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

[Magnus Danielson]

From: Don Tillman <don at till.com>
Subject: Re: Compensating multi-stage feedback (was: RE: all tranny vca+ )
Date: Sun, 2 Jul 2000 18:44:29 -0700 (PDT)

>    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.

I just wanted to point out this effect too. Let's not forget that this is part
of the full set of methods to break the signal quality.

> [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.

Right. Good point. I would classify it as being a component non-linearity but
the example is certainly one of those which would require specific attention.

>    > 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.

Certainly. Maybe we should tag it up and later investigate on it, OK?

>    > 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.

Well, changing the bias point is something that one has to consider as part of 
the total equation, if it is a good or a bad thing is another issue.

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

Well, at times off topic on this list can be rather rewarding.

Hey, look! We discuss and do not fight! ;)

Cheers,
Magnus