But the panel at SYNTHCUBE !
This build uses a CGS 202 PCB, a CGS 64 VCA for VC feedback, and a CGS 81 (or CGS 04) to mix the signals, as documented in the wiring diagram.
See Ken Stone’s page for further build information.
If you like you could use the other half of the CGS 81 as an attenuverter to invert the phase of the AUX signal (see diagram). Totally inverting the phase leads of course to cancellation, in this case maybe also changing the gain of the attenuverter would be interesting. Or you could use the CGS 81 to fade between LOWER and UPPER outputs (using a dual pot for AUX) for the feedback (I’ve not tried that myself though).
Listen to a demo of the feedback here!