SEM-style DIY filter

The Humpback multimode voltage controlled filter is a 2-pole, CA3080 based implementation of the classic state variable architecture. The controls of the filter are as follows:

Filter cutoff – a hardwired DC voltage frequency control
CV inputs – two frequency CV inputs are included on the panel and controlled by attenutators.
Outputs – lowpass, bandpass, highpass and notch are available simultaneously
Signal input – one signal input is available on the panel
Resonance-resonance control is included and is hot-rodded to allow self-oscillation. In self oscillation the filter will produce a sine wave output.

http://www.thonk.co.uk/shop/humpback/