Op-amp distortion, 1978 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 ProCo RAT.
One slow op-amp with huge gain, diodes to ground after it, a one-knob low-pass filter, and a JFET output buffer. At high gain the op-amp can’t keep up, and that’s a big part of the sound.
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. At high distortion settings the op-amp can’t keep up. Its output moves only 0.3 V per microsecond, so fast edges turn into ramps before the diodes see them. Probe the op-amp output and turn the distortion up, then tap U1 on the drawing and change the compensation cap. A bigger cap slows it more and softens the clip, and a smaller one sharpens it. The two ground legs on the minus input cut the gain below about 1.5 kHz, so the lows clip least.
Download.
Parts and what they do
Stock values for the Silicon version. Tap a part on the schematic to change it.
- R10, 47 Ω
- Supply resistor. With C11 and C12 it filters the 9 V line.
- C11, 100 µF electrolytic
- Supply filter cap.
- C12, 100 nF
- Supply bypass cap.
- R11, 100 kΩ
- Top of the 4.5 V divider.
- R12, 100 kΩ
- Bottom of the 4.5 V divider.
- C13, 1 µF electrolytic
- Filters the 4.5 V reference.
- R1, 1 MΩ
- Input pulldown.
- C1, 22 nF
- Input coupling cap. Against R2 it sets a corner near 7 Hz.
- R2, 1 MΩ
- Biases the op-amp input and sets the input impedance, about 500 kΩ with R1.
- R3, 1 kΩ
- With C2 it makes a filter against radio pickup.
- C2, 1 nF
- RF filter cap.
- U1, LM308
- The gain stage, a non-inverting amp with up to about 67 dB of gain. The 30 pF compensation cap, C3 on the board, is in this op-amp’s settings and sets the slew rate and bandwidth.
- Distortion, 100 kΩ pot, audio taper
- Wired as a variable feedback resistor. More resistance means more gain.
- C4, 100 pF
- Across the distortion pot. It limits the top end at high gain.
- R4, 47 Ω
- First ground leg. With C5 its corner is near 1.5 kHz, and it sets the maximum gain.
- C5, 2.2 µF electrolytic
- First ground leg cap.
- R5, 560 Ω
- Second ground leg. With C6 its corner is near 60 Hz.
- C6, 4.7 µF electrolytic
- Second ground leg cap.
- C7, 4.7 µF electrolytic
- Output coupling cap.
- R6, 1 kΩ
- Current-limiting resistor for the diodes. Their clipping happens across the voltage drop on this 1 kΩ.
- D1, D2, 1N914 / 1N4148, one each way
- Clipping diodes to ground. They clip the signal outright, unlike the 1979 overdrive’s diodes in the feedback. LEDs here are the common mod for a louder, more open sound.
- R7, 1.5 kΩ
- Sets the darkest the filter can go.
- Filter, 100 kΩ pot, audio taper
- A variable resistor in series with R7 into C8. Turning it up adds resistance and lowers the corner, from about 32 kHz at zero to about 475 Hz at full.
- C8, 3.3 nF
- Filter cap. The output buffer is fed from this point, so R7, the pot and C8 make a first-order low-pass after the clipping diodes.
- C9, 22 nF
- Coupling cap into the JFET buffer.
- R8, 1 MΩ
- Biases the JFET gate at 4.5 V.
- Q1, 2N5458
- Output buffer, a JFET source follower.
- R9, 10 kΩ
- Source resistor.
- C10, 1 µF electrolytic
- Output coupling cap.
- Volume, 100 kΩ pot, audio taper
- Output level.
Versions
- Silicon
- Negative ground. The 4.5 V reference biases the op-amp and the JFET buffer.
- OP07, later units, modeled after the ProCo RAT2
- The OP07 that replaced the LM308 once it went out of production, by 1996. It’s internally compensated, so the 30 pF cap does nothing. About the same slew rate, 0.3 V/µs, with 0.6 MHz of bandwidth. The RAT2’s status LED and its parts aren’t drawn.
- Red LEDs, 1989, modeled after the ProCo Turbo RAT
- The same circuit with red LEDs to ground instead of 1N914s. They clip near 2 V instead of 0.7 V, so the output is about 9 dB louder and the op-amp’s slewing is a bigger share of the sound. Early Turbo RATs used the LM308, like this build.
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.
Product and company names are trademarks of their owners. They are used here only to identify the circuits these models are based on; this site is not affiliated with or endorsed by any of them.
From Swell Amplification, swellamps.com.