I posted this picture before. Next module will be a (quad? 1184ish) VCA using these boards.
If you redraw the circuit with discrete resistors (rather than the 10K SIPs) you see that U4B is a differential input stage for CV±.
Since these were used in an audio console, I guess that one CV was connected to the the channel fader and the other CV input to the VCA master fader. I hope I can group 4 together and have a master volume/VCA control somehow.
Tested one with ±12V and the module passes audio 
but noticed that there is a 5V reference used. I’d better make a proper ±15V PSU for this.
