Compensating multi-stage feedback (was: RE: all tranny vca+ )

[Don Tillman]

   Date: Sun, 02 Jul 2000 22:28:24 +0200
   From: Magnus Danielson <cfmd at swipnet.se>

[Mixers...]

   I recall the issue about having capacitors in the signal path. 

I hear a more obvious effect than I'd expect from caps, but yeah,
it's entirely possible.

[The list...]

   Thanks! Note, the above list has very broad covering points and you
   must pin down each specific mechanism under them. I am sure we can
   come up with many, many, many of them. Actually, I think we
   _should_ come up with a lot of them and discuss each of them.

   > Here are my contributions:
   > 
   > Effects of junction capacitance varying with signal voltage

   Good point, covered by the linearity point

In some ways yes, there's a particular effect here though...  If you
look at the output of a power amplifier on a scope, let's say you're
driving it with a sine, and at one point in the cycle you see a little
"blotch" in the curve, the odds are that the junction capactance of
one of the output devices is changing dynamically with the waveform,
and at that one voltage the capacitance is such that the amp bursts
into oscillation.  Or near oscillation, depending on the "blotch".

I'm just pointing out a specific mechanism that happens a lot in power
amps.  It's not your typical nonlinearity.

   > Power supply interaction

   Uh, _what_ power supply interactions do you intend?
   Yeah, power supplys can surely mess things up.

I was primarily thinking of the effects of the effective series
inductance created by the major compensation pole in regulator chips.
There was some discussion a while back in high end audio circles about
evil effects of regulator chips, but I didn't follow it very closely.
I'm sure there are other effects.

   > Effects of bias changes changes with waveform transients

   Good point, covered by the linearity point.

I was also thinking of one particular effect here.  In a tube guitar
amp, a sudden transient will cause the grid to conduct on the positive
peak, charging the coupling capacitor, and changing the bias until the
capacitor has had a chance to discharge.  Eric Pritchard claims in
his tube emulation patents that this is an important part of the
characteristic tube guitar amp sound.  I don't know if I believe that
or not.  I know from personal experience that an extreme case of
coupling cap charge sounds awful, the signal can actually cut off
after a transient, but I can't say I've studied more subtle cases.

I'm sure I've gone way off topic at some point here.  :-)

  -- Don

-- 
Don Tillman
Palo Alto, California, USA
don at till.com
http://www.till.com