Filters

WR-904C

The WR-904C Filter Coupler - the third member of the 904 family that ties a WR-904A and WR-904B into a single CENTER + BANDWIDTH band-pass / band-reject pair.

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The WR-904C

The WR-904C is a filter coupler that ties an internal four-pole low-pass and a four-pole high-pass into a single voltage-controlled band-pass or band-reject topology. Two main controls run the pair together: CENTER sets the shared mid-point between the two cutoffs, and BANDWIDTH spreads them apart from (or pulls them toward) that center. Both have CV inputs that sum with the knobs.

The math is direct. Given a center voltage C and a bandwidth voltage B, the LP cutoff lands at C + B octaves above the base frequency, and the HP cutoff lands at C - B octaves above the base. As B grows, the band widens; as B shrinks toward zero, the band collapses to a single resonant peak; as B goes negative, the LPF and HPF cross and the band-pass becomes silent (a useful patch destination for gating filtered audio).

A three-position MODE switch picks the topology applied to the audio input: OFF (signal passes through unchanged), BAND PASS (HPF -> LPF in series, only the band between cutoffs survives), and BAND REJECT (LPF + HPF summed in parallel, the band between cutoffs is notched out). Two buffered CV outputs (CENTER OUT and BW OUT, the latter inverted) replicate the routing math so an external WR-904A + WR-904B pair can be driven the same way as the internal filters.

The WR-904C

The WR-904C has three controls: a three-position MODE switch (OFF / BAND PASS / BAND REJECT), a bipolar CENTER knob (±6 V), and a bipolar BW knob (±1.5 V, max ±3 octaves of spread). Two 1 V/Oct CV inputs sum into the CENTER (CNTR and CNTR 2); a third (BW) sums into the bandwidth. Internal LPF and HPF use the same Huovilainen-style ladder DSP as the WR-904A, with 2x oversampling. Base centre frequency is 200 Hz; ±6 V of CENTER covers roughly 3 Hz - 12.8 kHz. Two buffered CV outs (C OUT and BW OUT, BW inverted so the pair spreads symmetrically when both filters share the same C OUT) drive an external WR-904A + WR-904B pair if the internal filters are not wanted.

In a patch

The WR-904C's strongest patch is wah-style filter motion. Patch audio in, set MODE to BAND PASS, set BANDWIDTH to a small positive value (~0.5), and sweep CENTER with an LFO or envelope. The result is a single moving band that scans across the spectrum - a voltage-controlled wah. With a long, slow LFO it becomes a formant filter for synthesised vowel sounds; with a fast envelope it becomes a pluck or talkbox accent.

BAND REJECT mode gives a moving notch filter. Sweep CENTER with an LFO for the whooshing one-notch sound. Sweep BANDWIDTH instead and the notch opens and closes in place - useful for de-essing or scooping a specific frequency range out of a drone.

For external routing, ignore the internal filters: patch the CENTER OUT and BW OUT CV outputs into a separate WR-904A and WR-904B, route audio through them, and use the WR-904C as a pure CV controller. The buffered outputs make this swap trivial.

Inputs

IN (audio) — Signal input. Audio enters the internal HPF → LPF chain.
CNTR (cv) — CENTER FREQUENCY CV. Summed with the CENTER knob and CNTR 2 at 1 V/oct. Envelopes here give classic filter sweeps.
CNTR 2 (cv) — Second CENTER FREQUENCY CV (summing). Use this to mix an envelope and LFO without needing an external CV mixer.
BW (cv) — BANDWIDTH CV. Summed with the BW knob. Modulate to morph between narrow resonant passband and wide open.

Outputs

OUT (audio) — Composed filter output. BP = HPF → LPF cascade; BR = LPF + HPF parallel sum (notch); OFF = pass-through.
C OUT (cv) — CENTER CV output. Buffered sum of knob + both CENTER CV inputs. Drive an external WR-904A + WR-904B for a separate band-pass routing.
BW OUT (cv) — BANDWIDTH CV output, inverted (for driving the WR-904B HP cutoff alongside C OUT on the WR-904A LP). Front-panel coupling for an external pair.

Controls

MODE — Filter mode: BAND REJECT (notch), OFF (pass-through), or BAND PASS (LPF × HPF cascade).
CENTER — Center frequency offset from the 200 Hz baseline. Each volt is one octave. At 0 V the band centres at 200 Hz; at +4 V the band centres at 3.2 kHz.
BW — Bandwidth spread in octaves. +1.5 V = LPF 1.5 octaves above and HPF 1.5 octaves below the centre (3-octave band). Bandwidth extendable to 3 octaves. 0 V closes the band; negative values invert it.

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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