ra-lif
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Откуда: Омск (07.11.08 05:33)
s.gerus, попробуйте написать ЭТО на другом компе - может быть тогда это кто-нибудь прочитает
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http://www.passdiy.com/pdf/articles/distortion_feedback.pdf
Audio, Distortion and Feedback
Nelson Pass 11/1/08
Introduction
Audiophiles seem to revel in minor controversies – vinyl vs CD's, tubes versus solid state,
capacitor, wires, magic dots... and negative feedback.
At one extreme, the position is that “feedback makes amplifiers perfect”. At the other
extreme, “feedback is a menacing succubus that sucks the life out of the music, leaving a
dried husk, devoid of soul”.
The former viewpoint usually belongs to so-called “objectivists” who have a fine appreciation
for electronic theory and measurements. Their opposites would be the “subjectivists” who
emphasize the listening experience and often own tube amplifiers.
....
The problem is greatly compounded when complex signals consisting of many frequencies all
travel through the gain device at the same time, or when a simple signal is passed through a
number of nonlinear gain stages in series. Of course you can do both, and as we'll see later,
these can add up to a perfect storm of distortion.
....Harmonic Distortion and Sound
Many audiophiles believe that 2nd harmonic is to be preferred over 3rd harmonic. Certainly it is
simpler in character, and it is well agreed that orders higher than third are more audible and
less musical. However when given a choice between the sound of an amplifier whose
characteristic is dominantly 2nd harmonic versus 3rd harmonic, a good percentage of listeners
choose the 3rd.
I have built many examples of simple 2nd and 3rd harmonic “types” of amplifiers over the last
35 years. When I say “types” I mean that they used simple Class A circuits described as
“single-ended” versus “push-pull” and so tended to have a 2nd harmonic versus 3rd harmonic in
the character of their distortion, but were not made to deliberately distort.
Anecdotally, it appears that preferences break out roughly into a third of customers liking 2nd
harmonic types, a third liking 3rd harmonic, and the remainder liking neither or both.
Customers have also been known to change their mind over a period of time.
However the issue is partially obscured by the fact that the 3rd harmonic type amplifiers
usually have lower total distortion. Third harmonic usually appears with a negative coefficient,
resulting in what we think of as “compressive” - the example in figure 3. It's worth noting that
odd orders on nonlinearity also can be seen altering the amplitude of the fundamental tone
-something a distortion analyzer doesn't ordinarily display.
Audiophiles have been accused of using 2nd or 3rd harmonic distortion as tone controls to
deliberately alter the sound. I suppose that there are people who like it that way, but I don't
think this is generally the case. For reasons which will become clearer when we talk about
inter-modulation distortion, high levels of any harmonic become problematic with musical
material having multiple instruments, and the argument that 2nd or 3rd adds “musicality” doesn't
quite hold up.
The sound of 2nd order type circuits is often praised as “warm” and by comparison 3rd order
type circuits are often noted for “dynamic contrast”. 2nd order type amplifiers seem to do
particularly well with simple musical material, and 3rd order types generally seem to be better
at more complex music. Figure 4 shows a distortion curve of two power stages operated
without feedback – the blue is single-ended Class A, the red is a push-pull Class A.