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Vactrol Phaser

The phaser emulates the FP-777 Flying Pan circuit — four cascaded all-pass stages, each controlled by an independent vactrol (LED/LDR optocoupler) model. The vactrol modeling is the defining character of this phaser, producing the slow, syrupy sweeps and asymmetric response that distinguish optical phasers from their digital counterparts.

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Vactrol Characteristics

Each of the four stages models a discrete vactrol element with the following behavior:

• Asymmetric response — Fast attack (~3.5 ms) and slow release (150—1500 ms). The LDR charges quickly when the LED illuminates but discharges slowly, creating the characteristic lopsided sweep where the phaser snaps toward bright tones and drifts lazily back.
• Logarithmic resistance curve — The LDR resistance follows a logarithmic taper, not a linear one. This produces a perceptually even frequency sweep despite non-linear electrical behavior.
• Thermal memory effect — Each vactrol retains a small amount of residual charge from its previous cycle. At higher rates the stages never fully reset, shifting the operating point and subtly altering the sweep range from one cycle to the next.

The four stages use 47 nF capacitance each. Feedback is soft-clipped through a tanh saturator before returning to the input, preventing runaway oscillation while adding harmonic warmth at high feedback settings.

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Parameters

Per-band phaser controls. Parameter IDs use the bandN_ prefix (e.g., band1_depth, band3_feedback).

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LFO Section

The LFO that drives the phaser operates in one of two modes.

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Standard Mode

Six classic waveforms:

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Neural Mode

A proprietary modulation system that generates organic, non-repeating modulation curves. Unlike periodic waveforms that cycle identically every period, Neural mode produces evolving shapes with a more musical, breathing quality — subtle variations in timing, amplitude, and contour that never settle into a fixed loop. Neural mode is controlled by four dedicated parameters: