Hi guys,
reviving the project and this thread after a long hiatus.
Thanks again for all your input, it has really helped me so far.
I have something to show for as well.
The first prototype is working!
I have switched to the AD5290 digipot in the meantime. It´s daisychainable, and it was easier for me to wrap my head around that. It´s 15V only so I´ve decided to drive the SSL EQ card with 15V as well and sacrifice some headroom.
Also I have replaced the SSL 82e132 with a G series EQ 82e292.
Anyway I have now wired up the card, got the HMF section to work with regular pots and now already replaced the frequency pots with two 50k digipots, and it´s working beautifully!
I don´t get any digital noise or zipper noise when adjusting the frequency. Also I have written a pseudo log conversion of the 128 midi values to the 256 pot values and must say I am really happy with the resolution this gives.
I have used a regular pot without centertap for the gain and generally this seems to be working well.
What´s still weird is that I only get around 15 dB of gain, instead of around 20 which the circuit should do. Negative gain I can go until -20dB. Dunno what´s up with that. Maybe because of the missing center tap, or because I have preliminarily removed the neighboring OP amps, to only use the HMF section. Will see.
The sound is great! Next steps are to replace the other 2 controls for the HMF section with digipots (already got these parts), and then scale up to the whole EQ.
Attaching a pic of my quick and dirty (very dirty) prototype. Will switch away from the breadboard for the next step for less chaos and more reliability