State Variable Filter
The state variable filter is a second order (12dB/oct) filter with low-, high- and band-pass outputs and adjustable resonance. This is a classic filter design that appears in a number of synths, including the Oberheim SEM.
- Module size: 8HP (40mm)
- Power: 15mA (+12V); 15mA (-12V)
State Variable Filter
The project files, including schematic and layout, are available on github
Features
- AC-coupled line input (audio) with adjustable attenuation.
- Multiple control options for the filter cutoff frequency are summed:
- V/oct range from 0-9V corresponding to a filter cutoff ranging from <20Hz to approximately 16kHz.
- Two CV inputs with adjustable attenuation.
- Cutoff frequency offset adjustment with a potentiometer.
- Low-, high-, and bandpass and notch (bandstop) filter outputs (note: the band-pass output is not normalized to the resonance gain, so its amplitude will vary with the resonance control).
Following the notes from Eddy Bergman, I've opted to drop the fine-adjust for the cutoff frequency and replace it with a second CV input.
Documentation
Changelog
v1.2
- Fixed error in Q1 (matched PNP) orientation in schematic and PCB wiring.
v1.1
- Fixed wrong part for Q1 (matched PNP), replaced with DMMT5401.
- Fixed order of trim pots and summing resistors for CV inputs.
v1.0
- Initial board.
References
- Hal Chamberlain, Musical Applications of Microprocessors, 2nd Ed., Hayden Books, 1985
- Thomas Henry, "VCF-1", birthofasynth.com
- Eddy Bergman, "VCF-1 State Variable Filter by Thomas Henry" eddybergman.com
- Ray Wilson, "State Variable VCF 12dB/Octave with VC Resonance" MFOS
- Kevin Lightner, "docs/Oberheim/Oberheim_SEM1A" synthfool