Doepfer A-101-2v is a low-pass gate module for modular synthesizers in Eurorack format. Low pass gate means that the module can be switched between low pass filter and VCA. Doepfer A-101-2v is basically a -12dB lowpass filter, which can also be switched to VCA or a combination of lowpass and VCA. The controlling elements for the filter frequency and for the volume are so-called Vactrols. In the combination of low pass filter and VCA, frequency and volume change simultaneously. This means that the quieter the signal, the duller it gets. Furthermore, the module has attenuators for the externally supplied control voltage as well as the audio input signal, a manually adjustable resonance, which extends to self-oscillation, and two gate inputs, so that the module can also be controlled via external gate signals.

Vactrol

A Vactrol is a combination of light depending resistor (LDR) and light-emitting diode, which are housed in a common, light-tight housing. Increasing the LED brightness reduces the resistance of the LDR. This can be used for various circuits based on variable resistors. These include VCFs, VCAs, voltage-controlled phasers, trigger delays, slew limiters, LFOs, oscillators or envelope generators. These components are known for their"soft" sound, which is due to the fact that they are linear elements that do not cause distortion.