Two-stage op-amp overdrive, 1991 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 Marshall Bluesbreaker pedal.
A clean non-inverting gain stage, then an inverting stage with two pairs of silicon diodes in its feedback, each pair in series with 6.8 kΩ, and a tone control. Two diodes a side plus the series resistor make the clipping start late and round off gently.
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. A low-gain overdrive that many later pedals are closely based on. Most of the gain is in the first stage. Turning the gain pot down also puts up to 100 kΩ in series with the output, so low settings are quieter as well as cleaner. The second stage gains 47 until the diodes start to conduct around ±1.2 V, then about 1.5, so peaks flatten slowly instead of cutting off. Probe IC1b’s output and raise the gain: the waveform grows rounded shoulders long before it looks clipped.
Download.
Parts and what they do
Stock values. Tap a part on the schematic to change it.
- D5, 1N4001
- Reverse-polarity protection in series with the supply. It costs about 0.65 V of headroom.
- R14, 47 kΩ
- Top of the reference divider.
- R5, 47 kΩ
- Bottom of the reference divider.
- C11, 100 µF electrolytic
- Reference filter cap.
- R1, 2.2 MΩ
- Input pulldown. It keeps the input cap from clicking when switched.
- C1, 10 nF
- Input coupling cap. With R4 it makes a high-pass near 16 Hz.
- R4, 1 MΩ
- Biases IC1a’s input at the reference and sets the input impedance near 1 MΩ.
- IC1a, TL072
- First stage, non-inverting. The gain is 1 plus the feedback side of the gain pot over the gain leg below it.
- R2, 4.7 kΩ
- Gain leg resistor.
- R3, 3.3 kΩ
- In series with C2, and together they sit across R2. Above about 4.8 kHz they lower the leg’s resistance, adding a little gain at the top.
- C2, 10 nF
- In series with R3.
- C3, 10 nF
- The gain leg’s cap to the reference. With R2 it adds no gain below about 3.4 kHz at low settings, so the stage boosts the mids and highs far more than the bass.
- C4, 47 pF
- Across the feedback. It rolls off the very top.
- Gain, 100 kΩ pot, linear taper
- Gain pot, wiper on the op-amp output. One side is the feedback resistor, the other is in series with the stage’s output. Turning it up adds feedback resistance and removes series resistance, so gain and level rise together.
- C6, 220 nF
- Coupling cap out of the first stage.
- R8, 4.7 kΩ
- Series resistor between the stages.
- Leakage, 10 MΩ
- Not in the pedal. It stands in for capacitor leakage at a node that has no DC path while the effect is on.
- C7, 220 nF
- Second coupling cap.
- R7, 4.7 kΩ
- Input resistor of the inverting stage. With R8 (via C7) it sets the stage’s input resistance.
- IC1b, TL072
- Second stage, inverting, with the clipping in its feedback.
- R10, 220 kΩ
- Feedback resistor. Its gain is 220 k / 4.7 k = 47 before the diodes conduct.
- R11, 6.8 kΩ
- In series with the diodes. Once they conduct, the gain drops to about 6.8 k / 4.7 k instead of to zero, so the clipping stays soft.
- D1–D4, 2 × 1N914 / 1N4148, one each way
- Four 1N4148s, two in series each way. They start conducting near ±1.2 V.
- R9, 1 kΩ
- Series resistor into the tone control.
- Tone, 25 kΩ pot, linear taper
- Tone. The wiper slides between the op-amp side (bright) and the C8 side, which shunts treble to the reference (dark).
- C8, 10 nF
- Tone cap to the reference.
- R12, 6.8 kΩ
- With C9, a fixed low-pass near 2.3 kHz after the tone control.
- C9, 10 nF
- Fixed treble-cut cap.
- Volume, 100 kΩ pot, audio taper
- Output level, returned to the reference.
- C10, 100 nF
- Output coupling cap.
- R13, 1 MΩ
- Output pulldown.
About this version
- Original wiring
- Negative ground, 9 V, a TL072 dual op-amp, as drawn by GGG.
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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