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EQ/Filter1972· Musitronics (USA)

Musitronics Mu-Tron III

OTA-based envelope filter — Stevie Wonder's clavinet, Bootsy Collins's bass, Jerry Garcia's voice.

⏳
Schematic pending verification

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

An envelope-following resonant state-variable filter. The signal's envelope (from a precision rectifier) controls a CA3080 OTA which sweeps a 2-pole state-variable filter, producing the iconic 'auto-wah' quack. Switchable low-pass / band-pass / high-pass and high/low frequency range.

Signal chain preview

6 stages
Input
Op-amp input buffer + Gain control
Precision full-wave rectifier (envelope detector)
Low-pass smoothing → control voltage
Control voltage drives CA3080 OTA setting the filter cutoff
State-variable filter (2-pole) tapped for LP / BP / HP outputs
Output buffer
Output

Quick facts

  • Stevie Wonder's 'Higher Ground' clavinet is a Mu-Tron III. Bootsy Collins's Mu-Tron III tone defined funk bass.
  • Jerry Garcia ran one almost continuously in his rig in the late '70s.
  • Reissued as the 'Mu-Tron III+' and later cloned by Haz Labs and Mu-FX (founder Mike Beigel's company).

Controls

  • Gain (envelope sensitivity)
  • Peak (resonance)
  • Drive switch (up/down sweep)
  • Mode switch (LP/BP/HP)
  • Range switch (Low/High)

Mod recipes Pro

3 mods
  • 9 V conversion

    Run from a standard pedalboard supply.

    How: Add a charge pump for ±9 V rails; substitute LM13700 for CA3080 (more current handling). Re-trim Gain pot range.

  • Expression input for cutoff

    Use the pedal as a manual wah.

    How: Sum an external 0-5 V from a TRS expression pedal into the CA3080's Iabc node through a 100k resistor. The pedal sweeps the filter manually.

  • Dry mix

    Blend dry under the filtered signal — common request for funk bass.

    How: Add a 50k linear pot mixing buffered dry with the post-SVF output.

References