4x4 Matrix Audio/CV Mixer

I took inspiration for this design from Doepfer A-100. They outline the basic idea here as "Matrix Mixer II".  Each of the 4 inputs can be mixed into any four of the outputs. Each of the four output rows has two modes: i) Audio, where the volume is adjusted from 0 to Vcc and ii) Control voltage, where the signal polarity is reversed. The latter also has the effect of shifting the phase of the signal by 180 degrees.

I tested the circuit on the breadboard and made my own schematic in Kicad.

They recommended implementing the complementary parts of the circuit on different PCBs, but not one to shy away from a challenge I did mine on one single-sided board.  Oooh edgy!

Wickes Solid Oak Reducer 900mm (Product Code:138994)