Filters

WR-904A

A four-pole 24 dB/octave transistor-ladder low-pass filter with regeneration to self-oscillation.

The WR-904A

The WR-904A is a voltage-controlled low-pass filter built around a four-stage transistor ladder. Four cascaded one-pole stages produce a 24 dB/octave rolloff. A REGENERATION (resonance) knob sweeps the feedback path from a flat response up to a singing self-oscillating sine wave at the cutoff frequency. Three summing 1 V/Oct CV inputs control cutoff alongside a FCV knob and a three-position RANGE switch.

The ladder topology has a distinctive signature: the same negative feedback path that lifts a peak at the cutoff frequency also subtracts low-frequency energy from the signal as resonance climbs - the bass thins out near self-oscillation. This is intentional, not a defect; it is the response shape the four-pole ladder is known for, and the WR-904A keeps it.

Past regen ~0.9 the filter self-oscillates as a clean sine wave that tracks the cutoff CV. The audio input can be left unplugged and the WR-904A becomes a sine VCO tracking 1 V/Oct from any of the three CV inputs - useful for sub-bass, FM carriers, or sine-tone percussion. Output is ±5 V Eurorack level.

The WR-904A

The WR-904A has three controls: an FCV knob (0-10 V, with each volt = one octave), a three-position RANGE switch (SUB / MID / HI, each step shifting the base frequency by two octaves), and a REGEN knob from flat to self-oscillation. Three 1 V/Oct CV inputs sum with the FCV knob into the cutoff control. Self-oscillation tracks 1 V/Oct to about ±0.05 cents over the audible range so it can serve as a sine VCO. Output is ±5 V Eurorack level. The DSP is a Huovilainen-style ZDF ladder model with tanh saturation per stage and 2x oversampling; resonance feedback intentionally thins low-end energy as it climbs.

In a patch

The WR-904A sits at the subtractive heart of a voice patch. Wire a WR-921 SAW into the audio in, the audio out into a WR-902, and a WR-911 envelope into both cv-1 of the WR-904A (filter envelope) and a CV input of the WR-902 (amplitude envelope). The oscillator generates a harmonically rich saw, the WR-904A carves it into a sound, the WR-902 shapes it over time.

Self-oscillation is its own instrument. Crank regeneration past 0.9, unplug the audio input, and the WR-904A becomes a clean sine VCO tracking 1 V/Oct from any of the three CV inputs. Useful for sub-bass lines, FM carriers, or sine-tone percussion alongside a WR-923.

Pair the WR-904A with a WR-904B for a voltage-controlled band-pass - WR-904A for the high-frequency rolloff, WR-904B for the low-frequency rolloff, both tracking the same CV. Or use the dedicated WR-904C Filter Coupler to do the same routing with one knob each for CENTER and BANDWIDTH.

Inputs

IN (audio) — Signal input. Eurorack ±5 V audio. Designed for ~ ±5 V peak signals; push it harder for intentional distortion at the input stage.
CV 1 (cv) — Cutoff CV input 1 (1 V/oct). Typical routing: WR-911 envelope output for filter sweeps. Sums with CV 2, CV 3, and FCV knob.
CV 2 (cv) — Cutoff CV input 2. Patch keyboard or sequencer V/oct here so the filter tracks pitch - essential for maintaining timbre across the keyboard.
CV 3 (cv) — Cutoff CV input 3. Try an LFO here for filter wobble, or fast audio-rate modulation for metallic filter-FM.

Outputs

OUT (audio) — 24 dB/oct low-passed output at Eurorack ±5 V level. Self-oscillating sine when REGEN is past ~0.85 - works as a 1 V/oct oscillator in that mode.

Controls

FCV — Fixed Control Voltage. 0-10 V manual cutoff bias. Each volt doubles the cutoff frequency (1 V/oct). RANGE switch picks the baseline: SUB starts at 1 Hz, MID at 4 Hz, HI at 16 Hz.
Range — range switch (3 positions). SUB covers 1 Hz - 5 kHz (sub-audible filter rides); MID covers 4 Hz - 20 kHz (normal subtractive-synth operating zone); HI covers 16 Hz - 60 kHz (audio-rate FM or supersonic sweeps).
REGEN — REGENERATION (resonance). 0 = flat filter, 0.3-0.55 = musical emphasis, 0.55-0.85 = singing with noticeable bass thinning, 0.85+ = self-oscillation as a pure sine. Analog ladder behaviour: high regen eats bass because of the feedback-path topology.

Synths.pw and the Classic-series modules are not affiliated with, endorsed by, or sponsored by Moog Music Inc. Module numbers reference the late-1960s American modular tradition and are used in a historical sense.

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