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In Reply to: Re: On a different note: Response to Morricab on Amplifiers posted by morricab on February 13, 2007 at 02:59:04:
You seem to have your wires crossed with the basics and such makes me wonder why hold such a bigoted view of amplification."Well for starters crossover distortion is an Anharmonic type of distortion that is mostly non-existent in Class A and totally non-existent in single ended amplifiers."
This statements is wrong way round, a Class A circuit by definition cannot have crossover distortion and since a single-ended amplifier is necessarily Class A, the phrase "mostly non-existent" is effectively meaningless.
Moving on, a Class AB circuits are symmetric not asymmetric, the article you cited simply notes that asymmetric signals are capable of exposing more distortion than a symmetric signal. Crossover distortion is easily suppressed by correctly biasing the circuit, with good circuit design, crossover distortion can be suppressed to vanishingly low levels, i.e. levels at which it is barely measureable.
"Again, to whom? The oscilloscope or the listener. Linearity for the sake of linearity is NOT the goal. Audible distortion should be minimized not ALL distortion, the elimination of which is impossible with inherently non-linear amplification devices. "
What do you expect me to say to this comment? At any rate, it is an irrelevant here and a non-sequiter in the context of this thread. I started out by saying that zero feedback single-ended amplifiers are inherently less linear as they suffer from more distortion products and at higher level than P-P circuits with feedback, you raised the issue of crossover distortion in Class AB, however that is easily addressed with good design. So as it stands, I do not see that you have added anything to the discussion so far that undermines my original comments.
Music making the painting, recording it the photograph
Follow Ups:
"Class A circuit by definition cannot have crossover distortion "Wrong again. Sure a single ended Class A amp cannot have crossover distortion...its single ended but a complementary design can still have some distortion at zero crossing if the transistors are not perfectly matched (and they are never perfectly matched). If they were perfect complements of each other and running true Class A then there would be no crossover distortion but we are talking about real amplifiers, right?
"Moving on, a Class AB circuits are symmetric not asymmetric, the article you cited simply notes that asymmetric signals are capable of exposing more distortion than a symmetric signal. Crossover distortion is easily suppressed by correctly biasing the circuit, with good circuit design, crossover distortion can be suppressed to vanishingly low levels, i.e. levels at which it is barely measureable."Wrong again. Please look at page 3 of the article.
http://www.st-and.demon.co.uk/AudioMisc/distortion/page3.html
He states that asymmetric waveforms can heat transistors differently for each half of the wave cycle leading to dynamic differences in their bias. This leads to asymmetry in the output devices thus crossover distortion. I don't know what effects lead to asymmetry in SET amps or amps without feedback but it seems that music itself can lead to asymmetry in complementary Class AB amplifiers and that asymmetry is distortion. Bipolar transistors are especially temperature sensitive.
So the point is simple here, even with a PERFECTLY matched class AB design that measures Perfect with sine waves it will likely have distortions with real music due to asymmetry of the bias of the devices.
"Wrong again. Sure a single ended Class A amp cannot have crossover distortion...its single ended but a complementary design can still have some distortion at zero crossing if the transistors are not perfectly matched (and they are never perfectly matched). If they were perfect complements of each other and running true Class A then there would be no crossover distortion but we are talking about real amplifiers, right?"Okay.
"Wrong again. Please look at page 3 of the article.
http://www.st-and.demon.co.uk/AudioMisc/distortion/page3.html
He states that asymmetric waveforms can heat transistors differently for each half of the wave cycle leading to dynamic differences in their bias. This leads to asymmetry in the output devices thus crossover distortion. I don't know what effects lead to asymmetry in SET amps or amps without feedback but it seems that music itself can lead to asymmetry in complementary Class AB amplifiers and that asymmetry is distortion. Bipolar transistors are especially temperature sensitive.
So the point is simple here, even with a PERFECTLY matched class AB design that measures Perfect with sine waves it will likely have distortions with real music due to asymmetry of the bias of the devices."
You have your knickers in a twist, Certain non-linearities, such as what you have noted cause a departure from symmetry (see the previous page of the link you posted", however the The transfer curve of a complementary P-P circuit is symmetric. On the other hand, the transfer curve of a SE Class A is asymmetric, non-linearity simply results in yet more asymmetric distortion, its asymmetric transfer curve means, that is it unable to cancel out any distortion components, harmonic or anharmonics, in a zero feedback design, the non-linearity is even worse, as the open loop linearity is simply nowhere near what is achievable with a closed loop design. This where LeSurf original article comes in, a single-ended design is inherently less linear than a P-P design if both both are run in Class A, the SE with zero feedback even less linear than than an equivalent than equivalent P-P design with negative feedback, musically complex signal introduce new anharmonics components that an asymmetric design cannot cancel out. As you note, a Class AB circuit introduces the possibility of a new distortion products, however good design can reduce crossover distortion to barely traceable levels. Also a good design will ensure that a design the active devices are run at optimal temperature. At any rate, I saw a funny link for you to peruse, which mentions the some of the distortion products that may contribute to the 'unique tone' of zero feedback SE amplifiers. Be that as it may, the original still, a zero feedback SE amplifier is definitely less linear than a complementary Class A P-P circuit with negative feedback.
"So the point is simple here, even with a PERFECTLY matched class AB design that measures Perfect with sine waves it will likely have distortions with real music due to asymmetry of the bias of the devices. "
Strictly speaking, this is a non-sequiter issue and the statement is not clear at all, at least since the document you quote a very important assumption that is "quiescent bias current level of the AB output is small enough to be ignored" in all scenarios, firstly because Class AB circuit with negligible current defeats the whole point of having a Class AB circuit in the first place, in a decent Class AB, these distortions mechanisms (which includes crossover distortions) will be vastly reduced, because of the optimal biasing, for one bias current is never negligible.
Music making the painting, recording it the photograph
"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.
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