...


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## nikongod

fallenangel said:


> A side note on the design - I've never seen this kind of wiring on an input tube before.
> 
> It seems to me that you could get away with using a single 6922 tube for both channels here or maybe drive both triodes of each 6922 or put them in parallel? The input signal goes into Grid 1, out of Anode 1, then through a pair of 7K5 resistors where it hits B+ between them, then goes to the output tube, while there's a ground going to Grid 2, and meets that signal as well.  I'm confused to say the least...


 
  
 Its the same as a Melos. More or less. 
  
 There are so many reasons to use a long tail pair (LTP) input stage. 
 Lower gain
 Lower miller capacitance = wider bandwidth
 *generally* improved THD
 Improved PSRR
 Option of balanced inputs
  
 So much cool.


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## FallenAngel

That's nice to know that it's similar to the Melos, but still really confused on why the input tube looks the way it does...
  
 Shouldn't the second grid be joining the first grid through a resistor (equal on both sides), similar to this LTP example:


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## dsavitsk

fallenangel said:


> That's nice to know that it's similar to the Melos, but still really confused on why the input tube looks the way it does...




Phase


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## FallenAngel

Simple enough 
  
 Ok, then I'm absolutely clueless as to why it sounds very muddy in the low end. I'll go over the entire circuit with a fine toothed comb and fix any and all solder joints which look off to hopefully track down the noise issue. After that, I guess I'll replace the 0.22uF cap with something else that I have on hand, but aside from that I'm stumped. The same issue twice, even after rebuilding it... I'm thinking it's either the tubes I have or that cap.


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## dsavitsk

The next step really should be to measure voltages at the various nodes. Can you post a schematic with the measured voltages written in? If you have not worked with high-ish voltages much, it is a good idea to pick up a pile of $5 meters from Harbor Freight, connect them all with clip leads, then turn the amp on rather than poking around a live circuit.


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## FallenAngel

Voltages added in RED. Is there anything I missed?


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## dsavitsk

The voltages all seem to work out more or less -- there is about 1.8V bias on the first tube and ~1.2 on the second. More might be better, but that is high enough that you should not be easily over driving things.

Do you have the same issue in both channels? The E88CC is a pretty high Gm tube that is happy to oscillate if it is not well cared for. So that's one place to look. Make sure all the resistors are right on the pins, and that there are no long leads anywhere. 

The next step, assuming that everything is hooked up as it should be, would be to test each section independently. If you have a cmoy (or any other amp), run the input stage into it and see what you get. Similarly, try listening to just the output stage on its own, and see if you can pin down where the issue arises.

Going forward, if you plan on keeping the amp, there are a few changes I'd make. In particular, unless you are planning to add a feedback loop, I'd ditch the 6922 for something with a little less gain. -- 5687, 12AU7, 6SN7, another 6n6, etc. Second, I'd build a negative rail by adding 2 diodes to the rectifier and use that with a CCS in the tail of the driver tube. Then I'd scrap the RCRC section of the PS and add a small regulator, and probably increase the voltage and the plate resistors on the first stage. Just my $0.02.


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## 00940

dsavitsk said:


> Do you have the same issue in both channels? The E88CC is a pretty high Gm tube that is happy to oscillate if it is not well cared for. So that's one place to look. Make sure all the resistors are right on the pins, and that there are no long leads anywhere.


 
  
 +1. And the same for the 6n6p. I only see one grid stopper ?
  


> Going forward, if you plan on keeping the amp, there are a few changes I'd make. In particular, unless you are planning to add a feedback loop, I'd ditch the 6922 for something with a little less gain. -- 5687, 12AU7, 6SN7, another 6n6, etc. Second, I'd build a negative rail by adding 2 diodes to the rectifier and use that with a CCS in the tail of the driver tube. Then I'd scrap the RCRC section of the PS and add a small regulator, and probably increase the voltage and the plate resistors on the first stage. Just my $0.02.


 
  
  
 Agreed on all points.
  
 As is, the input stage is indeed extremely poor. With only 180r in the tail, it doesn't behave as a proper ltp and is extremely unbalanced. A quick sim gives a gain of 4 right now 
	

	
	
		
		

		
			





. A quick look at the curves doesn't place the current bias point in a very linear place either. So, if you don't have any other tubes, I'd at least put a ccs in the tail as suggested (about 4ma per tube, so 8ma), raise the input voltage at least up to 130V and change the load resistors to 10k. Not exactly how I'd design an amp but the reduction in distortion is dramatic in sims. Gain would be about 10x.


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## FallenAngel

Thanks for the input guys.
  
