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Surround Channels PCB:
This board contains six-channel preamplifier and set of relays switching between external decoder input or surround matrix placed on the PCB. The circuitry is based around another three PGA2310's in the same application as in main signal path.
It was created to enhance simple stereo signal with additional channles. As shown on the schematic it is based on set of PGA4134 opamps. Centre and LFE signal are created passing through adequatly band and low-pass filter. Both back are the same and are result of difference between left and right channel. Left and right surrround channels are created as a difference between back and adequatly left and right channels.
On the next page control...