Why needs to calibrate.
A PRM is set at minimal leak through of the input voltage. A leak can be heard if you using only one input. There is alway a leakage but in a normal situation with a signal at the audio input this must not be audible, A (analog) PRM has alway more leakage as a VCA.
How difficult is adjusting the PRM
This is quit easy, you need only your ears. The only problem is to locate the PRM because the PRM doesn't have control knobs
Adjusting the PRM
Each input (A & B) has his own trimpot. Switch the Fénix on 15 minutes before you start. It is advisable to do first a roughly adjustment and then the final adjustment. Put a audio sinewave at the A input (leave the B-input open) and listen to the output. Adjust the (A)trimpot for minimum leakage. Put the sinewave at the B input and listen to the output.Adjust the (B)trimpot for minimum leakage
10e PCB board with Ringmulator 1&2..