Why does Class A distortion increase with frequency?

[Mooly]

Hello Lumba Ogir and SandyK,
I have built Dougs designs, his blameless Class B being one of them. It was built on the official PCB,s and built EXACTLY to the construction details supplied, no component substitutions at all.
The wiring followed Dougs recommendations also included with the PCB's. It was used with a passive pre amp.
I still have the amp and dragged it out about 6 months ago to have another listen as my listening room had been redesigned and I now had much better speakers B&W703's.
I have to agree with your comments-- I found it "boring", there was no "soundstage" it was all two dimensional, instruments were "cold" no warmth or vibrancy. Going back to my MOSFET design was truly a revelation.
I would further add that in my experience designs like Dougs, and there are many that use similar topologies, do have this "same sound" about them.
Without wishing to detract from Dougs work-- and his knowledge of the subject is second to none I believe-- my conclusion is that technical perfection is not the whole answer to that "Magic" factor that some designs seem to have.
Regards Karl

[sandyK]

Mooly
Do you have a schematic of the actual amplifier you constructed, or does it appear in Issue 2 of the D.S. book ? If so, where ?

My email address is alexkethel@optusnet.com.au

Several DiyAudio members from Sydney have been able to make kits based on the D.S. topology really sing, with large 3D soundstages and a natural sounding warmth. Especially the Class A variants, however, Class AB versions can also be made to sound markedly better.

SandyK

[jacco vermeulen]

Originally posted by tcpip
is crossover distortion really the biggest source of distortion by far in a well-designed Class B amp?

Take a look at the Class AB power amp designs that do not include the output stage in the NFB loop.

[tcpip]

Originally posted by Nelson Pass
1) Possibly - I haven't thought about it that much because it's not
particularly important to me. My definition of a well designed amp
is a little different than Self's.

Accepted. We all know Self's approach is primarily driven by quantitative measurements.

2) If you mean Class B (with no bias at all), generally yes.

I don't know about the "no bias at all" bit. I meant a Class B amp with bias carefully adjusted to minimise THD at a few Watts output. This means there's some bias current flowing through the OPS devices, right? Is that what you mean by "no bias at all"? If yes, then that's what I meant too.

3) Take a look at the article "Leaving Class A" at www.passlabs.com.

Found the PDF, am reading it. Thanks a lot.

4) Yes, as I mentioned previously.

5) Yes, but it varies depending on the design.

I'll read your article and then return to this discussion. Thanks a lot. I'm pretty sure there's stuff in there I'll find very interesting.

[traderbam]

Originally posted by sandyK
In my experience they sound fine,but unexciting, with a relatively small soundstage. They are very clean sounding, which some people may not like. Neither do they have an artificial warmth.
SandyK

Originally posted by Mooly
I have to agree with your comments-- I found it "boring", there was no "soundstage" it was all two dimensional, instruments were "cold" no warmth or vibrancy. Going back to my MOSFET design was truly a revelation.

And yet....

Without wishing to detract from Dougs work-- and his knowledge of the subject is second to none I believe--

And so...

...that "Magic" factor that some designs seem to have.

What's the alternative conclusion that doesn't require magic?

[Mooly]

Whats the alternative conclusion --- that's easy --- Listen to an amp that you really enjoy. One that makes the music "come alive"
The conclusion is that many people don't seem to like "the piece of wire with gain" in much the same way that musicians prefer one instrument over another. An old Stradivarius or Testore wasn't designed with computer simulation, yet a cheap modern instrument probably was, and is probably technically superior. But which would you rather play ??

[tcpip]

Originally posted by Nelson Pass
3) Take a look at the article "Leaving Class A" at www.passlabs.com.

Read the article. It was an eye-opener. I didn't know Class A OPS had such low open-loop distortion. I think I'll go away and think some more about this entire Class A HF distortion thing.

What measured closed-loop distortion would you get with a high-bias Class A, if you applied a lot of GNFB? Do you have any distortion graphs for such cases handy? I know you don't prefer such amps, but I'm just curious to know what the "numbers" look like.

[traderbam]

Mooly,
No, no. Keep your wits about you. You are drifting into AKSA territory now.
You listened to Self's design and found it lacks magic. You assumed Self is a leading expert at making "technically" accurate circuits. Therefore, you've concluded, that technically accurate circuits don't sound great.
Are you sure?

[sandyK]

Not so long back, Silicon Chip magazine published a 20W/Ch Class A design which they stated was based around the work of Douglas Self. They quoted a typical distortion figure of .0006% at around 10W. (20HZ -20KHZ) This was WITHOUT close matching of devices.

SandyK

[tcpip]

Originally posted by sandyK
Not so long back, Silicon Chip magazine published a 20W/Ch Class A design which they stated was based around the work of Douglas Self. They quoted a typical distortion figure of .0006% at around 10W. (20HZ -20KHZ) This was WITHOUT close matching of devices.

Wow. That's impressive measured performance at 20KHz. Found the article. Will read more details at leisure. Thanks for the pointer. From some comments in some forums, it seems it's basically Self's Blameless, biased into Class A the way he'd demonstrated it in his book. Is this the case?