Filters

WR-923

The WR-923 - a utility module pairing two gentle 6 dB/oct filters with white and pink noise sources.

The WR-923

The WR-923 is a utility module that bundles four useful tools onto one panel: a single-pole 6 dB/oct low-pass filter, a matching single-pole 6 dB/oct high-pass filter, a white noise source, and a pink noise source. There is no voltage control on the filters - cutoff is knob-only - and no resonance.

The single-pole topology is intentional. A 6 dB/oct rolloff is the gentlest filter slope that still meaningfully shapes a signal - it removes spectral weight without imposing the strong character of a steeper filter. Patch white noise through the LP at 2 kHz and the result is the warm rolled-off hiss often associated with analog tape; patch the same noise through the HP at 2 kHz for airy cymbal brightness. These slopes are for tone shaping, not for voicing - reach for the WR-904A or a state-variable filter when resonance and steeper rolloff are needed.

Two noise outputs sit alongside the filters. White noise is a flat-spectrum random signal - equal energy at every frequency - and is the canonical source for snare drums, hi-hats, wind, and any unpitched percussive element. Pink noise has equal energy per octave (a -3 dB/oct slope) which is closer to natural sounds and human hearing. A fifth output, LF, is a heavily-smoothed white-noise stream useful as a slow random CV for filter cutoffs, oscillator drift, or organic modulation.

The WR-923

The WR-923 has two knobs: LP cutoff (10 Hz - 10 kHz, exponential) and HP cutoff (same range). Both filters are single-pole 6 dB/oct, knob-only, no resonance. Two audio inputs (LP IN, HP IN) and five outputs: LP, HP, WHT (white), PNK (pink), LF (low-frequency / red noise). Pink noise uses a Voss-McCartney 16-row sum approximation for a statistically correct -3 dB/oct slope. The LF output passes white noise through a 6 Hz one-pole low-pass with ~55x gain compensation, clamped to ±5 V. Audio outputs are at ±5 V Eurorack level.

In a patch

The WR-923 is the noise source for almost every percussion patch. White out -> WR-904A cutoff knob set high -> WR-902 -> WR-911 ADSR with fast decay = a snare drum. Same chain with the WR-904A high-pass (or the WR-923's own HP) and a longer release = hi-hat. Pink noise into a WR-914 with bands open at 500 / 1000 / 2800 = vowel-like breathing textures.

The filters are tone-shaping helpers, not voicing filters. Patch a thick SAW from a WR-921 through the WR-923 HP at 200 Hz to roll off the sub-bass before the main filter stage; patch a noise source through the LP at 4 kHz to soften the high end before sending it into a snare envelope. The single-pole slopes are gentle enough to add colour without destroying the source.

The LF noise output is a slow random CV. Patch it into a VCO pitch input at low depth for analog drift, into a filter cutoff for organic wobble, or into a quantizer for random melody lines. Pair with a clock and a sample-and-hold for stepped random rather than continuous.

Inputs

LP IN (audio) — Input to the low-pass filter. DC-coupled so CV works too.
HP IN (audio) — Input to the high-pass filter. Independent from LP IN.

Outputs

LP (audio) — 6 dB/oct lowpass output.
HP (audio) — 6 dB/oct highpass output.
WHT (audio) — White noise - full spectrum, equal energy at all frequencies. Bright / hissy.
PNK (audio) — Pink noise - -3 dB/octave rolloff. Warmer than white, closer to natural environmental noise (rain, wind).
LF (cv) — Low-frequency / "red" noise - heavily smoothed (~6 Hz bandwidth) slow random CV. Use for humanising pitch, drifting filter cutoffs, or any place a slow random-walk voltage is wanted.

Controls

LP — Low-pass cutoff. 10 Hz - 10 kHz exponential. No voltage control - knob only.
HP — High-pass cutoff. 10 Hz - 10 kHz exponential. No voltage control - knob only.

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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