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Getting Started: What is a lowpass gate (LPG)?

In past Getting Started posts, we’ve discussed mainly individual functions that you’ll find in Eurorack modules. Today, we’re discussing the lowpass gate, or LPG. LPG’s are a combination of three functions that are commonly packaged together as a standalone utility, and have been a modular synthesis staple for decades. They’re quite common in Buchla-inspired Eurorack systems, and are a wonderful tool for creating both percussive and melodic sounds.

The components of a lowpass gate

Lowpass gate modules generally have a simple user interface: an audio input and output, and a trigger input. Some will also have parameters that control decay time or filter type. 

From a more technical perspective, lowpass gates are usually made of three things: a VCA, a lowpass filter, and an envelope. The filter frequency and VCA level are controlled by the same envelope. This emulates the natural decay of organic sound: if you imagine hitting a frying pan with a spoon, the sound will become less bright as it decays in volume. Lowpass gates do a great job of recreating this same sort of harmonic response. 

Lowpass gates commonly use single-stage decay envelopes. This means that when they are triggered, the envelope instantly jumps to its maximum value, then decays back down to minimum over a period of time. In some lowpass gates the decay time is variable, but in more traditional designs decay time is often fixed.

Signal flow of a lowpass gate: oscillator goes into a filter and a VCA which are controlled by a decay envelope

Getting technical: what is a vactrol? 

When researching LPG modules, you may see some discussion of vactrols. Vactrols are an electrical component made popular in the early days of modular synthesis that create a natural-sounding decay envelope when a trigger is sent into them. They have the advantage of being a simple component to implement, but the disadvantage of having a wide variance in response and little to no control over decay time. Due to those disadvantages, as well as  environmental concerns around the heavy metals used to manufacture them and improvements in envelope circuitry, they are used less and less in modern LPG modules. 

Not all LPGs have built-in envelopes 

Some traditional lowpass gates, including early models from Buchla, featured only a linked VCA and LPF, and no envelope. They were designed to be used with an external envelope, similar to how many VCA modules are designed in Eurorack. It’s less common to see lowpass gates designed this way today, but they still pop up from time to time. 

Filter types in LPGs

It’s most common for lowpass gates to use lowpass filters, as the name implies. However, some units offer multiple types of filter – or a way to bypass the filter completely – for more sonic variety. A good example of this is the Make Noise QMMG, which includes modes for VCA only, VCA+lowpass filter, and VCA+highpass filter. 

Common applications of lowpass gates

Lowpass gates are a great way to create a plucky voice with any oscillator. Patch an oscillator into the audio input of the LPG, patch a pitch CV to the oscillator’s 1v/8va input, and patch a trigger signal to the LPG’s trigger input. 

A famous patch involving the lowpass gate is the so-called “Buchla bongo,” where a wavefolded sine wave (more on wavefolders in a future post!) is processed through a lowpass gate with a quick decay. This creates a wonderful sound that emulates something in the realm of a synthetic bongo.

How to patch a lowpass gate from scratch

If you don't have a dedicated lowpass gate module in your system, you can easily create a similar utility by patching a few utilities together. Simply patch a sound into a filter, patch the filter into a VCA, and mult a decay envelope to both  the filter cutoff and the VCA CV input. This also adds some extra flexibility to the timbre of your voice, since you can change the resonance, slope, and flavor of your filter depending on the modules you use. 

SUGGESTED MODULES

Sinc Bucina: A lowpass gate with a few different envelope options and multiple filter slopes

Rabid Elephant Natural Gate: a dual lowpass gate designed to emulate natural materials

Doepfer A-101-2: A traditional lowpass gate with resonance control.

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