Doepfer A-101-1v into a filter module for modular synthesizers in Eurorack format. It is the first Vactrol-based module in Doepfer's A-100 series and is based on the standard single-coupling filter circuit (non-inverting amplifier), whereby the different filter types are created by feeding the audio signal at different points of the circuit. This filter has its own input and a common output for lowpass, bandpass and highpass, where all three input signals appear simultaneously. Thus, three different audio signals can be fed simultaneously into this filter, which are summed up at the output and appear in different filtering (lowpass/bandpass/highpass). If the same signal is fed to all three inputs, different settings of the three input attenuators result in any combination of lowpass, bandpass and highpass. A notch filter is also possible. The filter inputs are normalized via switch sockets, if no signal is present at the input sockets for bandpass and highpass, the lowpass input signal is also present here.
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.