Doepfer A-120V VCF1 Low Pass Filter Vintage Black Panel
Module A-120 is a voltage controlled low-pass filter, which filters out the higher parts of the sound spectrum, and lets lower frequencies pass through. The Cut-Off Frequency determines the point at which filtering takes effect. You can control this manually, or by voltage control (filter modulation, for instance by an LFO). Three CV inputs are available, and the sum of the voltages from these affects the filter cut-off.
The module is based on a so-called "transistor ladder" design, with a cut-off slope of -24 dB/octave, as in various Moog synthesizers. That's what gives it its classic, legendary Moog sound.
Resonance (or Emphasis) is adjustable all the way up to self-oscillation - in which case the filter behaves like a sine wave oscillator.
Technical note: The module has been revised in 2009. The single transistors have been replaced by so-called transistor arrays which look like integrated circuits. But these are no "real" integrated circuits but contain nothing but 5 matched transistors. The new version of the module is still a transistor ladder based low pass filter!