303 circuit model

Source. Roland main board schematic, Feb 19 1982, with unclear values read from the TD-3 clone sheet. Parts on neither sheet are estimates. Running at 2× oversampling.

Pitch and VCO. A 6-bit R-2R ladder from 5.333 V gives 1/12 V per step. An exponential current charges C33 from 5.5 V to 12 V, then resets, smoothed with a polyBLEP.

VCF. Four coupled ladder stages with a tanh law per transistor pair, 18n bottom cap and 33n above, solved implicitly. Full feedback (k = 17) self-oscillates.

Envelopes and VCA. C62 decays through R138 + VR6 (200 ms to 2.5 s). Accent shorts VR6 and charges the sweep cap. The OTA VCA gets resonance compensation through C76/R100, and normal notes fade in through R_SOFT.

Mods. Dotted parts map mostly to Devil Fish controls: Normal and Accent decay, VEG decay, Soft attack, Slide time, Sweep speed and self-oscillating resonance.

Lexicon 224. Runs on its own 20 kHz clock with 12-bit gain-ranging converters, 16-bit words and a shared 32K-word delay memory. The core is Dattorro's published Griesinger-style plate network; the 224's own microcode is not public, so the hall, chamber, room and chorus variants are estimates.

RAT. LM308 stage with gain-bandwidth and slew limits from its 30p cap, 1N914 diodes to ground, then the 3.3n filter.