Ok I did some tests today, for anyone interested in making their own controller, here are my findings:
For this to work set the control inputs for expression rather than CV, then once you have built it go into the settings and set expression learn, turn pot from full to max, then exit and repeat for the other input.
The control inputs are TRS connectors, the sleeve is ground, the ring carries +5v, the tip is actual voltage input.
So it is a fairly simple job to make a controller knob box, you will need:
A die-cast metal enclosure - plastic would be ok, but a bit light and move around. Rubber feet also recommended.
2x 100k linear potentiometers - anything between 10k and 500k will work, but 100k gave the best response across the range in my tests.
2 TRS connectors, you could use jacks or sockets, I opted for sockets and using TRS cables to connect to the Rytm, if you just want permanent cables though use jacks wired directly to the pots.
Connect the sleeve of the socket to the leftmost pin on your pot, connect the tip to the middle pin on the pot, and connect the ring to the rightmost pin.
See the diagram below, looking at pot with legs facing down and with shaft of pot facing you.
Once I have built mine I’ll post a picture.
Should work with AH, A4mkII and ARmkII.
@eangman Maybe it would be handy to have the pinout for the control inputs in the manuals for AHmkI and AHmkII, ARmkII and A4mkII?