The A-120 is a 4-pole voltage controlled low pass filter , using a transistor ladder design with a cut-off slope of -24dB/octave as used in the original Moog Minimoog. So, this gives that 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.
The Cut Off Frequency determines the point at which filtering takes effect, this is controlled either manually or by voltage control .
Three CV inputs are available and the sum of the voltages from these control the filter cut-off.