DIY classic Serge-designed state-variable filter (VCFQ/QVCF) by Ken Stone / CGS

DIY Eurorack state-variable filter based on the classic Serge VCFQ (Variable Q) design and Ken Stone's CGS circuit. This versatile module features simultaneously available Low-Pass, Band-Pass, High-Pass, and Notch outputs. It offers manual and CV control over cutoff frequency and resonance (Q). Known for its smooth sweep, ringing percussive tones, and self-oscillation capabilities, it serves as both a flexible sound shaper and a clean sine wave generator.
https://sdiy.info/wiki/CGS_Serge_voltage_controlled_resonance_filter