Inductor wah with a volume setting

Pedal and tube amp circuits, solved part by part the way SPICE (the circuit simulator engineers use) does it, while you listen and watch. Turn the knobs, change any part, or tap a wire to put a probe on it.

How to use the bench

Every part is simulated. Nothing you hear is a recording.

  1. 1Pick a circuit

    Choose one from the menu, or type a name, maker or year in Find a circuit. The buttons under the menu switch between versions.

  2. 2Set the knobs and the input

    Turn the circuit’s knobs on the front panel. In the Input box, choose Guitar or Signal generator, and the note or chord.

  3. 3Press Play

    Press Play through the circuit to hear a picked note or chord. Play the string is the guitar by itself.

  4. 4Tap a part

    Tap any part on the schematic to change its value, or swap the transistor, tube or diode. The red numbers are DC voltages, and they update as you go.

  5. 5Tap a wire

    Tap a wire to put the probe there. The Probe graph and the spectrum show the signal there.

  6. 6Try a change

    Press Try it on a card under Try this to hear one change, before and after. Modify this circuit, below the schematic, adds or removes parts.

Good to know

Modeled after the Jen wah-volume pedal.

The Vox wah circuit with a two-pole mode switch. In Wah the treadle moves a resonant peak as on the Vox. In Volume the inductor’s top is lifted off ground and a 4.7 kΩ path joins the feedback cap, so the same treadle sets the level.

Input

Add

Peak voltage from the pickup for each note. Strum hard and a single coil puts out a few hundred millivolts. When you play a chord, its strings share this level.

How it’s played
Tap a part to change its value, or a wire to probe it. To add or take out parts, open Modify this circuit, below. Red numbers are DC voltages to ground, not warnings. Scroll or drag sideways to see the whole circuit.
Modify this circuit Add parts, take them out, or put one in series with another, then hear what it does. Tap to open.

Pick what to do, then tap the schematic. Your changes show in purple. Any change can go on a switch beside the knobs, from the menu in its row under Your changes.

Add a part

Tap where each leg goes. Tap both ends of a part to put the new one across that part.

Change a part

With Remove a part on, tap any part of the pedal or amp to take it out, and tap it again to put it back. Tapping a purple part you added deletes it.

Turn on Add in series with a leg and tap a part. In that part’s box, below the schematic, pick which leg to lift and what goes in the gap, such as a resistor between a cap and ground or a stopper in front of a grid.

Change a wire

With Cut into a wire on, tap any wire. Pick a part from the menu and it goes in the gap, wired in. Volume pot puts a pot there with the input on one end, the output on the wiper and the other end to ground.

With Run a wire on, tap a leg, then where the wire goes. Drag any piece of a wire you ran, or of an added part’s lead, to square it up or move it.

Keep and share

Your changes stay in this browser, on this device, for your next visit. Nothing is sent to us. Anyone using this browser can see them. They’re lost if you clear your browsing data or use a private window, so save a file to keep a copy. A shared link carries the circuit after the #, a part browsers never send to a server.

Test tone for the graphs
The graphs use a steady tone. Play sounds a plucked string that dies away.

Input

pickup, unloadedat the pedal input

Output

at the amp input

Probe

Spectrum

Frequency response

What the curves mean

The ink curve is the response to a very quiet signal, from the generator to the probe. The blue curve is measured with pink noise at the level you set, like an audio analyzer. When stages clip, the blue curve pulls away from the ink one and the gray coherence line (right-hand scale) drops below 1. Then no single curve tells the whole story. Both use the signal generator, whichever Input tab is picked.

Output

Bias

every voltage and current at rest

What to listen for. Flip the Mode switch and rock the Treadle knob. In Wah the circuit is the Vox wah: the treadle sets how much of the output Q2 feeds back through C2, moving the resonant peak. In Volume, C3 is lifted, so the inductor no longer has a ground for audio, and R13 with C6 feeds Q2’s emitter back to Q1’s base across the whole band. More feedback means less gain, so the treadle sets the level. Even at the heel some signal gets through, about 26 dB down from the toe. On the bench the treadle is held still for each note.

Download.

Parts and what they do

Stock values. Tap a part on the schematic to change it.

C1, 10 nF
Input cap. With the 68 k and the stage behind it, it cuts below about 200 Hz.
R1, 68 kΩ
Input resistor, most of the pedal’s input impedance. The feedback signal arrives at the base and works against R1.
Q1, BC109C
Common-emitter gain stage, biased from its own collector through R6, the inductor and R2.
R3, 22 kΩ
Q1 collector load.
R4, 470 Ω
Q1 emitter resistor. It sets the stage’s gain with R3.
R6, 470 kΩ
Bias feedback from the collector to the inductor’s top. With R8 it sets the base voltage through the inductor and R2.
R8, 100 kΩ
Bottom of the bias divider. It also keeps the inductor’s top near ground for audio in Volume, when C3 is lifted.
C3, 4.7 µF
Non-polar cap that grounds the inductor’s top for audio in Wah, so the inductor runs from signal ground to the feedback node.
R11, 220 kΩ
Across the switch contact, so C3 stays charged in Volume and the switch doesn’t click.
L1, 500 mH
The wah inductor, 500 mH. With C2 it sets the frequency range.
L1 winding, 60 Ω
The inductor’s winding resistance, a typical figure (wah inductors measure about 10 to 200 Ω).
R7, 33 kΩ
Q resistor across the inductor, damping the resonance. A larger value makes a sharper, taller peak.
R2, 1.5 kΩ
Joins Q1’s base to the bottom of the inductor, where the feedback arrives.
C4, 220 nF
Output coupling cap. The output comes from this point, before the treadle pot.
Treadle, 100 kΩ pot, linear taper
The treadle pot, assumed linear, heel down 0, toe down 10. It divides the output down before the feedback buffer. In Wah, heel down feeds back the most and puts the peak lowest. In Volume, heel down feeds back the most across the whole band, which pulls the level down.
C5, 220 nF
Couples the treadle wiper to the buffer.
R5, 470 kΩ
Biases Q2 from Q1’s collector.
Q2, BC109C
Emitter follower that drives the feedback from a low impedance.
R9, 1 kΩ
Q2 collector resistor, there to stop the follower oscillating.
R10, 10 kΩ
Q2 emitter load.
C2, 10 nF
Feedback cap, the capacitor of the resonance. Larger moves the whole sweep down.
R13, 4.7 kΩ
Volume path resistor. In Volume it joins Q2’s emitter to the feedback node across the band, so the feedback no longer peaks.
C6, 220 nF
Blocks DC in the volume path, so the bias doesn’t shift between modes.
R12, 220 kΩ
Across the switch contact, so C6 stays charged in Wah and the switch doesn’t click.

About this version

Jen, BC109C, modeled after the Jen wah-volume pedal
From a redrawn Jen schematic. FBC318C transistors on the drawing, the library’s BC109C in their place.

How this tool works. Every part is an equation, and the whole circuit is solved several times per audio sample: 192,000 samples a second while you drag a knob, 384,000 once you let go. Transistors use Ebers-Moll with gain, leakage, Early effect, base resistance, junction capacitance and temperature. Their charge storage is set at the bias point, close for small signals and rough while they switch hard. Tubes use Koren’s equations fitted to each datasheet, plus grid current. Diodes are exponential junctions with series resistance. Presets are typical of each type. Caps are ideal, and the battery or HT is a voltage behind a resistance.

Spot an error, or want a circuit added? Email info@swellamps.com. A schematic or a clear photo of one helps. Anything you send may be used to improve these models and our desktop app.

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