Module A-121 is a voltage-controlled multi-mode filter with a cut-off slope of -12 dB / octave. Four simultaneous outputs are available, each with different characteristics: low-pass, band-pass, high-pass and notch (or band reject). The cut-off frequency determines the point at which the respective filter effects appear. The frequency can be adjusted manually, or by voltage control (Filter modulation, for instance by an LFO or ADSR). Two CV inputs are available, whose control voltages are summed. Resonance (Emphasis or Q) can be adjusted manually, or by voltage control, right up to self-oscillation, in which case it will behave like a sine wave oscillator.

Technical notes: Module A-121-2 is the successor of the obsolete module A-121. But the circuitry is totally different from the A-121 which used the obsolete CEM3320 filter circuit. The sound of the new module A-121-2 is identical to filter of the Dark Energy II but has been expanded by the voltage-controlled resonance feature.