Hello Niels,

 >>  CB>> No, that's not what I want. What I need is to pass the signal
 >>  CB>> *identical* in both phase and amplitude on both inputs, and
 >>  CB>> reject anything that's slightly different in either.

 NP> Maybe my brain is not grabbing hold of things properly, but what you
 NP> are  trying to do seems to be like my "fake" Quadraphonic
system and
 NP> the idea may help.  I'm sorry but my brain can't pull it back to the
 NP> opamp level but here goes anyway...

[ascii deleted]

 NP>  The Above is a              The dotted section is added and only
 NP> normal Stereo wiring.        reproduces anything that is NOT in
 NP>                              both channels

Which is exactly the opposite to what I wanted.  I needed to
*remove* anything that is not (at identical volume) in both channels.

 NP> No doubt you have seen the setup before, and with the speakers placed
 NP> suitably gives a good surround sound and adds movement to the music
 NP> depending on the original L R balance of the instruments.

 NP> To me, it does a good job of removing the "difference
signal" out of
 NP> the main speakers and places it in the Aux speakers.

For a simple passive circuit, it does quite well, I've set it
up that way for a few locals here.

 NP> Whether the idea is usable in your application or not is not within my
 NP> current brain capabilities to calculate, but if it ain't then I have
 NP> just had some typing and ascii art practice. :-)

Looks like the latter is the case :)

What it's for is a "talkie booster" for a local here with a
reasonably bad case of "industrial deafness" - this means he
can hear well in a quiet room, but where there is a lot of
external noises he has problems understanding conversation.

With Dolby-encoded movies, the talking is exactly the same
levelwise on L and R, the music slightly different L and R,
and sound effects exactly the opposite L and R.  So a solution
was sought to keep only the "identicals" and throw away the rest,
allowing him to hear the storylines of the movies better...

The simple passive ASCII decoder you drew will boost the effects
and cut the talkies, exactly the opposite to what he wanted...

 NP> You may now tell a brain damaged old pensioner to crawl back into his
 NP> shell.

I can't do that - in a few short years I'll be one myself :-(

Regards,
Chris.

--- GoldED