This encoder takes input from your 4-channel tape recorder (what do you mean, you don't have one?) and matrixes it into either 2-channel QS format (suitable for old Sansui equipment, or just a passive Hafler hookup) or Dolby Pro-Logic II format (suitable for modern surround-sound receivers). The only difference between the two matrices I am aware of is that Dolby encodes left front and right front to hard left and right positions, without the "crossfeed" used in the more symmetrical Sansui scheme. Also I guess a real Dolby encoder has a center-channel input, but this has been omitted because center channels are dumb.

QS sample, channel identification
PL-II sample, channel identification
(both encoded with this unit)

Even if you don't intend to matrix-encode your recording, the 90-degree outphasing produced by feeding the same signal to all 4 channels is a cool effect to use when mixing in stereo. You can use one pair of phase-shift networks by itself without the rest of the stuff on the schematic. What tripped me up for a while is that the phase of the output of one network is totally nonlinear compared to its input. It is only a constant 90 degrees (over the whole audio band!) between the output of two complementary networks fed with the same signal.

I got the values for my phase shift network here. Math is hard

All op amps are TL074/TL072, supplied with +/- 12V.

Board top bottom

What it does: