Op-amp wah with a depth control, 1993 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 Ibanez WH10.

No inductor: two op-amps form a resonant filter, and the treadle pot in their loop moves the peak. A Depth pot mixes the filter with the dry signal, and a switch lowers the sweep for bass.

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. Q1 buffers the guitar and feeds two paths. One goes through Q3 as the dry signal. The other goes into a filter made of two op-amps that share one − input node. C8 and C9 are the filter’s timing caps, and the loop back from IC1B’s output runs through the treadle pot and R18. Less resistance there makes more feedback and moves the peak up. The peak stands about 18 dB over the input across the sweep. Depth and R16 mix the filter with the dry signal into Q4, the output buffer. With Depth at 0 the 10 kΩ pot still lets some filter through, so the effect never goes fully dry. 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.

C2, 220 µF electrolytic
Across the 9 V supply.
R19, 10 kΩ
Top of the reference divider.
R1, 10 kΩ
Bottom of the reference divider.
C3, 47 µF electrolytic
Filter cap on the reference.
C1, 100 nF
Input coupling cap.
R22, 1 kΩ
Series resistor at the input buffer’s base.
R2, 330 kΩ
Biases the input buffer at the reference. The input impedance is about 330 kΩ.
Q1, 2SC1815
Input buffer, an emitter follower (2SC1815). It feeds both the filter and the dry path.
R10, 22 kΩ
Input buffer emitter resistor.
R20, 1 kΩ
Series resistor into the filter.
C10, 2.2 nF
Couples the input into the filter. Small, so it also cuts the lows below the sweep.
R21, 15 kΩ
From IC1A’s + input toward the reference.
R14, 15 kΩ
In series with R21 to the reference. S2 shorts it in Guitar.
C11, 220 pF
With R23, from IC1A’s + input to the shared − node, a small treble path.
R23, 1 kΩ
In series with C11.
C9, 3.3 nF
From IC1B’s output back to IC1A’s + input, one of the filter’s two timing caps.
IC1A, JRC4558D
First op-amp of the filter. Its output is the wah signal.
C8, 10 nF
From the shared − node to IC1A’s output, the other timing cap.
R17, 47 kΩ
From IC1A’s output to IC1B’s + input.
R13, 6.8 kΩ
IC1B’s + input to the reference. With R17, a divider of about 1 to 8.
C12, 220 pF
With R24, from IC1B’s + input to the shared − node.
R24, 1 kΩ
In series with C12.
IC1B, JRC4558D
Second op-amp of the filter. Its output drives the treadle back to the shared − node.
Treadle, 50 kΩ pot, linear taper
The treadle, a 50 kΩ pot as a variable resistor from IC1B’s output to R18 (FREQ on the drawing). Less resistance, more feedback around the loop and a higher peak. Taper and direction assumed.
R18, 1.8 kΩ
In series with the treadle, so the loop never shorts. It sets the top of the sweep.
C7, 2.2 µF electrolytic
Couples the filter’s output to the mixer.
R15, 100 kΩ
From the filter side of the mixer to the dry signal, so the filter side never floats.
Depth, 10 kΩ pot, reverse audio taper
A 10 kΩ reverse-audio pot as a variable resistor from the filter into the output buffer, against R16 from the dry side. Turning it up lowers the resistance, so more filter and less dry.
Q3, 2SA1015-GR
Dry buffer, a PNP emitter follower (2SA1015) on the input buffer’s output.
R11, 10 kΩ
Dry buffer emitter resistor, to the supply.
R16, 10 kΩ
The dry signal into the output buffer.
Q4, 2SC1815
Output buffer, an emitter follower (2SC1815).
R12, 22 kΩ
Output buffer emitter resistor.
C4, 2.2 µF electrolytic
Output coupling cap.
R3, 100 kΩ
Output pulldown.
R5, 1 kΩ
Series resistor at the output jack.

About this version

Stock
Negative ground, 9 V. 2SC1815 and 2SA1015 as the drawing lists them, NJM4558 op-amps.

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.

© 2026 Swell Amplification LLC. All rights reserved. The simulator, its circuit models, drawings, presets and text are the property of Swell Amplification LLC and may not be copied, reproduced, redistributed or used to build another product without written permission. The original circuits are historical designs documented in public sources, and no claim is made to them. Everything here is provided as is, without warranty of any kind, for study and personal use. Results are simulations of typical parts, not measurements of any real unit. Don’t rely on them for the safety of any build or repair. Tube amplifiers carry lethal voltages.

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