MXR Phase 90
Four-stage JFET phaser — Eddie Van Halen's swirl.
We don't ship reference schematics until they've been hand-authored against authoritative sources (manufacturer service docs, ElectroSmash analyses, GEOFEX writeups) and reviewed in the simulator. Until then, the BOM, signal chain, controls and mod notes below are accurate, and you can still add this pedal to a project — it just won't include a circuit yet.
About
Four cascaded JFET-based all-pass filter stages whose cutoff frequencies are swept by a low-frequency triangle oscillator. The dry signal is summed with the all-pass output to create notches that sweep across the spectrum.
Signal chain preview
5 stagesQuick facts
- "Brown sound" album-Eddie tone leans on a script-logo Phase 90 set medium-slow.
- Script-logo (no feedback path) is more subtle; block-logo adds feedback for a more intense, vocal swirl.
- Uses matched 2N5952 JFETs as voltage-controlled resistors — matching is critical.
Controls
- Speed (500k reverse log)
Mod recipes Pro
4 mods- Script logo / R28 mod
Restore the smoother, more subtle 'script' phase character.
How: Remove R28 (the 24kΩ feedback resistor added in the block-logo revision). The pedal becomes the original 1974 script-logo voicing — less obvious sweep, lusher.
- Speed range expander
Slower minimum speed and faster maximum.
How: Replace R speed (500kΩ) with a 1MΩ reverse-log pot for slower slow speeds; or wire a switch parallel to R8 (LFO timing) to halve the LFO period for fast 'vibrato' speeds.
- Resonance / feedback
Add a 'depth' control for sharper, more vocal sweep.
How: Wire a 100kΩ linear pot from the output of stage 4 back to the input of stage 1 through a 10nF cap. Adds resonance (use sparingly — high settings self-oscillate).
- True bypass
Removes the JFET bypass switching, which can subtly color the dry signal.
How: Rewire jacks through a 3PDT footswitch. Disconnect the JFET switching circuit (Q5/Q6 area on the schematic).