doepfer-a-121-3

Doepfer A-121-3 Multi Filter

Four simultaneous filter outputs with voltage-controlled resonance

4 HP Eurorack module

The A-121 VCF 2 is a voltage-controlled multimode filter with a -12 dB/octave cut-off slope, simultaneously outputting low-pass, band-pass, high-pass, and notch (band-reject) signals. Built around the CEM 3320 filter IC, it offers voltage-controllable resonance up to self-oscillation, giving it a smooth, musical character suited to classic analog synthesis timbres, filter sweeps, and vocal-like formant effects.

Features

  • Four simultaneous filter outputs: low-pass, band-pass, high-pass, notch
  • -12 dB/octave cut-off slope
  • Voltage-controlled cut-off frequency (two CV inputs: FCV 1 direct, FCV 2 with attenuator)
  • Voltage-controlled resonance/Q (two CV inputs: QCV 1 direct, QCV 2 with attenuator)
  • Resonance adjustable to self-oscillation for use as sine wave oscillator
  • 1 V/octave tracking standard on FCV 1 and QCV 1
  • Audio input level attenuator to manage distortion or add overdrive
  • Built on CEM 3320 filter IC
  • Resonance is largely frequency-independent below self-oscillation threshold
  • Notch output produces phasing-like effect when f_c is LFO-modulated

Controls & Jacks (16)

  • Audio Level— Input signal attenuator controlling the level of the audio signal entering the filter
  • FCV 1— Cut-off frequency CV input (1V/octave standard)
  • Freq.— Cut-off frequency control for manual adjustment of the filter's center frequency
  • Audio In— Audio input to the filter
  • QCV 1— Resonance CV input (1V/octave standard)
  • FCV 2— Secondary cut-off frequency CV input, level controlled by FCV 2 attenuator
  • FCV 2— Attenuator for the FCV 2 voltage control input, controlling the intensity of frequency modulation
  • Q— Resonance (emphasis/Q) control, emphasizing frequencies around the cut-off point
  • Band— Band-pass filter output
  • Notch— Notch (band-reject) filter output
  • High— output jack
  • Low— Low-pass filter output
  • FCV2— Resonance (emphasis/Q) control, emphasizing frequencies around the cut-off point
  • Res.— Resonance (emphasis/Q) control, emphasizing frequencies around the cut-off point
  • HP— Secondary resonance CV input, level controlled by QCV 2 attenuator
  • QCV 2— Secondary resonance CV input, level controlled by QCV 2 attenuator

How-To Guides

  • Classic Filter Sweep
  • ADSR Envelope Filter
  • LFO Wah Effect
  • Self-Oscillating Sine Melody
  • Notch Filter Phasing
  • Spatial Spectrum Distribution
  • Vocal Formant Synthesis
  • Keyboard-Tracking Filter
  • High-Pass Body Removal
  • Voltage-Controlled Resonance Sweep

Explore the interactive A-121-3 Multi Filter panel on BleepDex — tap any control to learn what it does.

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