Class-A power amplifier with variable power supply

[davorin]

Has someone seen a schematic of a Class-A audio amplifier with constant Vce voltage?

I remember seeing 20 years ago a schematic of a commercial power amplifier where the output stage was class-A and the power supply driven by a Class-AB stage,
there having a constant Vce across the output stage of the class-A amplifier (think this also reduces distortion even more).

Not sure if it was a Yamaha amplifier...


As I just googled...think it's called Class-G now....there wasn't even Class-D back then (o;

[dannyf]

Easy: power the class A amp through a Class B amp whose inputs are tied together (with some offset obviously).

[davorin]

Seems to be called Class-H, not Class-G...man I've been long time absent into audio design (o;

Now comes the time consuming period to evaluate the right high-quality transistors ;-)

Just read a paper from Yamaha:

https://www.yamahacommercialaudiosystems.com/downloads/tech_docs/data/white_papers/yamahapoweramp_whitepaper_en.pdf

Though using Class-D for the power rail? Definitively a bad idea....though sure high efficiency...

[davorin]

As I've been now long time absent...are there any reading materials or even books regarding audiophile amplifier designs?

When I was in the apprenticeship I ordered shitloads of service manuals from high end products to study their design...
Like the famous Stax preamplifiers....though never figured out where they got their perfectly matched N/P channel JFET transistors from...normally P-channels are much worse than their N-channel counter parts...

[nanofrog]

As I've been now long time absent...are there any reading materials or even books regarding audiophile amplifier designs?

Might want to give these a look:
Audio Power Amplifier Design, by Douglas Self
Audio Power Amplifier Design Handbook, Douglas Self (older version)
Designing Audio Power Amplifiers, Bob Cordell
High-Power Audio Amplifier Construction Manual, by G. Randy Slone
The Audiophile's Project Sourcebook: 80 High-Performance Audio Electronics Projects, G. Randy Slone
INTRODUCTION TO ELECTROACOUSTICS AND AUDIO AMPLIFIER DESIGN, by W. Marshall Leach

Another person to look out for, would be Nelson Pass (amp designer). Here's a few links (best part, these are free):
PassDIY
First Watt
Pass Labs

[davorin]

Thanks for the great pointers...especially Nelson Pass...

Feels like coming home again after over 20 years when reading it (o;

Really good distraction doing all the digital stuff the past 10 years ;-)

[Mark Hennessy]

I have all those books (apart from the last one from Leach). Really, Douglas Self's work stands head and shoulders above the rest of them - his work is the most detailed, rigorous, original and insightful by quite some margin. All backed up with detailed measurements, and of course, he has put his designs into volume production. His other books are gold-mines as well. I much prefer his writing style too.

[nanofrog]

I have all those books (apart from the last one from Leach). Really, Douglas Self's work stands head and shoulders above the rest of them - his work is the most detailed, rigorous, original and insightful by quite some margin. All backed up with detailed measurements, and of course, he has put his designs into volume production. His other books are gold-mines as well. I much prefer his writing style too.

FWIW, Dr. Leach (now deceased), taught an audio course at Georgia Tech (Curriculum Vitae).

If you've not had the chance, you might want to take a look at Nelson Pass's articles as his designs have been manufactured since the '70's (wiki bio).

[Mark Hennessy]

FWIW, Dr. Leach (now deceased)

Oh - I didn't know that. What a shame. RIP.

The "Leach Amp" was one of the first DIY audio projects I ever saw on the internet - over 15 years back now.

If you've not had the chance, you might want to take a look at Nelson Pass's articles as his designs have been manufactured since the '70's (wiki bio).

Yes, I'm very aware of NP, and think it's great that he contributes so much to the DIY community.

His philosophy is one of "nice" distortion, rather than low distortion. Engineering a circuit to give the right spread of harmonics while managing other parameters like clipping behaviour is quite a skill, but personally I'd much rather reduce distortion to below what I can easily measure. My day job is in professional audio, and out of respect to the artists and the engineers who put the recording together, I view the hi-fi amplifier as something that should faithfully reproduce the recording, not a device to add to the performance. But that's just my view, of course

[nanofrog]

Oh - I didn't know that. What a shame. RIP.
Happened back in 2010 at the age of 70.

The "Leach Amp" was one of the first DIY audio projects I ever saw on the internet - over 15 years back now.

