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
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: