I’ve picked up a few AS3320s along with the rest of the Alfa chips, so I went looking for a nice low pass filter design using that chip to get me started. I found the AMS AM8320 “Prophet” filter. It looked pretty much what I was looking for and the schematic’s available (via the link above, I’m not sure if it’s OK to post here).
Edit: schematic posted (with Rob’s permission)
I reworked the circuit a little to fit my needs and swapped out some components, but it’s essentially the same as Rob Keeble’s circuit. I designed my own stripboard layout, which will fit, 2 boards across and 2 deep, into a 38hp panel.
I’m confident that the components will fit into the space now I’ve done the quad VCO as the spacing is fairly consistent throughout my panel concepts.
Starting with just one filter for testing, I marked out a board and built up the circuit.
I cut a temporary panel from cardboard, then managed to get some measurements wrong and screwed it up a little! Fine for testing anyway, so I wired it up.
First test went well. It sounds lush. I was instantly reminded of the Sequential filters – not sure how much of that is psychosomatic – I love it! The CV inputs work fine and it goes into self oscillation nicely. Only one issue with it; one of the inputs isn’t working, but I think that’s a soldering/grounding issue with the jack/pot because it works when I give it a wiggle! I’ll fix that when I move it to a more permanent home. Very happy with it overall – just another 3 to build…