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Rod Elliott on tube amp design

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We have been exposed to a philosophy of tube amp power supply design here on the forum which advocates very low inductance chokes, very low value filter caps, tube rectifiers, and high ripple.  I have expressed my personal disagreement with this philosophy.  Here, Rod Elliott explains very clearly (using proven electrical engineering principles) why such designs are inappropriate.  Scroll down to the section on power supplies for his explanations:

 

https://sound-au.com/valves/design2.html

 

The discussion is a bit technical, but the conclusions are very easy to understand.

 

 

Maynard

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the crux of the biscuit:

One thing that must be understood ... valves are non-linear, transistors (including FETs) are non-linear.  Live with it, and don't be fooled by claims that valves are linear, because they're not.  A valve is a comparatively poor voltage controlled current source, and a transistor is a very good current controlled current source.  The process of design is to create a voltage controlled voltage source (aka an amplifier), and while either technology can be used, ultimately (and I must include this with a touch of sadness), transistors win.  Such is life.

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1 hour ago, babadono said:

the crux of the biscuit:

One thing that must be understood ... valves are non-linear, transistors (including FETs) are non-linear.  Live with it, and don't be fooled by claims that valves are linear, because they're not.  A valve is a comparatively poor voltage controlled current source, and a transistor is a very good current controlled current source.  The process of design is to create a voltage controlled voltage source (aka an amplifier), and while either technology can be used, ultimately (and I must include this with a touch of sadness), transistors win.  Such is life.


But tubes have so much more charm😁.  Admittedly, I don’t agree with Rod on absolutely everything.  I am often asked why I bother with tubes given the performance available from SS devices.  The only answer I can give is that they attracted me from my first view of glowing filaments when the repairman came over to fix grandma’s console radio when I was 3 years old.  We all have our obsessions.....

 

 

Maynard

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Thank you for this thread, I was going to start one in a similar vein, but will hop on this one if you don't mind. I will start another one on the other topic I had in mind (the degree to which an amplifier plays in the total picture).

 

Has anyone heard a direct coupled amp or built one? I have, at RMAF, and I wasn't that impressed, but attributed more to the speakers that were hooked up to it.

 

In researching these kinds of amps I ran across this comment/observation: My one qualm with the LSES idea, is that by using a low-value choke and low-value cap- you're creating a resonant circuit that's likely got a corner frequency within the audio band.

I've heard tube amp power supplies with this issue- and my usual complaint, is reduced low bass energy and impact- and what bass is there, tended to be tubby and ill-defined. I've modified such designs, with additional capacitance and/or inductance- getting the resonant frequency below the audio band, and reducing the Q of the resonant circuit to a low enough level to not overshoot- and the bass has almost always improv

 

This is all way over my head, but what is a "resonant circuit"  and a corner frequency? (I get the "within the audio band part).

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3 minutes ago, dwilawyer said:

Has anyone heard a direct coupled amp or built one? I have, at RMAF, and I wasn't that impressed, but attributed more to the speakers that were hooked up to it.

 

Travis, by "direct coupled" do you mean "no series capacitors in the signal chain" or do you mean "no output transformers"? If the latter, then I have heard some of the Atma-Sphere OTL designs, and they are wonderful (IMHO).

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19 minutes ago, dwilawyer said:

Thank you for this thread, I was going to start one in a similar vein, but will hop on this one if you don't mind. I will start another one on the other topic I had in mind (the degree to which an amplifier plays in the total picture).

 

Has anyone heard a direct coupled amp or built one? I have, at RMAF, and I wasn't that impressed, but attributed more to the speakers that were hooked up to it.

 

In researching these kinds of amps I ran across this comment/observation: My one qualm with the LSES idea, is that by using a low-value choke and low-value cap- you're creating a resonant circuit that's likely got a corner frequency within the audio band.

I've heard tube amp power supplies with this issue- and my usual complaint, is reduced low bass energy and impact- and what bass is there, tended to be tubby and ill-defined. I've modified such designs, with additional capacitance and/or inductance- getting the resonant frequency below the audio band, and reducing the Q of the resonant circuit to a low enough level to not overshoot- and the bass has almost always improv

 

This is all way over my head, but what is a "resonant circuit"  and a corner frequency? (I get the "within the audio band part).

