The A-101-2 is a vactrol based combination of a Lowpass filter (LP) and VCA. The term “Low Pass Gate” was created by Don Buchla as used in his 292 module, but in contrast to the Buchla design the A-101-2 offers attenuators for both CV and audio input, a manual resonance control and two gate inputs to control the function of the module in addition to the manual toggle switch. The A-101-2 is essentially a 12 dB lowpass filter that can be switched to VCA or a combination of lowpass and VCA (the toggle switch function).
The controlling elements for filter frequency and amplitude (VCA) are so-called vactrols, using an LDR under control of a voltage driving an LED (the LED on the panel shows the state of the vactrol). The filter resonance will go up to self oscillation (manual control - not under voltage control). Vactrols are known for their smooth sound character, also the input will go into distortion with settings of the input attenuator over half way.