[Arny Krueger]

"Randy Yates" wrote in message


In the past we were told that negative feedback in an
amplifier (power amplifier) was bad.

It is pretty hard to build an amplifier without at least some local negative
feedback.

Our local Usenet audio group eggspurt on the evils of negative feedback is
an Aussie named Trevor Wilson. He understands that it is hard to build an
amplifier with at least some local negative feedback, and so in the past he
has criticized amplifiers with loop feedback. He represented an Aussie firm
named ME that built power amps with allegedly zero loop feedback. Needless
to say, they had to jump through some oretty expensive hoops to build amps
that way.

So, let's clarify this question to stipulate that it was loop feedback that
is the purported evil.

I believe the old
charge was that it produced excessive "transient
intermodulation distortion."

AKA TIM.

Or, as some have called it, slewing-induced distortion.

AKA SID.

Can someone please explain, using as much
engineering-speak as necessary (i.e., don't sugar-coat it
- assume an audience of electrical engineers) what this
was all about?

Transient intermodulation distortion (TIMD) in a feedback amplifier is
associated with nonlinearity in the stages that precede the forward-path
dominant pole. The forward-path dominant pole is usually introduced into the
amplifier's circuitry to render it stable under a wide range of
configurations and operations. Transient intermodulation distortion is
stimulated by input signals whose slew rate is in excess of that which the
amplifier can amplify linearly.

Transient intermodulation distortion is not necessarily caused by loop
feedback, and engineering design criteria have been established in the
literature to avoid it in audio amplifiers with loop feedback.