doepfer/a-121s
A-121s Stereo Multimode Filter
Dual multimode filter usable for stereo applications or as parallel/serial dual mono filters. Filter type continuously morphs from lowpass via notch and highpass to bandpass (-12dB/oct). Offers common and individual controls for frequency, resonance, and filter type, plus a frequency spread (Delta F) control.488
Capabilities 2
Jacks 12 — 10 in / 2 out
| Name | Signal | Voltage | Description |
|---|---|---|---|
| Inputs (10) | |||
| CFM1 | cv | — | Common frequency modulation input for filter 1, processed by a VC-polarizer controlled by the CFM knob. CFM2 is normalled to CFM1.488 |
| CFM2 | cv | — | Common frequency modulation input for filter 2, processed by a VC-polarizer controlled by the CFM knob. Normalled to CFM1.488 |
| Delta FM | cv | — | CV input for controlling the frequency spread (difference) between the two filters, processed by the Delta FM polarizer.488 |
| F | cv | — | Common frequency CV input for both filters, approximately 1V/octave.488 |
| In1 | audio | — | Audio input for filter 1. In2 is normalled to In1 so the same signal can feed both filters.488 |
| In2 | audio | — | Audio input for filter 2. Normalled to In1 so the same signal can feed both filters.488 |
| QM/TM1 | cv | — | Resonance and/or filter type modulation input for filter 1 (and/or 2), assignable via internal jumpers. Normalled to a fixed positive voltage.488 |
| QM/TM2 | cv | — | Resonance and/or filter type modulation input for filter 2 (and/or 1), assignable via internal jumpers. Normalled to QM/TM1.488 |
| SFM1 | cv | — | Individual frequency modulation input for filter 1, processed by polarizer SFM1. Normalled to a fixed positive voltage so SFM1 knob acts as individual frequency control when unpatched.488 |
| SFM2 | cv | — | Individual frequency modulation input for filter 2, processed by polarizer SFM2. Normalled to SFM1.488 |
| Outputs (2) | |||
| Out1 | audio | — | 488 |
| Out2 | audio | — | 488 |
Parameters 14
| Name | Type | Range | Behavior |
|---|---|---|---|
| CFM | knob | — | Common frequency modulation polarizer controlling two VC-polarizers for CFM1 and CFM2 inputs simultaneously. Center position is neutral. Allows the same modulation signal to affect both filters with simultaneous level/polarity control.488 |
| Delta F | knob | — | Manually controls the frequency spread (difference) between the two filters. At center position the frequencies are equal (when all other frequency parameters are at neutral).488 |
| Delta FM | knob | — | Polarizer controlling the level and polarity of the Delta FM CV input, allowing external CV to control the frequency spread between the two filters.488 |
| F | knob | — | Master frequency control for both filters simultaneously. Fully CCW is neutral position.488 |
| Level 1 | knob | — | Attenuator for audio input 1. Fully CCW is neutral (minimum) position.488 |
| Level 2 | knob | — | Attenuator for audio input 2. Fully CCW is neutral (minimum) position.488 |
| Link to 1 | switch | — | Toggle switch that links Type 1 knob to also control filter 2's type simultaneously; Type 2 knob becomes inactive when engaged.488 |
| Q | knob | — | Controls the resonance of both filters simultaneously. Fully CCW is neutral position.488 |
| QM/TM1 | knob | — | Attenuator for QM/TM1 CV input. Its function (resonance modulation and/or filter type modulation for filter 1 and/or 2) is set by internal jumpers. Fully CCW is neutral. Socket normalled to fixed positive voltage, so this knob also acts as manual Q/Type control when socket is unpatched.488 |
| QM/TM2 | knob | — | Attenuator for QM/TM2 CV input. Its function (resonance modulation and/or filter type modulation for filter 1 and/or 2) is set by internal jumpers. Fully CCW is neutral. Socket normalled to QM/TM1.488 |
| SFM1 | knob | — | Polarizer/attenuverter for the SFM1 input (individual frequency modulation for filter 1). Center position is neutral. When SFM1 socket is unpatched, acts as an individual frequency control for filter 1 via the socket's fixed positive voltage normalling.488 |
| SFM2 | knob | — | Polarizer/attenuverter for the SFM2 input (individual frequency modulation for filter 2). Center position is neutral. When SFM2 socket is unpatched, acts as an individual frequency control for filter 2 via normalling to SFM1.488 |
| Type 1 | knob | — | Continuously morphs filter 1 type from lowpass (L) through notch (N) and highpass (H) to bandpass (B).488 |
| Type 2 | knob | — | Continuously morphs filter 2 type from lowpass (L) through notch (N) and highpass (H) to bandpass (B). Has no function when Link to 1 switch is engaged.488 |
Firmware history 0 versions
No firmware history recorded.
References 1
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[488]
Doepfer A-121s Stereo Multimode Filter product pageproduct_page● verified 2026-05-17