|The Mixer / Output Module does two things, mixing of synthesizer sound sources and pre-amplifiing the result for PAs or headphones.
The circuit of the MSS2000 book proposes a six channel mixer with adjustable PA output, but I increased the number of channels to eight and added a headphone amplifier.
There are four pairs of input channels; the two input signals of each pair can be positioned in stereo space as you like controlled by pots.
The schema is part of the book "Formant Pro MSS 2000" by H.J. Helmstedt. The circuit for the headphone amplifier can be found here.