CEM 3350 Dual Filter
The CEM 3350 is an IC designed by Curtis Electromusic Specialties which contains a number of OTAs and supporting circuitry which simplify building a pair of state variable filters.
The configuration of the OTAs seems a bit odd at first, but doing the maths shows that depending on where the audio signal is injected, you can get a simultaneous Low Pass and Band Pass, or Band Pass and High Pass response from it, with control over the Q.
A fairly rough, somewhat dull, and incomplete run through of the maths can be found in this pdf.
I've made all the files for this open and licensed under Creative Commons.
The Kicad Project files with a schematic and layout here
I've not yet designed a panel for this, and the circuit has been lightly tested on a breadboard, but it's very much a work in progress.
Should really get this started again.