S205 Low Pass Gate

Product Description

 

(This module is included in the System B)

 

The term “Low Pass Gate” was first introduced by Don Buchla in his 200 series modular synthesizer. Its basically a low pass filter that is designed to work like a VCA. The idea stems from the fact that lots of percussive or acoustic instruments give higher frequency content as they are stroked harder and harder, changing BOTH the character and loudness of the sound.

 

The S205 is based on a Sallen-Key topology as most LPGs are. Internally it has a VCA in it, where both the cutoff and the VCA amp is controlled using the same knob/CV (labeled C/A). Most LPGs use Vactrols which act as an internal AD envelope generator. However, the vactrol is rather slow, it doesn’t work for high frequency modulation. So we decided not to use a vactrol emulation, as it can be easily simulated with an external ADSR module.

 

The S205 also includes a drive control. Notice that both the “Level” and “Drive” knobs/CV act together in changing the character of the overdrive. So the “Level” knob IS NOT ONLY for volume. The “Drive” works internally while the “Level” bloats the filter. This module can produce both clean and dirty results because of the high ranges used.

 

There is a switch to decide where to place the internal VCA. “Post” will put it after the filter, “Pre” will put it before the filter. And there is a third option for only using the filter without the internal VCA.

 

The overall design objective here is to be both similar and different. Similar to typical LPGs but also very different when you want it to be.