Pittsburgh Audio Lopass Gate
Lopass Gate (LPG)
Triple Mode Filter, VCA, Lopass Gate
The Pittsburgh Modular Lopass Gate module is a new take on a classic. Our intention was not to clone but to reengineer and expand on the original idea. Our module includes an amazing new self-oscillating resonance circuit and a quieter vactrol based VCA circuit with true unity gain. Voltage control of the module is handled using an invertible, attenuated input and ping input. The ping input takes any gate or square wave and converts it into a very tight trigger used to ping the vactrol.
In lowpass filter mode, frequency sweeps are warm and buttery and resonance is smooth and controlled until it gets near self oscillation. At this point the Pittsburgh Modular LPG module turns into a shouting beast. High resonance creates an edgier, shattered, more direct sound. It never loses control but is always teetering on the edge. Having both the smooth and snarling responses available creates a filter that is perfect for almost any situation.
In VCA mode, we worked very hard to reduce the bleed associated with most lopass gate circuits without resorting to tricks like adding an a traditional VCA after the vactrol circuit. After considerable effort, we succeeded in creating a vactrol based VCA that is identical or better in performance to our 2164 based Dual VCA module.
All the work we did on the filter and VCA come together with the lopass gate mode. The buttery warmth of the filter teams with the best attributes of the vactrol based VCA to create an impressive take on the original lopass gate.
HISTORY AND THEORY BEHIND THE LOPASS GATE
The lopass gate module is unique in the way that it simulates the characteristics of natural instruments. When used in lopass gate mode, louder sounds contain more harmonic content and quieter sounds contain less harmonic content. This is not the case when using a standard VCA such as the Pittsburgh Modular Dual VCA module. A standard VCA simply changes the loudness of the sound without changing the harmonic content. There is no reason that synthesizers need to simulate how acoustic instruments function but because of how it affects the harmonic content of sound, a lowpass gate sounds more organic or natural compared to a standard filter/vca chain.
The circuitry behind the lopass gate has evolved over the years to include resonance and more complex modulation options but the ability to switch between voltage controlled amplifier, low pass filter, and a third mode which uses both the VCA and filter together remains at the core of the modules functionality. A lot of the characteristics associated with the lopass gate circuit come from the use of vactrols. A vactrol is an optoelectronic device consisting of an LED and light detector enclosed in a light tight package. The elements of the vactrol are optically coupled and electrically isolated from each other. When the LED within the vactrol turns on, the light detector reacts very quickly creating a very sharp attack, however, when the LED turns off, the light detector within the vactrol does not react as quickly and closes more slowly. That slow decay or ringing is the quality most associated with the lopass gate. Therefore, using a very short trigger to ping the vactrol will not result in a very short sound, instead the result is an organic percussive sound with an natural sounding decay.
The lopass gate uses a Sallen-Key filter. The Sallen-Key filter is a 2 pole 12db filter with a very buttery, natural sound. In addition to the lopass gate, the Sallen-Key filter was used as the core of Korg's classic MS-10 and MS-20 filters. Although the Korg filters are remembered for their biting growl, that has more to do with the resonance then with the filter itself.