|
Propeller Head Plaza Technical and scientific discussion of amps, cables and other topics. |
|
|
|
Propeller Head Plaza Technical and scientific discussion of amps, cables and other topics. |
|
In Reply to: Re: On a different note: Response to Morricab on Amplifiers posted by theaudiohobby on February 15, 2007 at 02:43:18:
"The transfer curve of a complementary P-P circuit is symmetric"Perhaps only with a symmetric waveform as demonstrated by Lesurf. With real music there is a very real chance of the output stage of a Class AB amp to become asymmetric and produce crossover distortion due to bias drift. Who cares about "with a perfect signal"? We are talking about real world performance.
Look, I haven't read the article and I will assume, from reading Lesurf's other articles, that the man knows what he is talking about to the point that the asymmetry exists.
My point of looking at the other articles is simple, other amp topologies also can suffer from asymmetry, especially the most common amp of all the Class AB complementary transistor amplifier. The fact that it is signal correlated is worrisome. I think I have sucessfully made this point using Lesurf's article as a point of reference (same as you are using the hifi news article by the same author).
However, the question now becomes: which asymmetry is more obnoxious? Everyone knows that crossover distortion, even in very small amounts, is particularly bad sounding and engineers like Bob Cordell and John Curl both consider it to be the most important to eliminate because it can't be really treated with feedback (I read this from both of them posting in DIYaudio forum). Crossover distortion consists of lots of high order components and often gives the sound an edge or sterile sound. It can also make the highs sound grainy and low level resolution "veiled". What does the asymmetry of the SET produce? Does Lesurf say?
If you look at the the measurements of decent measuring SETs, like the one I showed further up in the thread, you don't see any anharmonic components in the 1Khz FFT spectrum...at least above the noise floor. You see neat multiples of the fundamental and only low order (at 10 watts there are some higher order as well but very low in level). So if there are anharmonic components, they are far lower in level than the harmonic components, which are not at such a harmful level to begin with.
Now you also see neat multiples in Class AB amps but of course the test is with a symmetric waveform. Put an asymmetric waveforms through the amp and maybe the tidy distorton spectrum changes as crossover distortion becomes a factor. Put the same asymmetric waveform through a no feedback SET and maybe it behaves the same.
There are other forms of asymmetry related to amps with a high negative feedback. One relates to what Otala described where back EMF from the speaker can go through the feedback loop and get reamplified. This will be highly asymmetric as the original signal gets heavily distorted by the speaker before going back into the amp and then being reamplified. Not only that it heats up the output stage transistors asymmetrically, especially with highly reactive speakers.
I know of one tube amp manufacturer who does Class A triode differential amplfiers. It is basically like push/pull but he has a special circuit that keeps dynamic track of the bias of the two tubes to keep them VERY close together. He told me that one of the reasons why push/pull tube amps don't sound as good as SETs at low levels was because of this bias mismatch. To this day his is the only PP like amp I have heard that has the low level resolution to go toe to toe with the best SET amps. Oh he uses no feedback, btw.
This post is made possible by the generous support of people like you and our sponsors:
Follow Ups
- Re: On a different note: Response to Morricab on Amplifiers - morricab 07:57:13 02/15/07 (0)
Top of Page ]
[ Contact Us ]
[ Support/Wish List ]
[ Copyright © 2024, Audio Asylum, all rights reserved ]
