I think having more small mixers in a synthesizer instead of one big multichannel one is more useful, as you can process mixed signal groups separately (for Stereo or panorama modulation for instance) and you can concatenate the mixers if you need more channels.
Each input has a level pot.
Schematic:
The schema is part of the book "Formant Pro MSS 2000" by H.J. Helmstedt.
Sound example:
Not necessary.
Reproduction hints:
None.