 

Yup I was given a schematic by Jeffrey and Dave of EMIA and built their DC IT amp and it is the one I have listened to the most. This one can run 45/2a3 but I want to scale it up and build a 300b/50 version... The power supplies end up being most of the amplifier.. You don't skimp here....

 

 

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14 minutes ago, Edgar said:

 

Travis, by "direct coupled" do you mean "no series capacitors in the signal chain" or do you mean "no output transformers"? If the latter, then I have heard some of the Atma-Sphere OTL designs, and they are wonderful (IMHO).

Of the Lofton-White variety.

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2 hours ago, babadono said:

A valve is a comparatively poor voltage controlled current source, and a transistor is a very good current controlled current source.  The process of design is to create a voltage controlled voltage source (aka an amplifier), and while either technology can be used, ultimately (and I must include this with a touch of sadness), transistors win.  Such is life.

 

What is the situation with hybrid solutions? With my Marantz PM11-S3 I only use the transistor preamplifier, I have decoupled the transistor power stage and connected 2 tube mono blocks as power amplifiers.  

 

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20 minutes ago, Edgar said:

 

Travis, by "direct coupled" do you mean "no series capacitors in the signal chain" or do you mean "no output transformers"? If the latter, then I have heard some of the Atma-Sphere OTL designs, and they are wonderful (IMHO).

E.G., Whammerdyne:

 

Direct Coupled – Capacitor-less design. The audio path is completely free of Class AB switching artifacts, Class D high-frequency noise of transitions between output pairs of devices. 

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18 minutes ago, seti said:

 

Yup I was given a schematic by Jeffrey and Dave of EMIA and built their DC IT amp and it is the one I have listened to the most. This one can run 45/2a3 but I want to scale it up and build a 300b/50 version... The power supplies end up being most of the amplifier.. You don't skimp here....

 

 

Ah Seti, my new best friend. 

 

 

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9 minutes ago, dwilawyer said:

E.G., Whammerdyne:

 

 

Got it. The only direct-coupled amps that I have heard were solid state. They are prone to DC offset at the output unless it is actively controlled.

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Some quotes I found interesting:

 

"The design and construction of any valve amp is a very expensive undertaking, and it wise to make as many decisions as possible before buying parts (which are themselves expensive).  Hopefully, the articles so far will be of some assistance, because despite their serious shortcomings compared to modern amps, valve amplifiers can be fun to play with.  There is an almost endless number of things that can be tweaked and will actually make a difference - often for the worse, but that's presumably half the fun."

 

This one got my attention:

 

"All in all, if you are after an amplifier that has minimal distortion (especially intermodulation), provides optimum damping for the loudspeaker, is relatively cheap to build and is comparatively safe and energy efficient, then valve amps are not for you.  Despite all the claims, if you do go to the trouble of building a valve amp that has distortion below 0.1% overall, a reasonable damping factor (at least 20) and has excellent overload performance with plenty of power reserve, it will be extremely difficult to hear the difference between that and a comparable transistor amp.  A good transistor (or MOSFET) amplifier with opamp preamp stages will cheerfully wipe the floor with almost all valve amplifiers, regardless of cost."

 

Obviously he is a SS guy.

 

"There is nothing that a valve amp does that can't be duplicated easily in a transistor amp, with the single exception of steadily rising distortion as power output increases.  While this can be done too, for anything other than a guitar amp there is very little reason to do so, because despite claims to the contrary, distortion in a hi-fi doesn't sound good - it sounds horrible, without exception.  Low distortion (and particularly intermodulation distortion!) are absolute requirements for high fidelity sound reproduction." 

 

PWK would agree on that!

 

 

 

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I can listen to SS but just short term. I've never been able to live with a SS amp for long. Granted valve amplifiers are fraught with issues but why do they sound so f'ing good. The specs and numbers suck by comparison but they sound awesome..... PP or SET.

 

 

"I have to tell you man. In listening to sound, I guess what I'm after is the closest thing that I can get to reality. Now, I know it's not going to be reality, cause the thing gotta go through wires and gotta go through filters and this and that. I understand all that. But what I really like is to get as close to the natural sound of the instruments as possible. That's why I like analog as opposed to digital. Because I don't give a shit what anybody tells you man, I know what you guys are going to tell me...'Oh yeah, but it's clean Ray!' Well it's clean but it don't got no balls!!!" - 1999 interview with Ray Charles

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7 hours ago, seti said:

Yup I was given a schematic by Jeffrey and Dave of EMIA and built their DC IT amp and it is the one I have listened to the most. This one can run 45/2a3 but I want to scale it up and build a 300b/50 version... The power supplies end up being most of the amplifier.. You don't skimp here....