Happened back on the 20th of November, 2010.

Found a few things if you're curious:
Obituary (local news paper)
Longtime professor passes away
Remembering W. Marshall Leach Jr. (and his low-TIM amp)
Alumnus, Longtime Professor Marshall Leach (Georgia Tech Alumni Magazine)

Here's a page on the ECE4445, Audio Engineering course he taught.

Yes, I'm very aware of NP, and think it's great that he contributes so much to the DIY community.

His philosophy is one of "nice" distortion, rather than low distortion. Engineering a circuit to give the right spread of harmonics while managing other parameters like clipping behaviour is quite a skill, but personally I'd much rather reduce distortion to below what I can easily measure. My day job is in professional audio, and out of respect to the artists and the engineers who put the recording together, I view the hi-fi amplifier as something that should faithfully reproduce the recording, not a device to add to the performance. But that's just my view, of course

Ah, didn't know your familiarity with Nelson Pass. 

I'm only into it as a hobbyist rather than a professional audio designer, but I'm of the similar/same philosophy (recreate source material 100% without adding, removing, or changing anything). 

[Mark Hennessy]

Thanks for the links. As I say, what a shame.

I'll make more of an effort to pick up his book. Unfortunately, it's very hard to find for a sensible price.

[nanofrog]

I'll make more of an effort to pick up his book. Unfortunately, it's very hard to find for a sensible price.

You're welcome. 

Unfortunately, all of his books are expensive. Even on abebooks.com, which is usually really good on pricing.

[akis]

Has someone seen a schematic of a Class-A audio amplifier with constant Vce voltage?

Can I ask why?

Class-A means the output device(s) are conducting for the whole cycle of the signal. Typically used for least distortion.

Constant Vce means that the hfe remains constant because if I remember correctly hfe is affected by Ic, Temperature and Vce in that order. But if Vce is constant then something else, eg the supply rails, has to increase/decrease in perfect unison with the signal. That means additional active devices in the output path - which negates the class A approach.

[davorin]

That's the trick....I remember seeing a schematic where Vce of the Class-A output stage was kept at a constant of 5V....and the power rails were feed by a another Class-AB stage...

Basically the output of the Class-AB stage was connected to ground of the +/-5V supply..therefore lifting the power rails continuously according to the audio input...

[dannyf]

I remember seeing a schematic where Vce of the Class-A output stage was kept at a constant of 5V

That has been given to you: your power supply is essentially an amplifier.

[davorin]

Exactly ;-)

The Class-AB amplifier is a power supply controlled by the same audio input....and a second power supply with constant 5V is used as offset so the Class-A transistors never are in the switched-off state...

I've also seen a design (think from Yamaha) where the output stage has two cascaded transistors on each side, for the positive and negative rail...
And the top/bottom transistors have a constant offset set by a 5V6 zener diode....

Damn...where did I hide my old schematics/service manuals (o;

[oPossum]

I remember seeing 20 years ago a schematic of a commercial power amplifier where the output stage was class-A and the power supply driven by a Class-AB stage,

I don't recall anything exactly like that, but Nelson Pass' STASIS series, and the Carver 'Magnetic Field' series have some similarity.

The STATIS uses a low current class A amp with a high current class B bootstrap.

The Magnetic Field amps use a PCR (phase control regulation - like a light dimmer) power supply to provide a variable voltage to the class B output stage

There where also Soundcraftsmen Class 'H' amps with a commutating (switching between 3 fixed rails) supply to the class B output.

[GK]

It was Technics and they called it Class A+. It is a different concept to class G or H. A class B amplifier drove the ground of a floating low voltage power supply for the power output stage of class A amplifier. Both the class B amplifier and the class A amplifier have the same closed loop voltage gain and are driven by the same input signal. Out of curiosity I have actually built a class A power output stage driven by a class AB amplifier that did 1000W into 2 ohms. Bias current for the class A output stage was 16A. I have since used the 6.3V transformers for the low voltage floating power supply for something else. I lost incentive to finish the project after posting preliminary details of the design and getting absolutely inundated with crap emails. However the rest of the bits are sitting under the bench. It's unfinished project #~368. I might get back to it one day. I have since devised a stable n-th order frequency compensation scheme. That, combined with the class A+ concept could make truly OTT power amplifier.

[Circlotron]

Where all the big boys hang out -> http://www.diyaudio.com