 I don't have other input stage tubes, except for some more 6N6P, but I do have a pair of 5963 (12AU7) in the mail right now.
  
 Sound-wise, it's in both channels.
  
 In terms of long resistor leads on the E88CC - the only long ones are the 180R tail resistors to ground and the 7K5 resistors are connected by a 2" wire to the caps, everything else is under an inch.
  
 I'll try to test the stages independently if I get some time this weekend, kind of busy lately. 
  
 In terms of "install this part here", how would I create a negative rail and install a CSS on the tail?
  
 As for the input voltage - I'm assuming you're saying replace the 7K5 resistors with 10K. Correct?
  
 Also, in terms of whether I'm keeping the amp - I'm completely impartial to keeping or rebuilding it into something else, but as I already have the casework for 4x 9-pin tubes and the 4-pin rectifier, it would be nice to keep that configuration.  Aside from that, anything goes. Of course keeping the transformer and PSU is a huge bonus, but I'm happy to modify the PSU, as long as I get to reuse some of the parts.


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## 00940

Since you asked for open-ended opinions...  if you can get about 180V from the supply (by increasing the cap in between the rectifier and the choke to 33uf... what's the dcr of your transformer secondaries ?), you'd be better off with a white-cathode follower output. I wouldn't reinvent the wheel and just use the aikido, as suggested by Broskie. Check for the schematic here: http://www.diyaudio.com/forums/tubes-valves/133201-6n6p-aikido.html#post1659341 , you want the one with 12au7-ecc99 (right column). Just replace r11 by a pair of diodes (Bijou-like, or SY-like, or Broskie-like, whoever you want to credit for that).
  
 You'll be able to keep pretty much all your casework intact.


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## dsavitsk

Have you tried shields on the tubes yet? I have seen the 6n6 make some pretty horrible noise without them.

Example of a negative rail here. http://tubelab.com/designs/tubelab-se/schematic/


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## 00940

To add something constructive. If you replace the ecc88 by a 12au7, you don't really need a negative supply for a CCS. With 4ma per section and the anode at about 100v, you have 3V of bias, just enough to fit a jfet ccs.
  
 So you could try to use the attached schematic.


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## dsavitsk

00940 said:


> To add something constructive. If you replace the ecc88 by a 12au7, you don't really need a negative supply for a CCS. With 4ma per section and the anode at about 100v, you have 3V of bias, just enough to fit a jfet ccs.




LM334 will work in place.

Actually, why are the two stages cap coupled? The easiest thing to do might be to put a CCS under the output tube (10M45 or DN2540 will work fine) and eliminate the cap and allow the thing to find it's own bias. You might need to tinker around with operating points a bit, but I'd think it would be worth the effort.

None of that helps with the noise problem, which I still say might be solved with some tube shields.


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## 00940

dsavitsk said:


> Actually, why are the two stages cap coupled? The easiest thing to do might be to put a CCS under the output tube (10M45 or DN2540 will work fine) and eliminate the cap and allow the thing to find it's own bias. You might need to tinker around with operating points a bit, but I'd think it would be worth the effort.


 
  
 So that the both stage can find their own bias ? 
	

	
	
		
		

		
		
	


	




    I don't see much hurt in a small coupling cap with a significant dc potential across it, especially when there's a big coupling cap at the end of the day. I'd think you'd gain more in maximizing the voltages at the anode of both tubes than trying to directly couple them and you need the cap for that. But that could very well be a matter of tastes.
  
 I agree on the solid state ccs though. A dn2540 set at 40 to 50ma could replace the 1k resistor and the cathode resistor adjusted for about 40v at the grid of the 6n6p (or top of the ccs, same thing).
  
  
  
 Wrt the noise... Would it be possible to have pictures of the current state of the amp ? Another thing: what did you do with pin 9 of the 6n6p ?


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## FallenAngel

Current stae of the amp:

  
 Pin 9 of the 6N6P is grounded.


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## FallenAngel

In terms of transforming this into an Aikido-like amp running off about 160V, what resistor values should I be using? I know this is asking for quite a bit, but the link (http://www.diyaudio.com/forums/tubes-valves/133201-6n6p-aikido.html#post1659341) shows it running off 250V, not 160V.


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## Artsi

Here is schematic that should do the trick.


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## 00940

Here's how I'd do the aikido on 160V. For a 12au7-6n6p combo (the 6922 has too much gain), keep all the values given by Broskie but make:
  
 r2-r4: 330R
 r11: replace it by two 1n4007 diodes in serie. No need for capacitor bypass anymore.
  
