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

[Don Tillman]

   Date: Fri, 30 Jun 2000 18:49:53 +0200
   From: =?iso-8859-1?Q?Ren=E9?= Schmitz <uzs159 at uni-bonn.de>

   The FET circuits we've seen lately were mostly not feedback (at least not
   over several stages), but still have very small THD. Devices with a square
   law characteristic (triodes, FETs) allow it to cancel most of the even
   order harmonics by symmetrical or push pull arrangements. 

Sometimes square-law devices *unintentionally* cancel their
distortions without feedback.

Consider the following very typical preamp topology:

  input -> gain stage -> volume and tone controls -> gain stage

You'll see this in most tube hifi and guitar amps.  If the gain stages
are inverting, square law, and have a gain of around 30 or so, the
square-law distortions from the first stage are mostly canceled out by
the square-law distortions in the second.  

Pretty cool, eh?

   IMO much of the "transparency" and "resolution" that is attributed
   by audiophile circles to these kind of circuits has to do with the
   absence of multi stage feedback.

I agree with this completely.

 -- Don

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