1. Scientific Bibliography, Patents & Workbench Measurements Implemented
• Stanford CCRMA — Scanner Vibrato LC Ladder (Kurt James Werner, W. R. Dunkel & F. G. Germain, DAFx-16, 2016): "A Computational Model of the Hammond Organ Vibrato/Chorus Using Wave Digital Filters". 18-stage, 19-node (v1..v19) M-derived LC ladder based on the inductance and capacitance parameters measured by Werner et al. and John M. Hanert's US Patent 2,509,923, solved in Warped-BLT state-space with resistive chorus mixing and continuous C² cubic B-spline electrostatic summing across the 16 stator plates.
• Stanford CCRMA & RE/P — Near-Field Rotary Acoustics (J. O. Smith III, S. Serafin, J. S. Abel & D. P. Berners, DAFx-02, 2002; C. A. Henricksen, Recording Engineer/Producer, April 1981): Based on "Doppler Simulation and the Leslie" (DAFx-02) and Clifford A. Henricksen's acoustic and polar measurements ("Unearthing the Mysteries of the Leslie Cabinet", RE/P 1981): near-field Doppler kinematics, 3 non-crossing acoustic reflection paths, Jensen V-21 phenolic throat compression + Bark formant filter H0(z), 15" Jensen C15L bass rotor, and walnut cabinet Helmholtz resonance.
• Aalto University (J. Pekonen, T. Pihlajamäki & V. Välimäki, DAFx-11, 2011; A. Carson, A. Wright, J. D. Parker & V. Välimäki, DAFx-24, 2024): "Computationally Efficient Hammond Organ Synthesis" (DAFx-11) and "Differentiable Grey-Box Modeling of Quasi-Periodic Audio Effects" (DAFx-24).
• Kon Zissis’s Public Generator & Manual Tapering Compilation (KonsData-20171011.xls — Public dataset of 100+ measured organs & 1965/1966 B-3/C-3 charts): Reference mV RMS output curve across all 91 wheels (including Alan C. Young’s 3.89 mV RMS 250 Hz valley on wheels #25–#36), the 61×9 Nichrome resistance wire matrix (104, 80, 50, 34, 24 Ω), shared generator pickup tonewheel robbing, and 6..64-turn primary taps on the independent T1 (Great + Pedals) and T2 (Swell + R1 / R2 || C1 secondary network) matching transformers.
• Faraday Law Alnico Pickups & 5 Generator Filter Families (US Patent 1,956,350 & factory service documentation): Pedal wheels #1–#12, flat-top reluctance profiles with copper eddy-current rings and shunt wire on #13–#43, series reactor wheels #44–#48, 0.255 µF LC filters on #49–#54, and 0.105 µF tuned / direct coils on #55–#91.
2. Lab Diary: The 7 Real Acoustic Traps & Dead Ends We Solved Along the Way
1. The Chorus C3 & Fast-Leslie "Yo-Yo-Yo" Formant Sweep: Early LC delay approximations and crossing rotary reflections caused a cyclic phaser/comb-filter hollowing on 88 8888 888. We solved it by implementing the 19-node tridiagonal LC ladder with C² B-spline stator plates and phase-aligning the 800 Hz LR4 15" drum crossover.
2. Quadratic (x²) Difference-Tone Intermodulation & Bass Bloat: Modeling asymmetric Class-A saturation in the T1 / T2 matching transformers, AO-28, and 6550 stages initially produced sub-harmonic difference tones (f2 − f1) that muddied close jazz chord voicings in the left hand. We separated independent low-frequency transformer core flux hysteresis (155 Hz) from odd/even tube stages with sample-rate-invariant DC blocking.
3. The Harmonic Percussion Marimba Trap (V7 12AU7A / V6 6C4): In one iteration we tested a woody 295/475 Hz resonator to thicken percussion — and realized it ruined the pure B-3 ping on fast bebop lines. We returned to the classic circuit: zero-latency Paliney #7 contact closure into transformer T4 in phase with the dry bus, variable-gain V7 12AU7A and V6 6C4 triode stage with dynamic Miller roll-off, physical 1' drawbar theft, and true T2 secondary network (R2 = 1 MΩ || C1 = 470 pF driving the AO-28 grid impedance) in Normal mode.
4. Restoring Chest Body (250–600 Hz) & Upper-Mid Openness (1.2–6.5 kHz): Cascading every real inductive roll-off darkened Full Organ. We audited every stage to eliminate internal comb paths and opened the Jensen V-21 phenolic throat up to 5.4 kHz.
5. Inline Pre-Leslie Console Spring Reverb Topology: Historical organs used two reverb setups: external stationary reverb channels (PR-40 / 122RV) or an inline Type-4 spring kit mounted inside the console between the AO-28 preamp and the 6-pin Leslie connector. SYNCHRONUS 91 models the inline console spring kit so the spring reflections enter the rotary cabinet and spin with the horn and bass rotor.
6. True Digital Headroom Without Hidden Compression: Recalibrated the rotary to unity gain (−5.4 dB) with 6 dB of output headroom so the safety ceiling never compresses normal playing.
7. Philly '56 Custom Console / 22H Rig: Inspired by hot-rodded Philadelphia consoles — 4 silver left-cheek pots (Bass, Treble, Perc Vol and 1.15 ms phase-aligned Direct DI Blend), A-100 octagonal Chorus Mix knob, and a 1-speed 22H rotary with a ~20-second free-wheeling lower drum coast-down.
3. Engineering Transparency: Workbench Tables & Matrix Resolution vs. Parametric Modeling
To keep CPU usage low (~3–5% on Apple Silicon) and run in real time with zero block latency, we clearly distinguish between what is computed from workbench tables / linear algebra and what uses efficient parametric models:
• Public Reference Tables & Real-Time Matrix Resolution: All 91 free-running tonewheel oscillators locked to Laurens Hammond’s 12 brass gear ratios; the 91-wheel mV RMS generator reference curve and 61 × 9 Nichrome resistance wire matrix (104..24 Ω) from the B-3/C-3 Service Manual and Kon Zissis’s compilation (KonsData-20171011.xls); exact 6, 8, 11, 16, 22, 32, 45, and 64-turn primary taps on the T1 / T2 matching transformers; Dan Vigin’s 4 Bakelite terminal strip ordering for adjacent-terminal crosstalk; and the 18-section LC ladder (500 mH / 0.004 µF) with 19 nodes and resistive dividers in the Scanner Vibrato solved sample-by-sample via the O(N) Thomas tridiagonal algorithm at 2x oversampling with C² cubic B-spline stator interpolation.
• Efficient Real-Time Parametric Models (No SPICE or 3D FEM): Vacuum tube and transformer stages (AO-28: 6AU6 / 12AX7A / single-ended 12BH7A, percussion 12AU7A / 6C4, power stage 6550, independent T1 & T2 matching transformers with R1 / R2 || C1 secondary loading, T4) use calibrated asymmetric waveshapers paired with dynamic TPT/ZDF filters (Miller roll-off and B+ sag) rather than iterative SPICE solvers; the Leslie 122 uses time-varying Doppler delay lines, 2-pole cascaded rotor ballistics and biquad formant filters (Smith et al., CCRMA) rather than block-based IR convolution; and the Type-4 spring tank uses a dispersive all-pass delay network tuned to Alan Young’s three spring delay times.