 

So did you order their Iron wrapped in .9999 pure AG to build those with?

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14 hours ago, dwilawyer said:

So did you order their Iron wrapped in .9999 pure AG to build those with?

 

The scary thing is Slagle of Intact Audio fame also wraps them in silver..

http://www.intactaudio.com/tran.html

 

I am getting interstage and outputs from Intact Audio. I can't afford silver but yeah it is .999 copper. Sadly I have a few projects to wrap up before I get to it. I'm rebuilding a Fisher X100, building a 112A Preamp and collecting parts for the PWK Main Amp/crossover. The PWK project is the most important right now.

 

 

 

 

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On 11/30/2020 at 4:22 PM, dwilawyer said:

Thank you for this thread, I was going to start one in a similar vein, but will hop on this one if you don't mind. I will start another one on the other topic I had in mind (the degree to which an amplifier plays in the total picture).

 

Has anyone heard a direct coupled amp or built one? I have, at RMAF, and I wasn't that impressed, but attributed more to the speakers that were hooked up to it.

 

In researching these kinds of amps I ran across this comment/observation: My one qualm with the LSES idea, is that by using a low-value choke and low-value cap- you're creating a resonant circuit that's likely got a corner frequency within the audio band.

I've heard tube amp power supplies with this issue- and my usual complaint, is reduced low bass energy and impact- and what bass is there, tended to be tubby and ill-defined. I've modified such designs, with additional capacitance and/or inductance- getting the resonant frequency below the audio band, and reducing the Q of the resonant circuit to a low enough level to not overshoot- and the bass has almost always improv

 

This is all way over my head, but what is a "resonant circuit"  and a corner frequency? (I get the "within the audio band part).


 

Chokes and capacitors form a resonant, or tuned, circuit at a particular frequency (depending on value) and exhibit a -3 db point at a particular frequency (the corner frequency).  You are familiar with this from speaker crossover networks which provide a chosen corner frequency.  In an amp power supply you can inadvertently choose values which have a corner frequency which is much higher than the lowest frequency you want to reproduce.  You can demonstrate the effect with this calculator (select solving for frequency and the appropriate units):

 

http://www.1728.org/resfreq.htm

 

Try specifying an inductance of 0.3H and a capacitance of 50 uf (typical lses values as I recall) and you will see that the frequency falls in the low bass area.  Compare that with more typical values of, say, 5H and 100 uf.  
 

The above is a very diluted explanation, of course, but should help a little.  I’ll try to explain a bit about Q when time permits.

 

 

Maynard
 

 

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22 hours ago, seti said:

I can listen to SS but just short term. I've never been able to live with a SS amp for long. Granted valve amplifiers are fraught with issues but why do they sound so f'ing good. The specs and numbers suck by comparison but they sound awesome..... PP or SET.

 

 

"I have to tell you man. In listening to sound, I guess what I'm after is the closest thing that I can get to reality. Now, I know it's not going to be reality, cause the thing gotta go through wires and gotta go through filters and this and that. I understand all that. But what I really like is to get as close to the natural sound of the instruments as possible. That's why I like analog as opposed to digital. Because I don't give a shit what anybody tells you man, I know what you guys are going to tell me...'Oh yeah, but it's clean Ray!' Well it's clean but it don't got no balls!!!" - 1999 interview with Ray Charles

 

Ray changed his mind somewhat as digital recording improved... I know I won't be able to find the interview  but I'll try...

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11 hours ago, Marvel said:

 

Ray changed his mind somewhat as digital recording improved... I know I won't be able to find the interview  but I'll try...

 

I'm sure but at what price? My favorite is when I someone talks of a SS as being tubelike as a compoliment... Save your money just use tubes..

 

 

 

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There are some things you gain with digital recording, if you can do the bandwidth. Just the lower noise floor and and zero wow and flutter. The maintenance on a large format RTR gets to be expensive enough.

 

That being said, Bruce Swieden used to record drums on tape and bound to digital for editing. Big fat drums... A lot of the MJ recordings are superb.

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