What’s POLEY?
This is a pole mixing filter based on the SSI2140 filter IC (remake of SSM2040). What’s different here is that you have pole mixing for different filter responses (bandpass, highpass, etc.), and compensation for level loss on higher resonance settings. The PCB is fitted for an expansion panel not yet designed) that gives you reso CV and more filter types.
Some audio demo from breadboard version first:
What I wanted, apart from different filter responses, was to utilize the internal resonance VCA of the 2140 for a trick I learned from SSI’s awesome paper AN701, namely that level loss on resonance can be avoided when using bandpass resonance. So, this is, strictly speaking, not passband level loss compensation by pumping more level in proportion to increased resonance, but actually bypasses the broader phase cancellations going hand in hand with classic LP feedback.
I implemented this trick in different filzers already, yet the challenge with the SSI2140 is that the resonance VCA input, unlike on the CEM3320, is prewired… now, if you can mix filter poles for different responses, you sure can mix filter feedback bandpass – yup! Only thing is that this requires you to attenuate the filter input a little bit more as things get more voluminious in contrast to passband level loss style resonance.
Proto Testing
Just got three nice proto boards from aisler, and whoop! Here’s more audio blooops
Stock modes: 4lp, 2bp, 1lp, 2hp
And the same with audio rate modulation
Modifications
At the moment I’m simply toying around, but might out some of these in an expander module that also offers reso CV and more filter responses.
Resonance diode limiting: Although you can tweak the maximum amount of resonance by means of R22, diode limiting the feedback path is also nice. Use D3 and R45 (not present on PCB) as per schematic. R45 is at 75k at my module. The lower the value of R45, the stronger the limiting effect.
4P out normalled to MOD input: Simply wire the 4pole out to the normal pin lug of the MOD in socket via a 100k resistor. If no cable is plugged into the MOD input, you can use the bipolar mod amount knob for dialling in self-FM-ing madness. Listen to some responses with that mod: