March 18, 2011
The Lopass Gate is widely recognized as a sweet sounding filter with a Vactrol-induced slew to it. Set into the top switch position, it is a Sallen-Key filter very similar to the Korg MS-10/MS-20 filter. The LED/photocell elements, known as Vactrols, add something around 10ms of slew on the attack and 100ms of slew on the decay. This is more than an OTA filter for sure, but what if there was some REAL slew on the CV?
Then, there's the Buchla 192. A simple, nonresonant, 2 pole lopass filter, featuring an all discrete transistor signal path with the control elements made from photocells and incandescent lamps in a little bit of heat shrink tubing! The original run (it's quite rare, but I don't know how many are out there) of this module didn't even have a CV input. It has a slew of several seconds when the cutoff is changed.
The one that landed on my workbench was living behind a Dual Reverb front panel with the labels scratched off. It was not functioning. Since I discovered at least one dead "optical element" and this device was such a hack anyway, the owner and I decided to mod it for actual VTL5C3/2 Vactrols. After all was said and done, it is a very interesting sounding lopass. Sort of like a 292 in lopass mode, but with an all discrete signal path.