MidCentury Modular Low Pass Gate Quick Start Manual

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Low Pass Gate

INTRODUCTION

Low Pass Gates were first developed in the 1970s by

Don Buchla, and were a crucial part of early West Coast

synthesizers. They have one knob which controls both

amplitude and harmionic content simultaneously.

The key component behind the panel of an LPG is

called a vactrol. A vactrol consists of a light emitting

diode placed next to a light detecting resistor, sealed

inside a light-proof enclosure. Audio is passed through

the LDR while control voltage is used to light the LED.

As CV increases, the LED emits more light which allows

more audio to pass through the LDR.

There is a great amount of variation from one vac-

trol to the next. As a result, no two LPGs sound exact-

ly the same. In addition, vactrols have an imperfect

response to CV. When an LPG is pinged with a quick

trigger, sound is not cut off immediately but instead it

rings out and decays in a natural way. The sound of a

low pass gate has often been compared to a physical

material being struck or plucked.

The MidCentury Modular Low Pass Gate provides

manual and CV control over the cutoff, and a ping in-

put. There are also controls to set the amplitude and

width of ping triggers, as well as an accent CV input.

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Page 1 The sound of a low pass gate has often been compared to a physical material being struck or plucked. The MidCentury Modular Low Pass Gate provides manual and CV control over the cutoff, and a ping in- put.
Page 2 MODULE OVERVIEW INTRODUCTION...
Page 3: Module Overview
MODULE OVERVIEW Audio enters the module through the audio in- put (6) and comes out through the audio output (10). The Cutoff knob (1) provides manual control over the cutoff frequency and amplitude simultaneously. CV control over these parameters is also available via the Cutoff input (8), and CV can be attenuated with the as- sociated knob (5).
Page 4 CALIBRATION To calibrate this LPG, you will need a screwdriver, an LFO, and a sound source. White or pink noise is pre- ferred but a VCO will work too. There are four trimpots and one jumper on the back of the module. You will need to access these while the module is powered.
Page 5: Technical Specs
CALIBRATION Before adjusting T3, set the Cutoff knob to a point where you can hear subtle changes to the cutoff. If using white noise, putting the Cutoff knob near 10:00 works well. Make sure Trig Height is still fully counter clockwise, and then turn the Oomph knob all the way up.