4PM CV in for Cutoff, Res, VCA?

Started running other osc’s through the 4PM filter and thought “Boy, I’d love this as a filter module”. Then I took a look at the Ripples schem, and there you have the exact same filter (minus all the crazy switching). Now, I’m wondering how one could mod the 4PM to take external CV for cutoff, res and VCA and keep the unit in tact otherwise.

You could insert a signal into the inverting input of the corresponding cv opamps from the VCF, RES and VCA lines from the chip but you would probably have to limit the signals to -5v/5v.
Maybe use a 3PDT switch to switch the inputs from the chip to the incoming signals.

Anyone try or ponder this yet?

> You could insert a signal into the inverting input

… through a resistor.

> but you would probably have to limit the signals to -5v/5v.

Not at all! The op-amp inverting input would still stay safely at 0V. Circuits in which the external CV is fed to a virtual ground through a resistor are “safe” from the point of view of excessive input voltages.

Thank you! Very excited by this news. Now to see what I can cram in the case…

I was able to stuff 5(!) jacks in the regular 4PM case - 2 for cv out and 3 cv in. Only one on the left side because of the Arduino header connections, but the I managed to get 4 on the right side - the only obstacles on that side are caps. I used the Tayda switch jacks.

I opened up an Ethernet cable to get really thin wire and attached that on the bottom side of the PCB.I found it easiest to attach the wire to where the cap intersects with the inverting input (C19, C18, C5) for each CV circuit. I used 33k to a jack for the cutoff and resonance CV in, and 47k for the VCA CV in, It works like a charm and sounds amazing!

Also, since the Shruthi doesn’t turn on the Osc’s unless it receives a midi note, you can just fire the unit up and use it as a filter/VCA module with no midi connected.