The FET at the input acts like a buffer. The op-amp with the diodes adds rich overtones to the signal, followed by a lowpass/tone altering-filter. The last circuit is the line driver with two outputs. You need two identical circuits for stereo.
I recomend using better op-amps for this circuit.
For exampel: NE5532,TL072... Check the modified version of TM-3!