Inductor wah with a volume-pedal mode, 1970 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.
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.
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.
3Press Play
Press Play through the circuit to hear a picked note or chord. Play the string is the guitar by itself.
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.
5Tap a wire
Tap a wire to put the probe there. The Probe graph and the spectrum show the signal there.
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
- Not sure what a control does? Hover over it, or hold a finger on it, for a short note.
- For step-by-step walkthroughs, open Start here at the top of the page.
- To put together a pedal or preamp of your own, use the Builder tab.
- Your switches stay where you left them the next time you open that circuit, in this browser only.
- A tube amp opens whole, with its power amp. Pick Preamp only in the Power amp menu to hear the preamp alone, or another power amp to try this preamp on it.
- The graphs use a steady test tone. Play sounds a picked note that dies away.
- Part values are typical of their type. Each circuit’s notes say which schematic it’s based on.
Modeled after the Colorsound Wah-Swell.
The inductor wah circuit with its first transistor’s emitter grounded and no damping resistor across the inductor, so the peak is sharp. A switch turns the treadle into a volume pedal and grounds the feedback.
Try this
Common mods
Changes people often make to this circuit. Try it plays the note before and after, at the same loudness. Keep it adds the mod to Your changes, where it can go on a switch.
Input
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.
Your build
Stack blocks from the guitar down to the output. Tap + Add a block wherever you want one. The coupling caps and the supply are wired in for you.
Your blocks. These are blocks you saved with Save block, values and part changes included. Add one to any build of the same kind with + Add a block. They stay in this browser.
Your builds stay in this browser, on this device, and 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 to a file to keep a copy. Open a file, or drop the .bench file on the page, brings it back on any device.
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.
Tap where each leg goes. Tap both ends of a part to put the new one across that 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.
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.
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.
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. In Wah it works as the Vox circuit: 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. Q1’s emitter goes straight to ground and nothing sits across the inductor, so the stage has more gain and the peak is sharp, standing about 19 to 20 dB over the input across the sweep. Swell turns the same pot into a volume pedal on Q1’s output. Press "Sweep the treadle" to see the peak at six positions. The model leaves out the inductor’s core losses.
Download.
Parts and what they do
Stock values. Tap a part on the schematic to change it.
- C1, 10 nF
- Input cap. With the 100 k and the stage behind it, it cuts the lows.
- R1, 100 kΩ
- Input resistor, most of the pedal’s input impedance. The feedback arrives at the base and works against it.
- Q1, BC109B
- Common-emitter gain stage with its emitter straight to ground, so it has more gain than the Vox’s with 470 Ω.
- R3, 22 kΩ
- Q1 collector load.
- 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.
- C3, 10 µF electrolytic
- Grounds the inductor’s top for audio.
- 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, not on the drawing. With no damping resistor across the inductor, it and the loads around it set how sharp the peak is.
- R2, 1.5 kΩ
- Joins Q1’s base to the bottom of the inductor, where the feedback arrives.
- C4, 220 nF
- Coupling cap from Q1’s collector to the treadle pot’s top.
- Treadle, 100 kΩ pot, linear taper
- The treadle pot, 100 kΩ, assumed linear, heel down 0, toe down 10. In Wah it divides the output down before the feedback buffer. In Swell its wiper is the output.
- R11, 100 kΩ
- Series resistor from the pot’s top to the output in Wah. With the amp’s 1 MΩ and R13 it drops the level about 1 dB.
- R13, 100 kΩ
- Output pulldown at the jack.
- C5, 220 nF
- Couples the treadle wiper to the buffer.
- R5, 470 kΩ
- Biases Q2 from Q1’s collector.
- Q2, BC109B
- Emitter follower that drives the feedback cap from a low impedance. Its collector goes straight to the supply.
- R10, 10 kΩ
- Q2 emitter load.
- C2, 10 nF
- Feedback cap, the capacitor of the resonance.
- R12, 47 kΩ
- With C6, a 47 kΩ load on the feedback node in Wah. Swell shorts it.
- C6, 10 µF electrolytic
- In Swell it grounds the feedback node for audio, so there is no resonance and Q1 runs as a plain gain stage.
About this version
- BC109B
- The drawing names no transistor. BC109B values stand in, as in the Clyde McCoy build of the inductor wah.
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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