Catalyst Audio - Mig VCF

The MIG VCF is a self-oscillating, voltage-controlled filter from Catalyst Audio and Electroserf, built as an 18dB-per-octave Sallen-key design around three 1J24b tubes — two on the panel, one hiding behind it. Each one has a different job: the first sets and amplifies the input level, the second runs in a high-impedance setup feeding the filter core, and the third amplifies and shapes the resonance signal. The nonlinear behaviour of those tubes is where the sound comes from — resonance that turns fierce while still "musical".

High-pass and low-pass outputs are available at the same time, the high-pass made by inverting the low-pass and summing it back to cancel out the lows. The dial that rewards some deeper inspection is Res Mode, which crossfades the resonance circuit's input between the low-pass and high-pass taps and noticeably changes the texture of the resonance. Drive sets how hard the second tube pushes the core, and it leans heavily on the input level — the two together are where a lot of the character lives. Frequency, Drive and Resonance all take CV with attenuverters.

One practical note, true to the Sallen-key topology: self-oscillation needs the resonance path not to be drowned out, so if it won't sing, ease back the input level or the Drive. This is a filter for anyone after tube grit and a vocal, slightly unhinged resonance rather than something surgically clean.

Features:

• Self-oscillating vacuum tube filter, 18dB/octave Sallen-key design
• Three 1J24b tubes: input/level, filter core, and resonance shaping
• Simultaneous high-pass and low-pass outputs
• Drive sets the level into the filter core (tube #2)
• Res Mode crossfades the resonance source between low-pass and high-pass
• Frequency, Resonance and Drive controls
• CV inputs with attenuverters for Frequency, Drive and Resonance
• Aggressive yet musical resonance
• NOS Cold War-era Soviet pentode tubes
• 100% analog