The Doepfer A-101-2 is a classic Low Pass gate design - a combination of Low Pass Filter and VCA. It is built around a Vactrol - a combination of light source and photo sensitive receptor, which acts like a VCA with a slow decay or 'ring' to it.
It can function as either or both effects. It also has an additional trick up its sleeve in that you can select LPF, VCA or the combined effect (known as Low Pass Gate or LPG) by feeding a combination of 2 gate signals into the unit. This can lead to rhythmic, pulsating sounds from the unit.
The A-101-2 is easy to use, and can function as a straight forward filter module, or (by setting the resonance to minimum) can deliver the classic Buchla LPG patch. It is a very cost-effective unit, in that it can respond just to a clock pulse or gate, and does not require an envelope generator for the VCA to cut the output to zero.
The only reservation I have with this unit is that the Vactrols employed tend to deliver a faster decay than LPG's from other makers I have tried. On the other hand, I have read that there can be individual differences between 2 Vactrols, so that another A-101-2 may sound quite different.
