This OTA Voltage Controlled Filter uses the same filter structure as the venerable Korg MS 20 Filter. The filter is switchable between Lowpass and Hi Pass modes, and features an updated diode gain stage in the resonance section. This circuit was adapted by Sagittronics, who does excellent work.
The VCF will overdrive and distort in both the HP and LP modes, and the resonance can self-oscillation. As with the MS 20 Filter, the Low Pass slope is 12dB/octave and the High Pass slope is 6dB/octave. The CV input is an attenuverter, allowing for dynamic control-voltage of the cutoff.
A VCA is your friend
As with most OTA filters, (as well as diode ladder, cascade, CEM, and SSM designs,) wherein resonance is achieved via negative feedback, the non-resonanced signal (the passed signal) will drop. When used in conjunction with a VCA, this is trivial.
There are ways to compensate for this by injecting some of the original signal into the feedback loop, or adding extra gain circuitry, but this has its own issues, and was avoided in this design, in large part for authenticity. In the “normal” mode, there is a normal volume drop, and in the “extended resonance mode” you will see a greater volume drop to the “passed signal.” Due to the high resonance gain, there is a large drop, again, if using a VCA, this is trivial.
All jacks, knobs, and other components are soldered directly to the PCB. There is no wiring, making this kit ideal for those who have built a simple module before, and are looking for a high quality, legendary filter in an easy to build package.
We recommend this kit as the third or fourth module you build. It is easy, and can be built in a single sitting. If it works, you are ready to build other modules.
The Eurorack DIY OTA Filter Kit Contains everything you need to build your DIY Analog Synth Module:
High Quality 3.5 mm Jacks
A Eurorack Aluminum Panel
High quality Alpha Potentiometers
Mounting Screws and Power Cable
All components required to build the OTA Multimode VCF.