Analogue Systems RS-120 Comb Filter

The RS-120 is quite an unusual filter because unlike other filters it cuts several notches into the audio signal which result in a comb-like look of the frequency spectrum. Soundwise it is perfectly suited to create phaser and flanger sounds.

The filter is based on a bucket-brigade-device delay with very short times that clearly changes the phase of the signal. The feedback of this delay is used here as a resonance parameter that goes up to self-oscillation.

There are two audio inputs with level control. Unity gain is reached at the pot´s position 4, beyond thsi point the filter will be overdriven. A control sets the delay time from 2.5ms to 25ms and in addition there are two CV inputs for controlling delay time, one with fixed 1V/oct and a variable one.