 The problem is R11. With only 80V across each tube, you're very close to bring the grid to 0V to get any decent current flowing. From the published curves, a 1.4V drop across r11 should give you about 15ma flowing.
  
 I must confess it's a mystery to me why Artsi requires 5W resistors. 0.6W resistors are sufficient.
  
 At the risk of repeating myself, if going the aikido way, my point of view is that your PS is loosing way too much voltage. 3uF as first cap is just a waste. With your transformer, 5u4gb-33R (at each diode plate)-22uF-10H/160m-220uF followed by 220R-100uF by channel should give you a decent 210V to 220V supply without overtaxing your rectifier tube. Especially since the aikido stage cuts your standing current by two (about 20mA per channel). If going that road, make r2-r4 470R and use 3 diodes in serie.


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## Artsi

00940 said:


> I must confess it's a mystery to me why Artsi requires 5W resistors. 0.6W resistors are sufficient.


 
 I have just calculated simply 160v and around 0.017A through 6N6P. 160x0.017=2.72w or have i calculated totally wrong? It could work with burning hot 0.6w resistor too.
  
 With my DIY amp's 6AS7 150v and 0.07A =10.5W, and 10W resistor goes very hot, or have i understood these wattages totally wrong and 0.6W would work better with my DIY amp?


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## 00940

Under normal working conditions*, there's never 17ma/160V across those two resistors. The cathode one for example will dissipate 43mW (R*I²,0.017² * 150). That's the good thing with WCF outputs, all the heat is dissipated by the tubes. That's why Broskie specifies only 0.5W or higher.
  
 And neither will you have 17ma with a 150r cathode resistor with a 6n6p at 80V (that'd be -2.5V at the grid, you need 110-120V across the tube for that)
  
 * The only conditions where I could see those resistors dissipating a lot of heat would be a failure of a triode as short. At this point, whatever their ratings, they'll go poof with the tubes.


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## FallenAngel

You were asking about my transformer earlier, it's this Antek (totally cheap) - http://www.antekinc.com/as-1t200-100va-200v-transformer/
 So we're expecting about 190VAC output at full load, rectified to around 270VDC, with the tube giving a 50V drop, that's an optimistic 220V before filtering.
  
 I was not clear about your PSU description, what do you mean by "33R (at each diode plate)"
  
 In terms of the full PSU, a diagram would really be appreciated, it's not super clear in words (and I'm a visual person, just don't tell anybody since people seem to enjoy my writing).
  
 I'll try to upload a modified Aikido-like schematic soon with proposed values.


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## 00940

Ok, here's a drawing. The 33r resistors are only needed if the secondaries' DCR are too low. If the 200V secondaries each measure above 33r (just a simple dmm check), the 33R resistors aren't necessary. See: http://www.freewebs.com/valvewizard/fullwave.html


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## FallenAngel

Resistance between the secondaries is around 32R.
  
 I already have a 100uF/400V cap for each channel as per amp schematic, should I replace it with a 220uF?
  
 Come to think of it, could I just leave the PSU mostly intact except for the 33uF cap instead of the 3.3uF (probably will just add a 33uF cap in parallel, easier, and I have no other use for a 3.3uF huge film cap like that), and changing the 100uF cap in the PSU to a 220uF?
  
 What's the purpose of the 4x15K 0.6W (60K / 3W) resistors?
  
 The 100uF/100V cap is a complete mystery as well...


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## 00940

fallenangel said:


> Resistance between the secondaries is around 32R.


 
  
 If it is 32R from CT to each end, you can reduce the two 33r to 10R or 22R. If it is 32R from the extremities, keep 33R.
  


> I already have a 100uF/400V cap for each channel as per amp schematic, should I replace it with a 220uF?
> Come to think of it, could I just leave the PSU mostly intact except for the 33uF cap instead of the 3.3uF (probably will just add a 33uF cap in parallel, easier, and I have no other use for a 3.3uF huge film cap like that), and changing the 100uF cap in the PSU to a 220uF?


 
  
 The caps values after the choke aren't extremely critical. With unregulated supplies filtered by LC or RC cells, you have a trade-off. It is good to have a final RC filter per channel to reduce crosstalk. But if you put too high a resistor and too low a cap, you actually increase the ripple (because of the varying current draw) compared to the ripple after the choke (it's an easy sim in ltspice if you wanna see it). So put a decent value cap after the choke (100uF is fine) and the biggest cap you can after the 220R resistors going to each amp.
  


> What's the purpose of the 4x15K 0.6W (60K / 3W) resistors?
> The 100uF/100V cap is a complete mystery as well...


