Propeller Head Plaza Technical and scientific discussion of amps, cables and other topics.

---

[ Follow Ups ] Thread: [ Display  All  Email ] [ Propeller Head Plaza ]

---

[ Alert Moderator ]

---

Re: On a different note: Response to Morricab on Amplifiers

Posted by morricab (S) on February 12, 2007 at 02:37:46

In Reply to: Re: On a different note: Response to Morricab on Amplifiers posted by theaudiohobby on February 11, 2007 at 15:36:19:

Well since I can't get a copy of the article for Hifi News (it isn't even available yet on their website), I went in search of the man Jim Lesurf. I found a surprising number of intersting articles on his websites.

One that shows that music has lots of assymmetrical waveforms:
http://www.st-and.demon.co.uk/AudioMisc/asymmetry/asym.html

Now he also has an article on Asymmetry in amplifiers on his site (I assume a sort of precursor to what he published in hifi news):
http://www.st-and.demon.co.uk/AudioMisc/distortion/page1.html

What I find interesting here is that this discussion of Asymmetry centers around push pull or complementary amplifiers and not single ended amplfiers.

Here are some interesting comments from this article:

"The purpose of what follows is to outline some of the amplifier effects which might need to be probed to identify non-linearities which affect musical reproduction. Then go on to consider an alternative method that might provide measured results more relevant to musical performance."

"In general, musical waveforms often have the following medium-term and short-term properties:
The frequencies involved tend to have a relatively simple harmonic (or near-harmonic) relationship.
The amplitude tends to vary, with brief transient events appearing as well as a steadier level.
The waveforms often show distinct asymmetries
The form factor (peak to mean level ratio) tends to be somewhat higher than for a sinusoid, even during the body of a sustained note.
"

This is his argument why using sine waves or even square waves to measure distortion behavior in amplifiers is ultimately inappropriate.

"It is a common observation in the audio world that “Class A” amplifiers often “sound better”. Similar claims are often made for valve (tube) amplifiers. There may be various reasons for these beliefs. However it also seems to be the case that some amplifiers that are neither Class A nor employ valves are also sometimes judged to “sound good’’."

"The diagram above shows a pair of transistors used as an output stage which I’ll assume is “Class AB” with negligable quiescent current...This means that if these rail fluctuations couple or influence the amplifier output they may well introduce distortion. Ideally, no such unwanted couplings will occur. However in some cases, when the signal waveforms are symmetric, the coupling contributions from the positive and negative rails might tend to cancel each other...The consequence may be that – when testing with simple symmetric waveforms like a single sinewave – these various distortion contributions largely cancel, hiding the effects from detection. The consequence may be that there is a risk when developing or testing using simple symmetric waveforms that we get a low apparent non-linearity value, but when asymmetric waveforms are present, higher levels of overall distorion may arise."

"The key point to draw from the above examples is that Class AB designs can have various distortion mechanisms whose contributions may depend upon the details of the signal. In particular, upon its symmetry. Tests using entirely symmetric waveforms may not expose these problems in a specific design. Class A systems may be able to avoid some significant signal distortion mechanisms, but this may not be evident when comparing various amplifier topologies simply using sinewave tests. Hence employing symmetric waveform tests alone may not help us assess the relative merits of the different amplifier arrangements."

This is about your typical Class AB feedback amplifier. As you can see the normal hifi amp also will produce "anharmonic" distortions with real music. Likewise, it will produce its own special kind of distortion called Crossover distortion. This is MOSTLY eliminated with Class A amps but not completely because matching of transistors is never perfect. According to Lesurf it is not fixable with negative feedback.

Here is a more detailed analysis of one possible cause for this asymmetry:

http://www.st-and.demon.co.uk/AudioMisc/distortion/page3.html

His comments on Negative feedback:

"That the main limitations are of two kinds
The risk of oscillations or instability
Class B type distortions may not be reduced
We can use feedback with care to improve the performance of an amplifier, but it should be clear that although some feedback helps it is no substitute for designing or using an amplifier whose open loop behaviour is as good as possible. "

"In practice the situation where this arises is when we have used a pure class ‘B’ amplifier which will exhibit cross-over distortion. In these cases the feedback will reduce the distortion level for reasonably powerful signals, but small signals will experience high levels of gain error, and hence high distortion even when feedback is used. For this reason class ‘B’ should be avoided – especially when we are dealing with small signals – if we wish to avoid distortion."

"so if we allow the system to be ‘near’ oscillation we may find that it does oscillate under some circumstances when certain loads are attached...An amplifier system which has a flat, unpeaked, response, with no risk of unwanted oscillations irrespective of the output load is said to be Unconditionally Stable. For obvious reasons this is highly desirable if it can be achieved...To avoid these problems and ensure stability we have to adopt two approaches. One is to build an amplifier that works as well as possible without feedback, and then only apply a moderate amount. (Note that the terms “well as possible” and “moderate” here are matters of personal judgement not strict engineering.)"

If the amp is stable enough and distortion low enough then none is likely the right amount.

I have seen several measurements in Stereophile where amps exhibit crossover distortion and according to Lesurf this is not fixed with negative feedback.

It appears that PP amps have problems with asymmetric behavior as well, particularly if they are not running true Class A, which is nearly all of them.

This post is made possible by the generous support of people like you and our sponsors:

Atma-Sphere Music Systems, Inc.

---

[ Top of Page ] [ Contact Us ] [ Support/Wish List ] [ Copyright © 2025, Audio Asylum, all rights reserved ]

---

[ General ] [ Speakers ] [ Tubes ] [ Vinyl ] [ Digital ] [ Hi-Rez ] [ Video Asylum ] [ Cables ] [ Tweaks/DIY ] [ Music ] [ Films ]

---