Oscillators
WR-921A
The WR-921A Oscillator Driver - a control-only module that sums pitch CVs and pulse-width CVs and broadcasts them to a bank of WR-921B voice oscillators.
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The WR-921A
The WR-921A is a control-only module - it has no audio output. Its job is to sum incoming 1 V/Oct pitch CVs, add a manual FREQUENCY offset, sum incoming pulse-width CVs, add a manual WIDTH offset, and broadcast the two composite control signals to a bank of paired WR-921B voice oscillators. Three summing frequency inputs (F1, F2, F3), two summing width inputs (W1, W2), two CV outputs (FREQ, WDTH).
Centralising pitch summing makes wide unison cheap and stable: a single WR-921A drives many WR-921Bs through a mult, so adding voices doesn't multiply the calibration burden. Each WR-921B's own FINE knob and RANGE switch can be set independently while the master pitch comes from one place.
A RANGE switch picks the FREQUENCY knob's span: SEMI covers ±12 semitones for fine intervalic tuning, OCT covers ±6 octaves for coarse transpose. The WIDTH knob (10% - 90%) plus the two width CV inputs sum into a 0-1 pulse-width CV on the WIDTH OUT bus. Both outputs are clean DC-coupled CVs at 1 V/Oct (FREQ) and 0-1 (WIDTH).
The WR-921A
The WR-921A has three controls: a FREQ knob (-6 to +6), a RANGE switch (SEMI for ±12 semitones, OCT for ±6 octaves), and a WIDTH knob (10% - 90%). Five CV inputs sum into the two outputs: FREQ OUT is a 1 V/Oct composite pitch CV, WDTH OUT is a 0-1 composite duty-cycle CV. There is no audio path. The FREQ OUT runs at 1 V/Oct for cross-module compatibility (some external 921B-style oscillators expect 0.5 V/Oct; with those, a volt of CV will produce two octaves of pitch shift).
In a patch
The canonical WR-921A patch is keyboard -> WR-921A F1 -> WR-921A FREQ OUT -> mult -> N x WR-921B CONTROL. Every WR-921B follows the keyboard at 1 V/Oct, but each voice's FINE knob and RANGE switch can be set independently. Detune them slightly and the result is analog unison. Set them to different RANGE positions and the same keyboard yields octave doubling.
The width chain is the global PWM bus. One LFO into W1 makes every paired WR-921B pulse-width-modulate in lockstep. This is musically very different from each WR-921B running its own LFO - the unison stays phase-coherent and the chorusing is rhythmic rather than randomised.
If only one voice is needed, skip the WR-921A entirely and use the standalone WR-921 instead - it has its own pitch summing on board.
Inputs
- F1 (cv) — Frequency CV input 1 (1 V/oct). Typical routing: keyboard or piano-roll V/oct for pitch.
- F2 (cv) — Frequency CV input 2. Patch a sequencer row for melodic lines, or an envelope for pitch ramps.
- F3 (cv) — Frequency CV input 3. LFO here = vibrato across the whole oscillator bank at once.
- W1 (cv) — Width CV input 1. Summed with W2 and the WDTH knob. LFO here gives global PWM.
- W2 (cv) — Width CV input 2. Layer with W1 for complex PWM motion.
Outputs
- FREQ (cv) — Composite pitch CV out (1 V/oct). Patch into one or more WR-921B CONTROL IN jacks, typically via a WR-994 or WR-CP35 multiple for fan-out.
- WDTH (cv) — Composite duty-cycle CV out (0-1 range). Patch into one or more WR-921B WIDTH inputs for synchronized pulse-width across the bank.
Controls
- Freq — OSCILLATOR DRIVER FREQUENCY. Manual pitch offset summed with the CV inputs. Scale selected by the SEMI/OCT switch.
- Range — Frequency knob span. SEMI = +/-12 semitones (2 octaves total, fine tuning), OCT = +/-6 octaves (12 octaves total, coarse transpose).
- Width — RECTANGULAR WAVE WIDTH. 10-90% manual duty-cycle offset for the paired WR-921B RECT outputs. Sum with W1 and W2 CVs for modulation.
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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