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Circuits

Coils and the wah

A coil is a part of its own in a circuit now. With a capacitor beside it, it rings at one frequency, and that ring is the voice of a wah pedal. This page builds one: two transistors, a 500 mH coil and a pedal you can rock with an LFO.

New words

A coil (an inductor) is wire wound round and round. It lets slow changes through and pushes back against fast ones: a capacitor's opposite.

A coil and a capacitor together resonate: they pass energy back and forth and ring at one frequency, f = 1 / (2π √(L C)).

Coil Wah's Patch: two Oscillators into CoilWah, a copper circuit box with its schematic in small, an LFO wired into its pedal, then the VCA; a keyboard below.
Play the keys: a clavinet-like pulse through the wah, an LFO rocking its pedal once a beat. Double-click CoilWah to open its parts list. Open the example sound: Coil Wah ↗ (opens in a new tab)

1What a coil does

Current through a coil builds a magnetic field, and the field fights any change in the current. So a coil is a plain wire for steady current and for low notes, and a growing obstacle as the frequency rises. A capacitor is the other way round: it blocks steady current and lets high frequencies through.

Write a coil as L, the two points it joins and its value in henries:

L1 out l 500mH

A setting can turn a coil too, in henries:

setting coil: 500mH, from 100mH to 2H
L1 out l coil
  • Values as a parts list writes them: 500mH, 500m, 1H, 100 mH. A small m is milli.
  • A circuit holds up to 4 coils. A coil adds no work for each sample beyond a capacitor's, so it costs what a filter does.

2A coil and a capacitor ring

Put a coil and a capacitor side by side and they swap energy: the capacitor charges, the coil's current carries the charge back, and so on, at one frequency, f = 1 / (2π √(L C)). A sound fed in through a resistor comes out loudest at that frequency and quieter either side of it: a band-pass. The bigger the resistor, the narrower the band (its Q) and the longer it rings.

A 500 mH coil with 22 nF rings at 1.52 kHz. The pictures are an Analyzer's Response after the wah's circuit: the same peak, moved by the pedal.

The Analyzer's Response after Coil Wah's circuit with its pedal at the heel: a peak near 560 Hz.
The pedal at the heel (47k): the peak sits low, near 560 Hz, and rings longest. (The orange line is the phase: it turns fast because the circuit runs oversampled, a short delay.)
The Analyzer's Response after Coil Wah's circuit with its pedal at the toe: a wider peak near 2.3 kHz.
At the toe (220R): the peak near 2.3 kHz, wider.

3A wah round a coil

The classic wah pedals tune a coil this way, and Coil Wah does it from textbook parts:

  1. Q1 is a buffer: its emitter follows the sound, so what comes after it doesn't load what comes before.
  2. R3 feeds the coil L1 (with RL, its own 60 Ω of wire) and Ctune: the tuned circuit. Its top, out, is what the module plays.
  3. Q2 amplifies that point about 30 times, upside down. Through the pedal's pot (Ra and Rpedal) some of it comes back to Ctune's far end.
  4. The more comes back, the bigger Ctune looks to the coil, so the lower it rings. pedal turns from 100k (the heel, about 530 Hz) down to 100R (the toe, about 2.6 kHz).
CoilWah · the parts list
# A wah with a coil, as the classic pedals have one: a 500 mH coil (L1, with its own 60 ohm of wire) and Ctune make a# tuned circuit that rings at f = 1 / (2 pi sqrt(L C)). Q1 is a buffer for the sound; Q2 is a gain stage whose output# comes back to Ctune's far end through the pedal's pot (Ra and Rpedal). The more of it comes back, the bigger Ctune# looks to the coil, and the lower it rings: pedal turns from 100k (heel, about 530 Hz) down to 100R (toe, 2.6 kHz).# An LFO wired into pedal rocks it across the whole range once a beat, as a foot on the pedal would.circuit CoilWah  setting input: 0.1 V, from 0.01 V to 1 V  setting output: 0.02 V, from 0.005 V to 0.4 V  setting pedal: 3.3k, from 100k to 100R  V1 vcc gnd 9V  C1 in b1 47n  R1 vcc b1 470k  R2 b1 gnd 470k  Q1 vcc b1 e1 2N3904  Re1 e1 gnd 10k  C2 e1 a 1u  R3 a out 47k  C3 out b2 220n  R4 vcc b2 470k  R5 b2 gnd 82k  Q2 c2 b2 e2 2N3904  Rc vcc c2 10k  Re2 e2 f 220  Re3 f gnd 1k  Ce f gnd 10u  C4 c2 d 1u  Ra d w 1k  Rpedal w gnd pedal  Ctune out w 5.6n  L1 out l 500mH  RL l gnd 60end
Coil Wah's schematic: Q1 as a buffer, R3 into the coil L1 with RL and Ctune, Q2 with its collector through Ra and Rpedal back to Ctune, and the supply V1 9 V.
The same parts list, drawn: the coil is the four loops.

4Rocking the pedal

In Coil Wah an LFO is wired into pedal: at depth 1 it rocks the pedal across its whole travel, once a beat, as a foot would. Slow it to a bar for a lazy sweep, or set its shape to Square for a two-note wah.

In Envelope Wah an Envelope opens it on every note instead, the quack of an envelope filter on a bass. Its sine sub goes round the wah, so the low end stays.

The Patch: two Oscillators into CoilWah, a circuit box with its schematic in small, and an LFO wired into its pedal input; then the VCA.
  1. The wah: a circuit.
  2. Its pedal, wired.
  3. The LFO that rocks it.

Try it

  1. Open Coil Wah and play a few notes.
  2. In the parts list, change Ctune out w 5.6n to 10n: the whole sweep moves down.
  3. Change RL l gnd 60 to 300: the ring softens, a gentler wah.
  4. Turn input up toward 1 V: the transistors start to bite.
Open the example sound: Coil Wah ↗ (opens in a new tab)

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