Lowpass Gate

Manufacturer: MRB

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Included in the MRB Luxury Filter Bundle



The MRB Lowpass Gate is an interpretation of a modified Buchla lowpass gate. A lowpass gate is essentially a 2-pole voltage controlled lowpass filter. The original was not capable of resonance, but later hardware versions by others as well as this one, have the feature. What makes them unique is the use of a Vactrol as the frequency control element. A Vactrol is comprised of an LED shining on a light dependent resistor (LDR) packaged together in a sealed black plastic cylinder. The brighter the LED, the less the resistance, the higher the filter tunes. The characteristic sound loved by one and all is caused by the LDR's slow response to rapid changes in light intensity. The response is asymmetrical in time, responding much faster to light increases than decreases. If the control voltage briefly flashes the LED, the LDR responds quickly, but takes a while to get used to the darkness again. As the LDR does this, the filter closes down relatively slowly (about 250ms for the Vactrol most often used) yielding a percussive ping in the process. Because an LDR is a pure resistance and not an active element, a Lowpass Gate exhibits no distortion and yields a very clean plucky sound.


  • INIT. CV -- Initial control voltage to the Vactrol model. Clockwise opens the filter.
  • RESONANCE -- Q of the filter. Functional in VCF mode only.
  • ATTACK -- Light adaptation time of the Vactrol model, 1-50ms (filter opening time).
  • DECAY -- Darkness adaptation time, 25-1000ms (filter closing time).
  • CV -- Vactrol modulation (summed with INIT. CV)
  • RES CV -- Resonance modulation
  • DEC CV -- Decay modulation
  • IN -- Audio input
  • OUT -- Audio output

The ATTACK and DECAY controls offer timings way outside the range found in actual Vactrols. Using these controls near their extremes is definitely not "stock".

The MODE switch selects the 3 circuit configurations as found on the original. VCF mode, selects the lowpass filter with resonance and is the mode most often used. BOTH mode is kind of a hybrid between a VCF and a VCA. It's a VCA that gets duller as it gets quieter. VCA mode provides no filtering and has a percussive response. VCA and BOTH mode do not offer resonance and the RESONANCE knob will have no effect.

The suggested method for use is the following: Select VCF mode. Turn INIT. CV all the way down. Patch a very brief AD-type envelope into CV and turn the CV knob up to where you want the gate to open. The brief envelope is used to "strike" or "ping" the Vactrol. The DECAY control is then dialed to taste. The DECay CV input can be used to modulate the timing from brief blips to long tails. Turn up the ATTACK control to soften the "hit". Playing with your strike envelope's decay time, the CV input level and the DECAY time iteratively will get you where you want to go. The plucky "Buchla Bongo" sound can be elusive and depends on the above controls as well as the sound being used. I have found that exact 50% square waves with a hint of sawtooth sounds good. Just a triangle wave sounds sweet too. Generally, low or no RESONANCE is used for this sound.

A dynamic envelope, such as the MRB Velocity Envelope generator is a great choice for the strike.