 
  
 With an aikido, the top tubes' cathodes are sitting at B+/2. To avoid stressing the tubes, it's best to bias the heaters somewhere around B+/4, so that the cathode to heaters voltages aren't too big. So the 15K resistors are a voltage divider (so you cannot use a single resistor but you could use 47K-15K) tied to the heaters CT (or fake CT with resistors). The 100uF cap in // filters that reference point. Its value isn't critical (voltage rating is).
  
 It was already explained at length by Nikongod at the beginning of the thread.
  
 Edit1: wrt the transformer: I see two 6.3VAC windings, no 5VAC for the rectifier ? Do you have resistors to drop the excess 1.3V ?
 Edit2: I'm going to be a bit nasty: you could keep the tube rectifier for the nice look  but disconnect it and replace it below the hood by a pair of uf4007. Voila, instant 50V gain.


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## FallenAngel

It's 32R between the secondary to CT (well, since it's a dual winding transformer, it's for each secondary). I should have some 10R / 5W resistors around.
  
 Looks like I'll keep PSU to 200uF (bypassed by 1uF) after the choke. I'll add a 100uF/100V cap between the heater supply resistors and ground.
  
 Would it be fine to just put 2x 100uF/400V caps per channel after the resistors? Just so I can skip placing an order and use parts on hand.
  
 Edit1: I have another 5V transformer for the rectifier. The 6.3V windings are AC heaters for the 4 signal tubes.
 Edit2: Uhm... very evil


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## 00940

> Originally Posted by *FallenAngel* /img/forum/go_quote.gif
> 
> I'll add a 100uF/100V cap between the heater supply resistors and ground.


 
  
 I just re-read the first posts. If you already had a 10uf to 33uf capacitor there, as suggested by Nikongod, it's enough, it's not very critical. The virtual CT of the heaters must be held at 40 to 60Vdc though, that's the critical point.
  


> Would it be fine to just put 2x 100uF/400V caps per channel after the resistors? Just so I can skip placing an order and use parts on hand.


 
   
 Sure.
  
 Quote:


> Edit1: I have another 5V transformer for the rectifier. The 6.3V windings are AC heaters for the 4 signal tubes.


 
  
 Oh, ok, didn't catch that.


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## FallenAngel

How does this look?
  

  
  
  
 Aikido schematic

  
 EDIT: Updated Schematic after 00940 post


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## 00940

Mostly fine (if a bit weirdly drawn   ). Two little things: the 100R resistor doesn't need to be 5W and the two 1meg resistors at the output should go before the output cap, not after.


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## FallenAngel

I just figured 100R/5W since it said it had to be rated for B+, if it's fine, I'll just use a 1/4W as the rest.
  
 Updated the schematic. Not sure of the last bleeding resistor is necessary or even should be 1M, I see anywhere from 10K-1M here.


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## 00940

No, it is said that the caps with * should be rated for b+. For resistors, the * means they have to be of "good quality". I'd rather use 0.6w resistors rather than 1/4w though, as suggested by Broskie.


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## FallenAngel

I usually use Vishay Dale RN60, they technically 1/4W, but I'm pretty sure they're actually 1/2W.
  
 Just to confirm, what rating should the 100R resistors for the heaters be?
  
 I'm probably going to a local electronics store tomorrow to pick up the parts I'm missing.


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## FallenAngel

The new amp is on the work bench and I'm building it this weekend. 
  
 Just a little question; can I make the assumption that anything connected via 1M can be safely connected to something going to that destination (Anode to B+ connects through 1M resistor to signal, I'm planning to just connect the 1M between the pins and run a wire to B+).


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## FallenAngel

Just built rebuilt the PSU by adding a 33uF cap before the choke in parallel with the 3.3uF, added the 2x 10R/5W resistors on the output of the transformer and wired up the 100uF/100V cap with 15K/45K resistor divider.
  
 Testing just the PSU, unloaded, I got 274V DC before the choke and 273V DC after... which looks quite excessive.
  
 The amp is ready to be plugged in and tested, just wanted to verify that this isn't a little crazy first.


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## wakibaki

This is typical of CLC filtered PSUs. A supply with capacitor input filter CL will have an output voltage of ~1.41*RMS voltage input _for all capacitors over a certain size_. A supply with choke input will have an output voltage of ~0.9*RMS voltage input. If a parallel cap is put in front of the choke, then as the capacitor size increases, so the output voltage increases asymptotically to 1.41*RMS input. This is sometimes used to control the output voltage.
  
 These figures above are approximations, say nothing about the ripple, choke resistance, load.
  
 w


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