Inductor wah with a 25 k treadle, early 1970s style

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 Maestro Boomerang EG-1.

Two transistors around a 500 mH inductor, as on the Vox wah. A 25 kΩ treadle pot sets how much of the output goes back through a 0.01 µF cap, and Q1 runs with a 120 Ω emitter resistor, so the stage has more gain.

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. It works as the Vox wah does. Q1 amplifies, the treadle pot sends part of the output to Q2, and Q2 drives the 0.01 µF feedback cap into the bottom of the inductor, which moves the resonant peak. The parts around it differ: Q1 has only 120 Ω under its emitter, so the stage has much more gain and the output is loud, about 28 dB over the input at the peak. The pedal clips on strong notes near the peak. The 25 kΩ treadle pot also loads the output more than the Vox’s 100 kΩ. Press "Sweep the treadle" to see the peak move.

Download.

Parts and what they do

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

C1, 47 nF
Input cap.
R1, 48 kΩ
Input resistor, most of the pedal’s input impedance. The feedback signal arrives at the base through R2 and works against it.
Q1, BC109B
Common-emitter gain stage. With 10 kΩ over 120 Ω it has a lot of gain before the feedback pulls it down.
R3, 10 kΩ
Q1 collector load.
R4, 120 Ω
Q1 emitter resistor, small, so Q1 has more gain than on the Vox circuit.
R6, 1.5 MΩ
Bias from the collector to the inductor’s top, and from there through the inductor and R2 to the base. It is the only bias path, so the stage biases itself.
C3, 6 µF electrolytic
Grounds the inductor’s top for audio, so the inductor runs from signal ground to the feedback node.
L1, 500 mH
The wah inductor, 500 mH. With the feedback cap it sets the frequency range.
L1 winding, 60 Ω
The inductor’s winding resistance, a typical figure. The drawing has no value for it.
R7, 47 kΩ
Q resistor across the inductor, damping the resonance. Larger makes a sharper, taller peak.
R2, 4.7 kΩ
Joins Q1’s base to the bottom of the inductor, where the feedback arrives.
C4, 1 µF electrolytic
Output coupling cap. The output comes from this point, before the treadle pot, so the level doesn’t follow the treadle.
Treadle, 25 kΩ pot, linear taper
The treadle pot, 25 kΩ, heel down 0, toe down 10. It divides the output down before the feedback buffer. Heel down feeds back the most and puts the peak lowest. Taper and direction are assumed. It also loads the output with 25 kΩ.
C5, 1 µF electrolytic
Couples the treadle wiper to the buffer.
R5, 620 kΩ
Biases Q2 from the supply.
Q2, BC109B
Emitter follower that drives the feedback cap from a low impedance.
R10, 8.2 kΩ
Q2 emitter load.
C2, 10 nF
Feedback cap, the capacitor of the resonance. Larger moves the whole sweep down.

About this version

Stock, BC109B
The drawing marks both transistors P-2356, a house number. BC109B values stand in.

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.

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