Blended op-amp overdrive with germanium diodes, 1994 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 Klon Centaur.

A buffer, then two paths: an op-amp gain stage into germanium diodes to ground, and a clean path set by the other half of the dual gain pot. A summing op-amp mixes them, then a treble control and the output. The last two op-amps run off a charge pump, swinging from about −8 to +15 V.

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. The gain knob turns two pots at once: one raises the clipping stage’s gain, the other turns the clean path down. At low settings you hear mostly the clean, full-bass path with a little clipped signal on top, close to a clean boost. At high settings the clipped path takes over. Probe the diodes, then the summing op-amp’s output, and sweep the gain to hear the two trade places. The clean path is filtered too. It carries the bass the clipping stage throws away, so the mix is thicker than either path alone. With the charge pump the summing and tone stages have so much headroom that only the diodes clip.

Download.

Parts and what they do

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

R29, 27 kΩ
Top of the 4.5 V reference divider.
R30, 27 kΩ
Bottom of the reference divider.
C18, 47 µF electrolytic
Reference filter cap.
R1, 10 kΩ
Input series resistor.
C1, 100 nF
Input coupling cap.
R2, 1 MΩ
Biases the buffer at 4.5 V and sets the input impedance near 1 MΩ.
U1A, TL072
Input buffer, a unity-gain follower. Everything after it sees a low source impedance whatever the guitar does.
C3, 100 nF
Coupling cap into the gain stage.
R6, 10 kΩ
Series resistor into the gain op-amp. With C5 across it, the highs get through more easily.
C5, 68 nF
Across R6.
Gain A, 100 kΩ pot, linear taper
First half of the dual gain pot, with its wiper to 4.5 V. Turning it up lowers the resistance in the inverting leg (more gain) and raises the resistance shunting the input (less loss).
R10, 2 kΩ
Sets the maximum gain.
R11, 15 kΩ
In the inverting leg, bypassed for mids and highs by C7.
C7, 82 nF
Across R11. The gain is higher above about 130 Hz.
U1B, TL072
The gain stage, on the plain 9 V supply.
R12, 422 kΩ
Feedback resistor.
C8, 390 pF
Across the feedback. It rolls off the top.
C9, 1 µF
Coupling cap to the diodes.
R13, 1 kΩ
Feeds the diodes.
D2, D3, 1N34A germanium, one each way
Germanium diodes to ground, one each way. They’re the only clipping in the pedal.
C10, 1 µF
Coupling cap from the diodes.
R16, 47 kΩ
The clipped path into the summing node.
C11, 2.2 nF
With R15 it lets some treble of the clipped signal across to the blend network.
R15, 22 kΩ
In series with C11.
R7, 1.5 kΩ
Clean path 1. With C16 it’s a low-pass at about 106 Hz, so this path carries only the bass.
C16, 1 µF
Clean path 1 filter cap.
R19, 15 kΩ
Clean path 1 into the summing node.
R5, 5.1 kΩ
Clean path 2 from the buffer.
C4, 68 nF
Across R5.
R8, 1.5 kΩ
Shunts clean path 2 to the reference.
C6, 390 nF
With R9, a second shunt that cuts more of the lows.
R9, 1 kΩ
In series with C6.
Gain B, 100 kΩ pot, linear taper
Second half of the dual gain pot, turned by the same knob. It picks off less clean signal as the gain goes up.
R17, 27 kΩ
Clean path 2 into the summing node.
C12, 27 nF
With R18, a treble path around R17.
R18, 12 kΩ
In series with C12.
U2A, TL072
Summing amplifier (inverting) on the charge-pump rails.
R20, 392 kΩ
Summing feedback resistor.
C13, 820 pF
Across the feedback. It rolls off above about 495 Hz, which tames the clipped path’s fizz.
R22, 100 kΩ
Tone stage input resistor.
R21, 1.8 kΩ
From the summing output to the treble pot.
Treble, 10 kΩ pot, linear taper
Treble. It moves the C14 injection point between the input side (more treble) and the output side (less).
R23, 4.7 kΩ
From the treble pot to the tone output.
C14, 3.9 nF
Treble cap, from the pot wiper to the tone op-amp’s input.
U2B, TL072
Tone stage (inverting, unity gain in the mids) on the charge-pump rails.
R24, 100 kΩ
Tone stage feedback resistor.
C15, 4.7 µF electrolytic
Output coupling cap.
R25, 560 Ω
Output series resistor.
Output, 10 kΩ pot, linear taper
Output level.
R28, 100 kΩ
Output pulldown.

Versions

Klon
Negative ground, 9 V.
“Silver”
Values reported for silver units: the two treble-pot resistors swapped (4.7 kΩ on the input side, 1.8 kΩ on the output), 47 Ω instead of 2 kΩ setting the maximum gain, 10 kΩ and 4.7 kΩ in the clean-blend legs, 560 pF across the summing feedback, and an audio-taper output pot. Whether production changed is disputed.

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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