This is my first filter design for Eurorack. I seem to always use my multi-mode filters (built from kits) in lowpass mode so that's what I've designed for myself. I didn't want to jump into a multimode filter yet, it just complicates things I don't have a good grasp on yet. The module uses 8HP.
For this filter I'm using the AS3320 filter chip which I picked up along with various other chips in that series from Electric Druid. When I went to figure out the circuit I ran into the biggest road block which is that the data sheet is nearly useless. It shows a sample circuit for low pass, high pass and other use cases but they are a tangled mess of wires, very poor layout for helping anyone understand the chip. In an age where the data sheet doesn't need to fit in a single page, there's no excuse for providing such bad documentation. Rant over. These chips are clones of CEM3320 and the full data sheet for that is somewhat better. Here's where I found it.
In the CEM3320 data sheet I found the math and fine details needed for figuring out values for the passive components, we must assume that the math works the same for the AS3320. Along the way I went back over to Electric Druid and discovered a fantastic writeup and a reorganized schematic that really helped clear up what this chip does for various filter designs in the wild.
https://electricdruid.net/multimode-filters-part-1-reconfigurable-filters/
https://electricdruid.net/multimode-filters-part-2-pole-mixing-filters/
That write up was hugely helpful. The diodes in my typical power supply on modules drops the nominal 12V to about 11.5V. The data sheet's values are for 15V. Armed with the CEM3320 data sheet I did all the math to figure out component values for my supply voltage. After that I did a breadboard version to verify the design.
A couple values I was a bit shaky on because my design isn't like anyone else's so there are a couple trim pots in the resonance circuit to allow some adjustment. There are a lot of text notes in the schematic (link below) about how I arrived at the various component values.
The CV input circuit and Frequency/Resonance control pot circuits are a circuit I've used in several designs with the Frequency pot flipped to make a more intuitive control. I expect 10V CVs in my system and the chip expects a much smaller voltage max so there's a voltage divider in the path before the op-amps and another after them to manage that. Here's a simulation of it. On the resonance the input is expecting a current range. The RCC1 and RCC trim pot produce this current from the resonance CV. RCC1 is mainly there to prevent overcurrent on one end of the trim.
Like the VCA design, I used one op-amp on the input to invert the signal getting filtered and another inverting op-amp on the output resulting in a net non-inverted output.
When I got the PCBs I assembled several.
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