PCB for a 4x4 passive matrix mixer.
The board for this module differs from the abortive first run in that a) the pot layout is actually correct (mea culpa ...) and b) there are now four jumpers, one per input, so you can decide whether you want the inputs to be DC coupled (just put a wire bridge in) or AC-coupled (add a 100nF capacitor)
If you're not clear on how a matrix mixer works then Wikipedia actually has a decent article on the subject - basically you can route multiple sources to multiple destinations and each can have it's own mix, hence all the pots. Signal routing can be immensely confusing to begin with, and the best way to understand it is to refer to the schematic. Finally, since the mixer is passive, you may find it useful to run your inputs through a buffer beforehand if you're piping the outputs to a VCO or similar.
Since it is not economical for me to keep huge numbers of pots in stock, pre-built versions of this module will be available as custom orders only.
100% through-hole components and an ideal beginners project - the BOM and schematic can be found here: https://bitbucket.org/yorkmodular/matrix-mixers/src/master/
Approximate board dimensions: 55 x 96mm