IIRC, Dynaco had this accommodated in their pre-amps at one point in time (many, many years ago). I could be wrong.
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My Fisher tube integrated had a center channel power output in the 60s. You can do this with ANY preamp. Get 3 high quality Y connectors, male to two female. Plug one into the right channel and use one of the female plugs to run your regular interconnect to the amp, do the same on the left channel. Then run the ICs from the other two plugs to the third Y connector and plug it into your center channel amp. It carries a combined L+R signal. You are simply doing externally what the other preamps do internally and will have no more signal degradation than they do, assuming high quality connectors.
Stanwal...If you use a simple Y interconnect you have shorted the Left and Right together. ALL channels will be Left plus Right... in other words Mono. To mix Left and Right so as to get a center signal without screwing up the Left and Right you must use a resistor network.
In my opinion the simplest and best way to drive three front speakers is to use a stereo power amp with the signal of one channel inverted. Bridge the amp with your center channel, and reverse the speaker wires on the channel that you inverted. Your amp will see a low impedance load, but I never had a problem even with 4 ohm speakers.
Another way is to connect the three speakers in a Y configuration. This will not lead to a low impedance load,
Many years ago I experimented with non-standard connections for speakers.
Using 2 amps I had a standard pair of L/R speakers in front using the first amp, my brothers SAE. No problem.
Next, I connected a pair of speakers IN SERIES between the 2 hots of another amplifier. These speakers I stuck in the BACK of the room, maybe 10 feet apart. This amp was my Kenwood from my first 'serious' effort at better sound. The effect was startling. Any music with phase information 'intact' sounded really live. As a matter of fact, albums recorded live were scary. The music would come from the front pair, with little or no sound from the back. As soon as the crowd began clapping, you were in the MIDDLE.
Current over-engineered music lacks many phase cues which renders this probably a moot technique and makes my absolute phase switching on the amp so much extra circuitry.
OH, the technique to which I refer? I got the idea from SQ / QS 'matrix' sound at the dawn of '4 channel'. I think Dynaco (?) sold an adaptor which did the same thing, from a single amplifier.
Magfan...What you did was basically the "Dynaco" setup, also attributed to Hafler. The Dynaco setup also included a Y-connected center front, which increased the load impedance of the front speakers so as to compensate for the series-connected rears that were bridged across the amp. Dynaco also suggested a "blend" resistor at line level to restore Left/Right separation which is reduced by the Y connection, but I always prefered the sound without the resistor.
With certain recordings the results were superb.
I remember ALL the early 4ch stuff being just phase extraction from the original 2ch material. On recordings with mixed or screwed up phase information it worked only ok if at all. I remember LIVE recordings with few mics sounding best since the phase is generally preserved.
For the DIY crowd, BE CAREFUL if you try any of this stuff. If your AMP doesn't have common ground you could fry your outputs and upstream.
There WAS 1 discreet 4ch system, from specially encoded vinyl which needed a special cartridge with freq response to 40khz plus. I heard a demo with 4 people speaking 4 languages, 1 from each speaker. Trouble came in when the HF encoding started to etch away. Someone like me, who never mastered the art of keeping vinyl perfect it was a hopeless proposition. CD's, for better or worse came along just in time.
Magfan...There were two basic approaches to early multichannel...matrixing with many variations but most notably SQ, and the discrete system you mention. Information for the rear channels was carried by FM modulation of a carrier between 20 and 40 KC. Because it was FM there was no requirement for low distortion in the 20 - 40 KHz range, so long as the carrier frequency could be tracked. The need for response to 40KHz spured great improvements in phono cartridges which benefit us today although the multichannel scheme was a bust.