Jon Yaeger wrote:

Brian,

This is a long shot . . . the positive feedback may be there to increase
gain (doubtful?) but maybe it's a form of "pre-distortion" if it occurs 180
degrees out of phase from the output. The purpose of "pre-distortion" is to
lower overall distortion, but the audiophool in me shudders from the very
concept.

Erno Borbely employed it in an article in Glass Audio he wrote about an SE
design (1996, Vol.8, No. 5).

Just a guess . . . .

I'd like to see a schematic of this amp posted at ABSE or somewhere,
so we could see it.

The error signal contains the inverse of the distortion in the output.
So it could be applied in some way to a preceeding gain stage,
and thus cancel the natural open loop distortions of the amp.
Perhaps it ain't as wacky as it seems.
But not an audiophile solution, more that of a pure engineer,
ie, a mean and tricky solution to a tecnical problem.
I wonder how stable this amp is with no R load at the output,
and just a pure capacitance, say 0.22 uF, which can make many
FB amps oscillate strongly at some F above the AF band..

Patrick Turner.




Jon

From: (Brian)
Organization: http://groups.google.com
Newsgroups: rec.audio.tubes
Date: 24 Nov 2003 21:44:18 -0800
Subject: Positive Feedback in P-P 6V6 Amp

Let me clarify the circuit: The negative feedback from the
output-transformer secondary goes to the stage that precedes the phase
splitter, in the usual way. The positive feedback that originates at a
6V6 grid goes to the stage that precedes the stage where the negative
feedback is applied. The loops are not nested. This is why I mentioned
that the positive feedback applies a somewhat distorted signal, since
that signal is taken from within the negative-feedback loop but
applied outside it.

Weird, huh?

Brian