I read in sci.electronics.design that Kevin Aylward kevindotaylwardEXTR
wrote (in
i.net) about 'A little feedback worse than none at all?', on Wed, 19
Nov 2003:

I have *already* explained, that yes, if the feedback is *low*,

The proliferation of high-order harmonics is also a characteristic of
large amounts of feedback. They don't disappear as you increase the
feedback: each one is reduced but combinations spawn even higher orders
of harmonics and intermodulation products.

than it
can sound much worse. However, if one gets into the *total* THD/IMD
figures of 0.01%, then feedback is great. End of story.

IF (and it's a big IF) you really achieved 0.01% IMD, then you'd have a
case. But if you get 0.01% THD by putting lots of feedback around a not
very linear amplifier, then you will NOT get 0.01% IMD.

We added a weighted harmonic distortion option to IEC 60268-3 precisely
because IC manufacturers were offering audio chips with this problem;
poor open-loop linearity but lots of feedback. Good THD figures but very
poor IMD. The chip manufacturers refused to accept that IMD was of any
significance (well they would, wouldn't they) so we 'gave' them weighted
THD instead.
--
Regards, John Woodgate, OOO - Own Opinions Only. http://www.jmwa.demon.co.uk
Interested in professional sound reinforcement and distribution? Then go to
http://www.isce.org.uk
PLEASE do NOT copy news posts to me by E-MAIL!