AI004 OTA Voltage Controlled Filter

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. The build guide is located here.

https://youtu.be/oHHq2-ASvoQ

A Loving Tribute to the MS 20 Filter.

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.

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 PCB
• A Eurorack Aluminum Panel
• High quality Alpha Potentiometers
• Mounting Screws and Power Cable
• All components required to build the OTA Multimode VCF.

Oscillator Outputs Matter

The AI004 was/is designed to be used with an oscillator with a 10v peak to peak waveform. That is the "standard" in other formats for an oscillator.

In Eurorack there are no standards for this. Here is a scope showing 4 different oscillators in my rack. The green signal is 8v peak to peak, the blue is about 5, the yellow is a measley 4v, and the red looks like 5 or 6. These oscillators CAN be used with the AI--4, but your oscillator output will absolutely, 100% affect the tone of the filter, especially one like this in which the resonance is keyed onto the input signal.

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.