note about lowpass gates
2006-05-16 by Gary Chang
I wanted to post a note about the "Lowpass Gate" and its usage throughout electronic music, for the purpose of clarifying why some of us have obsessed about them.... From the 200 Series Brochures, circa 1970s: "The Model 292[Quad Voltage-Controlled Lopass Gate], with the capability of simulating the spectral and amplitude changes that accompany a receding sound source, is one of several Electric Music Box modules that can deal with spatial localization and movement." There were three opreation modes for these devices - filter, vca and dual. Obviously, when in dual mode, the lopass gate operates as both - a filter(6db/oct) and a VCA. This dual mode is the sound of Mort Subotnick's electronic realizations of the 70's. It has a characteristically "woody" chirp to its transient character. It is this mode that most of us who used the old Buchlas miss, hence the quest for "LowPass Gates". The issue with the recent versions of this circuit is that most of the versions of the the LP gate is that they are too slow. When hit with a short transient, they tend to ring excessively, making the "uber marimba" patch made famous by Subotnick all but impossible without slapping a VCA after the gate to shorten the sound. The old Buchla had only a 1v ptp audio signals, so the vactrol didn't have as far to go to open all the way up, compared to the 10v ptp audio signals found on Wiard and many other systems, so it was very quick. As a matter of fact, even the new 200e has very slow lopass gates, needing a second gate in VCA mode to shorten the ring. This is not typical of the 70's 200 series 292 gates. Then in comes the Borg2, and we have a perfect 10v ptp lopass gate - even better than the original (which are 1v ptp and had a "dual linear" - as opposed to logarithmic cv curve). I also love to use the original Borg Filter for multichannel panning with the JAG, because it rolls off the distant channels (just like the 227 quad locator of the Buchla 200 system), while opening up to full bandwidth where the sound is nearer. So, that's the lowdown on this LowPass Gate business.... gary