I read in sci.electronics.design that Kevin Aylward kevindotaylwardEXTR
wrote (in
et) about 'A little feedback worse than none at all?', on Sat, 15 Nov
2003:

Thing is, I've come across a reference to a study done by P. J.
Baxandall in the '70s, in which he showed that using less than 15dB
or so of feedback can actually INCREASE the generation of
higher-order distortion products.

ANY amount of feedback causes this;

Ahmmm, a bit misleading john. If the feedback is large enough,
essentially, all distortion will be reduced in practise.

No, not misleading at all. You should read more carefully. PJB was
quoted as:

INCREASE the generation of higher-order distortion products.

Note 'higher-order'. Your explanation below actually shows how
intermodulation between low-order harmonics (and including the
fundamental) creates higher-order ones that wouldn't be there if there
were no feedback!


The deal is that with *low* feedback, a pure square law device will have
its input mixed with its output causing generation of 3rd harmonics, and
higher, which were not present in the original amp. However, if the FB
is large enough, the resulting distortion can easily go to 0.01%.
Sure, if the original device was *pure* square, than 0.01% 3rd, is still
larger than zero. The reality however, is that all practical devices
have odd harmonics as well, such that large feedback will result is a
net total reduction of the original distortion at all harmonics of
relevance.

What is *relevant* is the nasty noise created by the high-order
intermodulation distortion. Because some intermodulation products due to
different orders add in-phase, the intermodulation products can be much
larger than the harmonics. And, being unrelated harmonically to the
fundamental, they sound much worse than even high odd-order harmonics.

Attempts at weighting harmonics in terms of their subjective nastiness
has more or less settled on weighting by the square of the order. See
IEC/EN 60268-3, although this weighting was proposed (by E R Wigan,
IIRC) long before it was accepted into the standard. Naturally, weighted
harmonic distortion doesn't appear in manufacturers' specifications,
because the figures are often unattractively large.
--
Regards, John Woodgate, OOO - Own Opinions Only. http://www.jmwa.demon.co.uk
Interested in professional sound reinforcement and distribution? Then go to
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