Tron
Mu-Tron Phasor II-inspired optical phaser engineering baseline.
Tron is Greybound's optical-style phaser model. The reference target is the Mu-Tron Phasor II family: a 1970s phaser associated with organic lamp/LDR-style motion, rate/depth/feedback controls, and a smoother sweep than a sharper one-knob JFET phaser.
The implementation does not copy a factory schematic. It is a graybox model that captures the useful engineering behavior: buffered input, smoothed optical modulation, six moving all-pass stages, bounded feedback, wet/dry mix, and low output impedance.
Baseline Assumptions
- Input impedance is high enough for guitar or pedal outputs.
- Output impedance is low enough to drive another pedal, amp input, or FX return.
- Modulation belongs to private pedal state: LFO phase, optical smoothing, all-pass memory, and feedback memory.
- The optical control path moves slower than the raw LFO, giving a rounded organic sweep.
- Pre-amp placement is the reference rig because a phaser before amp drive is the classic guitar use case.
Controls
rate_hz: LFO rate in Hz, clamped to0.03..12.0.depth: optical sweep depth.feedback: resonant feedback around the phase network.mix: wet/dry blend.bypass: routing state; private modulation state remains owned by the pedal instance.
Current DSP Approximation
The current implementation uses:
- source/load input boundary and light input coupling,
- sine LFO with nonlinear lamp drive,
- one-pole smoothing to mimic optical lag,
- six first-order all-pass stages with spread center frequencies,
- feedback around the phase-shift path,
- dry/wet summing with mild output makeup,
- output lowpass and headroom guard.
This is not a SPICE model and it is not component-exact. It is a musically useful phase network that gives Greybound a stable organic phaser while leaving room for future circuit-level validation.
Reference Direction
Useful future reference work:
- collect public Mu-Tron Phasor II and optical phaser topology notes,
- identify practical all-pass center-frequency ranges from measured sweeps,
- compare 4-stage and 6-stage variants,
- validate feedback range against self-oscillation and output headroom,
- render sweep spectrograms to verify moving notch behavior.
References
- Effects Database: Musitronics Mu-Tron Phasor II describes the Phasor II as an electro-optical six-stage phase-delay network derived from the Bi-Phase lineage.
- Mu-Tron Phasor II quick start guide documents the modern control surface: stage, depth, rate, and feedback.
- ElectroSmash MXR Phase 90 analysis is useful contrast for the simpler four-stage JFET phaser topology and dry/phase-shifted output mixer principle.
Validation Gates
The model is not component-exact until:
- rate accuracy is measured over long renders,
- all-pass notch movement is measured across depth settings,
- feedback is validated for stability and musical resonance,
- bypass and active output levels are matched within a useful range,
- pre-amp and FX-loop placement are compared with renders,
- measured hardware or SPICE captures define a specific reference unit.