Mixed-mode feedback gain formula

[antomas]

Please, does anybody know how to compute the voltage gain of this circuit?
http://sound.westhost.com/p56-f2.gif
The page describing it can be found here:
Variable Amplifier Impedance
Thanks to anybody willing to answer.
Massimo

[djoffe]

see enclosed pdf

[antomas]

Originally Posted by djoffe

see enclosed pdf

Dan,
Great stuff! Thousand times thanks!!!

The reason for asking about the formula is that I'd like to try this circuit (with a higher Zout) to implement the concept described by Nelson Pass in this document to drive a Fostex FE206 in sealed box.
http://www.passdiy.com/pdf/cs-amps-speakers.pdf
As a chipamp I'd like to use the popular LM3875, keeping into account the minimum voltage gain needed to prevent instability. Hope the mixed mode feedback shall not make things harder.

Eventually this system could provide decent sound with little build effort.
Time will tell.

Again, heartfelt thanks.
Massimo