psudo Center channel from stereo

This is is one way: http://www.ampzilla2000.com/trinaural.html

Another way is to build a simple mixer to combine the L/R signals without shorting them.

Kal

This guy has some kind of a diagram that shorts the two negatives from the amps outputs. This is risky. I remember measuring power supply voltage between the two minus posts on some amps in the '70s and '80s. I think a friend blew an amp doing this. It might be cheaper just to buy an old Pro-Logic (receiver/processor), as these can be found cheap nowadays. In this guys post, he says to contact the manufacturer before trying his circuit. [http://kantack.com/surround/surround2.html]

I tried several different ones over the years for fun. I did have a Dynaco ( Panor owned at time I believe ) that did have a five channel, that included the center speaker. I can't remember exactly what the center channel sounded like, but I don't think it was the matrix sound, like the typical rear four four channel one. These(matrix 4 channel only) can have a lot of ambient, reverb type of sound, depending on the recording. This link I posted above may be it. If you could find the diagram (with resistors to relieve the load ) or one of these, it may fit the bill. [http://www.enjoythemusic.com/magazine/equipment/899/decoder.htm]

The Hafler circuit gives you a difference signal (intended for pseudo-surround) and you want a summed signal for a center.

Kal

They had resistors in the one I had. It was to help to keep your amp from having to low a resistance on it. [http://www.enjoythemusic.com/magazine/equipment/899/decoder.htm]

There was the "Hafler Hookup" from a long time ago that provided a pseudo quadraphonic sound with two extra speakers and a jumper setup between the main speakers. Could that be what you're thinking of? You can Google or Wiki it.