In November 1966, a junior electronics engineer named Bradley Plunkett was assigned an unglamorous task: find a cheaper replacement for the Jennings mid-range boost switch used in Vox amplifiers. The switch let players bump the midrange in three positions. Plunkett's solution was a simple potentiometer connected to a transistorized circuit. When he turned the pot while testing the circuit with a guitar signal, it made a sound nobody expected. The circuit was sweeping a resonant peak across the frequency spectrum, and it sounded uncannily like a human voice.
That accident became the wah pedal.
What the Name Was Borrowed From
The term "wah-wah" was already in use before 1966. Brass players, particularly jazz trumpeters and trombonists, had been using the Harmon mute since the 1920s to produce the same vowel-like sound manually. The Harmon mute fits inside the instrument's bell and produces a nasal, constricted tone. Moving a hand over its stem opening sweeps the resonance of the resulting acoustic cavity, producing the characteristic "wah" sound Miles Davis made famous on recordings like Kind of Blue.
Plunkett's circuit was solving the same acoustic problem electronically: varying the frequency of a resonant peak. The brass player moves a hand. The guitar player rocks a foot pedal. The physics is nearly identical.
The Circuit Is a Bandpass Filter
The core of every wah pedal is an LC resonant circuit, where an inductor (L) and capacitor (C) form a tuned bandpass filter. At the resonant frequency, the filter produces a sharp peak in gain. Above and below that frequency, output falls off steeply on both sides.
The Dunlop Cry Baby GCB-95, the canonical wah pedal, sweeps this peak from roughly 450 Hz to 1.6 kHz as the treadle moves from heel to toe. The resonance peak itself has a Q factor of approximately 10. In filter terms, Q = fc / bandwidth: a Q of 10 at 1 kHz means the -3dB bandwidth is only 100 Hz. That is a very narrow, very pronounced peak.
This is not an accident of the circuit's design. It is the whole point.
Human vowel sounds are produced by the vocal tract shaping resonant peaks called formants. The first formant (F1) for typical vowels falls between 300 Hz and 800 Hz. The second formant (F2) falls between 800 Hz and 2500 Hz. A wah pedal that sweeps a single resonant peak from 450 Hz to 1.6 kHz is moving through exactly the range where F1 and F2 live in human speech. The brain interprets the sweep as vocal articulation because the spectral signature matches it.
The Inductor Is Not Optional
What makes the wah circuit interesting is that the bandpass filter is built around a real physical inductor, not simulated with capacitors and op-amps as most audio filters are. Inductors are comparatively rare in audio circuits for good reason: they are physically larger, more expensive, and their impedance behavior is nonlinear in ways that are hard to predict at manufacturing scale.
But those same nonlinearities are what give the wah pedal its character.
The original Thomas Organ Clyde McCoy wah used an inductor with a round, toroidal shape with a hole in the center, now called the halo inductor for its appearance. It produced a smooth, dark sweep. When supplies of those inductors ran short, manufacturers turned to Italian-made Fasel inductors, available in red and yellow versions. The red Fasel has a brighter, more aggressive response with pronounced harmonic distortion. The yellow Fasel produces a smoother vintage character. Both have a DC resistance of around 16 ohms on modern production units, compared to lower-resistance vintage units, and that difference in copper resistance changes the Q of the resulting filter.
Guitar players have been modifying wah pedals by swapping inductors since the 1970s. The inductor is, in a real sense, the voice of the pedal.
Why the Potentiometer Does Not Change the Inductor
The clever part of the circuit is how the treadle pot controls the filter frequency. You cannot easily change an inductor's value electrically. What the circuit does instead is use the pot to vary the effective capacitance of a transistor stage in the feedback path. As the transistor's collector current changes, it appears to the inductor as though the capacitor has changed value, even though the physical capacitor is fixed. The resonant frequency shifts accordingly.
This means the treadle is not controlling an acoustic parameter directly. It is controlling a transistor bias point that reshapes the filter's transfer function. The inductor is always the same physical component. The circuit only tricks it into behaving as though its resonant partner has changed.
Digital Wah and the State Variable Filter
The standard DSP approach to implementing a wah effect is the Chamberlin state variable filter, a two-integrator topology described by Hal Chamberlin in his 1980 book Musical Applications of Microprocessors. The core loop:
high = in - q*band - low
band = f*high + band
low = f*band + lowwhere f = 2 sin(pi fc / samplerate) controls the center frequency and q is a small positive damping value (lower means more resonant, more vocal). The bandpass output is taken from band. Sweeping fc from 450 Hz to 1600 Hz with q near 0.1 produces a mathematically clean wah sweep.
The problem is that "mathematically clean" is not what people like about wah pedals. The Chamberlin SVF is a perfectly linear filter. A real inductor is not. At signal amplitudes typical of guitar output, the inductor core enters mild saturation, producing second and third harmonics that add brightness and edge to the sweep. The filter's output impedance interacts with the next stage in a frequency-dependent way. None of this is in the SVF equations.
More accurate models use wave digital filters (WDFs), which model the circuit topology directly, including the nonlinear inductor. This adds computational cost but captures the behavior that makes different inductors sound different.
Auto-Wah: The Envelope Follower Variant
In 1972, Mike Beigel and Aaron Newman founded Musitronics and built the Mu-Tron III, the first commercial auto-wah. Instead of a foot pedal controlling the filter cutoff, an envelope follower tracks the amplitude of the input signal and drives the filter frequency directly. When you play loudly, the filter opens up. When you play softly, it closes down.
The Mu-Tron III used optocouplers for control, which gave it a slightly laggy, organic response different from a voltage-controlled circuit. Stevie Wonder used it on "Higher Ground" in 1973, making the pedal an overnight sensation. Bootsy Collins and Jerry Garcia were also prominent users.
The Mu-Tron III's state variable filter ran in a configuration that allowed the envelope to sweep both the center frequency and, indirectly, the Q, producing a more complex spectral movement than a simple frequency sweep.
Modern auto-wah plugins implement the same concept with configurable attack, release, sensitivity, and filter type parameters. Some also offer pitch-tracking modes, where the center frequency tracks the fundamental of the input, not its amplitude.
What the Circuit Is Teaching You
The reason to understand wah pedal physics is not nostalgia for analog circuits. It is that the wah effect demonstrates a principle that applies across many audio contexts: narrow, swept resonances produce vocal quality because they mimic the formant structure of the human voice.
The same principle explains why a formant filter in a synthesizer can make a pad sound like it is singing. Why a swept bandpass in a noise layer can sound like breathing. Why the phaser's chain of allpass filters, which produces a moving comb-notch rather than a moving peak, sounds less vocal than a wah: it is sweeping notches, not peaks, and the human auditory system does not map notches to speech.
A Q of 10, sweeping from 450 Hz to 1.6 kHz, through an inductor that adds harmonic content. That is the specification for sounding human.