Filters
WR-904B
The WR-904B - a 24 dB/octave high-pass companion to the WR-904A, designed to chain into a voltage-controlled band-pass.
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The WR-904B
The WR-904B is a four-pole transistor ladder voltage-controlled high-pass filter with a 24 dB/octave rolloff. Three summing 1 V/Oct CV inputs feed the cutoff control alongside a FCV knob covering 0-10 V of manual bias and a two-position RANGE switch (LOW: 4 Hz - 20 kHz, HIGH: 10 Hz - 50 kHz).
The WR-904B has no resonance control. It is a pure ladder HPF designed for tonal sculpting and for chaining with the WR-904A low-pass to form a voltage-controlled band-pass. Internally the high-pass shape is computed as input - lowpass(input) from the same ladder topology, giving the right phase and a clean, gentle character without resonance peaking.
Use it whenever a patch needs subtractive shaping rather than resonant cutoff: scoop bass rumble out of a noise source, remove low-end before the main filter stage, or chain it after a WR-904A to form a moving band-pass. Output is ±5 V Eurorack level.
The WR-904B
The WR-904B has two controls: an FCV knob (0-10 V, one octave per volt) and a two-position RANGE switch (LOW: 4 Hz - 20 kHz, HIGH: 10 Hz - 50 kHz). The default position is HIGH because that is the typical audio-band setting; LOW exists for sub-audio CV filtering and for chaining at very low cutoffs in band-pass topologies. Three 1 V/Oct CV inputs sum with the FCV knob into the cutoff. The DSP runs the ladder at the high-pass cutoff and computes input - lowpass(input) for the 24 dB/oct shape, with tanh saturation per stage and 2x oversampling. No resonance control. Output is ±5 V.
In a patch
The classic WR-904B patch is the band-pass chain. Send audio into a WR-904A, the WR-904A output into a WR-904B, and share the same CV across both cv-1 inputs. Set the WR-904A cutoff above the WR-904B cutoff. A single CV now sweeps both - the band between the two cutoffs is what passes. The dedicated WR-904C Filter Coupler automates this routing with one knob each for CENTER and BANDWIDTH.
Standalone, the WR-904B is a bass-cleaning utility. Patch a WR-923 noise source through it to scoop low rumble before sending the noise into a snare or hi-hat patch. Patch a sub-bass through it with the cutoff set just above DC to remove power-supply hum without affecting tone. Or sweep a slow LFO into cv-1 for tape rumble-style filter motion on a drone.
For more aggressive high-pass duties - resonant sweeps, steep cuts, sharp transients - reach for a generic state-variable filter. The WR-904B is gentle by design.
Inputs
- IN (audio) — Signal input. Accepts Eurorack ±5 V audio. The WR-904B is DC-coupled, so slow CV passes through as well.
- CV 1 (cv) — Cutoff CV input 1 (1 V/oct). Envelopes here give inverse high-pass sweeps.
- CV 2 (cv) — Cutoff CV input 2. Keyboard V/oct for pitch-tracking timbre.
- CV 3 (cv) — Cutoff CV input 3. LFO for wobble effects.
Outputs
- OUT (audio) — 24 dB/oct highpass output at Eurorack level. No resonance peak - the signal rolls off cleanly below cutoff.
Controls
- FCV — Fixed Control Voltage. 0-10 V manual cutoff bias (1 V/oct). RANGE switch picks LOW (4 Hz baseline) or HIGH (10 Hz baseline).
- Range — range switch. LOW spans 4 Hz - 20 kHz; HIGH spans 10 Hz - 50 kHz (1.5 octaves higher).
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