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L-1 Mutant Vactrol Filter

L-1 Mutant Vactrol Filter


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This Mutant Vactrol Filter is fully functional and in great condition with minor rack rash.

The Mutant Filter is so named because at its heart is the Sallen-Key structure of two Buchla Low Pass Gates (actually "not" as it turns out) with the embellishments of an expo current sink and a resonance clipper similar to that found in the Korg MS-20 filter. It is not a clone or an adapation - it's more of a mutation, an almagm metasticized from disparate elements. The result is a filter that can be sublime, organic, raucous or downright mean. At times it can be sublimly organic in its raucous mean-ness. Its sounds are legion, and the samples on this page are just a taste of the beauty and weirdness that can be wrought from it.

The resonance limiter was taken from Rene Schmitz's 'Late MS-20 Filter' design. I used regular signal diodes instead of the green LED's of Rene's design mainly because I couldn't find any green LED's around here at the time I was putting it on breadboard. Green LED"s would work just fine. The expo control is taken directly from Rene's 4069 VCO design.

I have an added switch which allows the lower half of the resonance signal to be clipped. Surprisingly, this is the tamer resonance mode (and sounds quite pleasing). When the resonance is not clipped, the filter will more freely self-oscillate and render some of the wilder MS-20 like resonance tones.

The Mutant Filter has six modes of operation:

Mode 1 - 12 dB low pass with resonance tapped from the 24 dB low pass point.
Mode 2 - 24 dB low pass.
Mode 3 - 12 dB High Pass with resonance tapped from the 12 dB band pass point.
Mode 4 - 12 dB Band Pass.
Mode 5 - 12 dB High Pass with resonance tapped from the 24 dB high pass point.
Mode 6 - 24 dB High Pass.