Op-amp fuzz and wah, 1973 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.
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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
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5Tap a wire
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6Try a change
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Good to know
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Modeled after the Fender Fuzz-Wah pedal.
A fuzz and a wah in one treadle, built on one dual op-amp. The fuzz half is an inverting stage with 10 MΩ of feedback into a pair of diodes, mixed with clean signal from a phase splitter. The wah half is a bridged-T band-pass whose grounded pot moves with the treadle. With the wah off the treadle is a volume pedal.
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.
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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.
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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. Use the Fuzz and Wah switches and rock the Treadle knob. The fuzz stage has so much gain that the op-amp output is close to a square wave, and the diodes after R18 square it further. Signal Mix sets how much of that is heard against the clean copy from Q1’s emitter. The wah stage is a non-inverting amp with a bridged T in its feedback: at one frequency the T passes little back to pin 2, so the gain peaks there. The treadle trades R19’s resistance between the T’s two arms, moving the peak from about 400 Hz to about 1 kHz. At the very end of the track one arm is shorted, the peak disappears and only a broad treble lift is left. A real treadle stops short of the end. The peak is sharper than on an inductor wah. With the wah off, the treadle turns R12 and the pedal is a volume pedal.
Download.
Parts and what they do
Stock values. Tap a part on the schematic to change it.
- C3, 100 nF
- Input coupling cap to Q1.
- R4, 220 kΩ
- Top of Q1’s base divider.
- R5, 100 kΩ
- Bottom of Q1’s base divider. With R4 it sets the base near 2.8 V.
- Q1, 2N3904
- Phase splitter. With equal 10 k loads on collector and emitter there are two copies of the guitar at about unity gain. The collector’s drives the fuzz, and the emitter’s is the clean side of Signal Mix.
- R7, 10 kΩ
- Q1 collector load. It is also the source resistance the fuzz stage sees, so with R15 it sets that stage’s gain.
- R8, 10 kΩ
- Q1 emitter load, equal to R7.
- C6, 100 nF
- Couples the collector to the fuzz stage.
- C7, 100 nF
- Couples the emitter’s clean signal to Signal Mix.
- R11, 10 kΩ
- Holds the fuzz stage’s inverting input near the reference for DC.
- R10, 10 kΩ
- Feeds the reference to the fuzz stage’s + input.
- A1B, MC1458
- The fuzz stage, inverting, with R15’s 10 MΩ over about 10 k of source: far more gain than the 9 V can carry, so the op-amp clips hard.
- R15, 10 MΩ
- Feedback resistor of the fuzz stage. Smaller means less gain and less fuzz.
- C10, 22 pF
- Across R15, it rolls the fuzz stage’s gain off above about 700 Hz, which keeps the fuzz from turning harsh.
- R13, 100 kΩ
- Half of a DC feedback path. With C9 grounding the middle for audio it holds the output at the reference.
- C9, 25 µF electrolytic
- Grounds the middle of R13 and R16 for audio, so they set the DC only.
- R16, 100 kΩ
- The other half of the DC feedback path.
- R18, 56 kΩ
- Series resistor into the diodes. With them it squares the clipped output further and sets the fuzz level.
- C15, 10 µF electrolytic
- Blocks the 4.5 V so the diodes clip around ground.
- CR1, 1N914 / 1N4148
- Clipping diode for one half of the wave. No type is given, so a 1N914 is assumed.
- CR2, 1N914 / 1N4148
- Clipping diode for the other half.
- Fuzz Volume, 1 MΩ pot, audio taper
- Fuzz Volume, 1 MΩ, taper assumed audio. Sets the fuzz level into Signal Mix.
- Signal Mix, 1 MΩ pot, audio taper
- Signal Mix, 1 MΩ, taper assumed audio, between the fuzz and the clean signal. Toward 10 it picks more fuzz. On the drawing, clockwise picks more clean.
- C1, 22 nF
- Coupling cap into the wah stage.
- R1, 820 kΩ
- With R6, a divider that drops the signal about 18 dB before the wah stage, which has a lot of gain at its peak.
- R6, 120 kΩ
- Bottom of the input divider, and the wah stage’s DC bias from the reference.
- A1A, MC1458
- The wah stage, non-inverting. Its feedback to pin 2 runs through a bridged-T network that passes little at one frequency, so the gain peaks there.
- R9, 100 kΩ
- From pin 2 to the reference. With R14 it sets the gain away from the peak, about 2.
- R14, 120 kΩ
- Feedback resistor beside the bridged T.
- C8, 220 pF
- Rolls off the wah stage’s highest frequencies.
- C11, 22 nF
- Bridged-T cap from pin 2. With C12 and C13 it sets where the peak can sit.
- C12, 22 nF
- Bridged-T cap between the two pot ends.
- C13, 22 nF
- Bridged-T cap to the output.
- R17, 10 kΩ
- Keeps a resistance in the T when the treadle shorts the other half of R19, so the peak stays bounded.
- Wah Control, 10 kΩ pot, linear taper
- Wah Control, 10 kΩ in the treadle, taper assumed linear. Its wiper is grounded, so the treadle trades resistance between the two arms of the bridged T and moves the peak. Toe down assumed clockwise.
- C14, 1 µF electrolytic
- Output coupling cap of the wah stage.
- Wah Volume, 1 MΩ pot, audio taper
- Wah Volume, 1 MΩ, taper assumed audio. Sets the output level with the wah on.
- Volume (treadle), 1 MΩ pot, audio taper
- Volume, 1 MΩ, taper assumed audio, turned by the treadle with R19. It sets the output level with the wah off.
- R2, 10 kΩ
- Top of the reference divider.
- R3, 10 kΩ
- Bottom of the reference divider, making half the battery for both op-amp halves.
- C4, 25 µF electrolytic
- Across R2, filtering the reference.
- C5, 25 µF electrolytic
- Across R3, filtering the reference.
- C2, 50 µF electrolytic
- Supply filter across the battery.
About this version
- Fuzz-Wah, 1973, modeled after the Fender Fuzz-Wah pedal
- From the Fender factory drawing 011747. A 2N3904 stands in for the 2N3392.
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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