The low-pass-gate is a traditional Buchla design that has some mysterious qualities to it. For decades it has mostly been seen in Buchla synthesizers and luckily is now available in various forms, formats and styles. But what is a low-pass-gate? It is an electrical circuit that combines elements of amplifiers and low-pass filters. Therefor it will be used to determine the amplitude of a sound and its sonic complexity.

Due to its technical structure it is possible to just send trigger to the CV inputs of low-pass gates and still get envelope-shaped results. This is because low-pass gates usually use vactrols –  photoresistive opto-isolators, or or opto-couplers. What is that? Basically vactrols are light-emitting diodes (LED) or another light-emitting component and a semiconductor-based photoresistor. With these two components you have a source of light and a detector of light, both sitting in a closed housing, electrically isolated from each other. When you send a voltage into a vactrol – the light-emitting part – will emit light and the detector will receive this light = this opens the gate. And because this special construction is some kind of slow, it has a special natural sound to it. And as the light received by the detector is opening the VCA and controlling the filter cutoff at the same time sounds   at low amplitude will sound duller. And this is where the magic happens, this behavior in combination with the “natural” respond of vactrols make low-pass gates sound so nice.

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Make Noise – Optomix (Rev.2)

All time classic: The Optomix is might be THE module people think of first when talking about low-pass gates in eurorack. It has been around for years and has built a large fan base. The Optomix by Make Noise actually features two low-pass-gates utilizing 4 vactrols to provide simultaneous voltage control over amplitude and frequency content of a signal. In Rev. 2 the damp circuit was updated to accept audio as well as CV for patching side-chain compression. Additionally Optomix offers a summing stage complete with an AUXiliary IN allowing for the chaining of multiple units to create larger mixes.

Rabid Elephant – Natural Gate

The Natural Gate might be the most advanced low-pass gate when it comes to engineering complexity. Due to this it has some unique features exceptional sound design capabilities. For example, it is possible to choose between three “material characteristics”, each of them responding differently to excitation signals. As the trigger frequency increases, the low-pass gates open up more and more. Additionally the decay varies depending on the pitch of incoming audio material. Thanks to this behavior Natural Gate has a very pleasant sound and a very natural sound. Its two channels also featuring ducking compressor, wave shaper or envelope functionality.

Doepfer A-101-2 Vactrol Lowpass Gate

The Doepfer A-101-2 Vactrol Lowpass Gate is a single low-pass gate with the ability to also be used as VCA or low-pass filter – and it features resonance control that not many low-pass gates provide. It has a rather aggressive sound, especially with lots of resonance. What also sets the Doepfer apart from others is that you can set the mode of operation not only manually but also via gates, which is the reason for two gate inputs; this is very interesting in combination with clock dividers or trigger sequencers.

  • Gate 1 high & gate 2 low = low pass mode
  • Gate 1 low & gate 2 high = VCA mode
  • both Gate 1 & 2 high = combo mode

Metasonix RK5 Dual Low Pass Gate

The Metasonix RK5 Dual Low Pass Gate is a very special module. It takes the concept of a low-pass gate, invented in the 1970s and uses technical solutions from the 1950s – vacuum tube audio circuits. The module offers a very special warm and gently distorted sound with a rather long decay. It is the perfect module for those who want to explore “new” low-pass gate timbres and want to create unique, somewhat dirty Buchla Bongos and other percussive sounds.

Verbos Electronics – Amplitude & Tone Controller

The Verbos Amplitude & Tone Conroller consist of a entirely discrete VCA combined with a vactrol controlled low-pass filter. The input stage lets you dial in a very pleasant distortion if you want it to. The sound is gorgeous somewhere between wood and acid. As all Verbos modules this is a very high quality module with a classy vintage sound. And you don’t only get the sound but also the look – how beautiful it is!

Make Noise – DynaMix

The DynaMix is the second Make Noise module in this overview, as Make Noise is constantly developing new modules featuring vactrols and keep exploring what low-pass gates can be. It is also as a two-channel low-pass gate with integrated mixer. Instead of vactrols, transistor circuits are used to control the integrated filters and VCAs. This results in a harsher and faster responds than with an integrated vactrol. Its sound is comparatively hard, but nonetheless very musical and the circuit is capable to gently distort the sound when you want it.

This is by no means a complete overview of the available modules or a full review of the modules discussed. The what about… texts are here to give you new ideas and maybe another perspective on things. All these modules offer more features than we have talked about in this post. For further information please click the provided links or – even better – visit the shop and ask the SchneidersLaden experts in the showroom.