Op-amp and CMOS overdrive, 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, not measured from one unit. Each circuit’s notes say which schematic it’s based on.
Modeled after the Electro-Harmonix Hot Tubes.
An op-amp gain stage, then two CD4049UB inverters run as amplifiers that clip, a tone control that fades between a bass and a treble path, and a third inverter to make up the level.
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. IC1A buffers the guitar and IC1B turns it over at a gain of 3.75. Drive is a variable resistor in series with R5 ahead of the first inverter, whose gain is R7 over the two: about 0.07 at 0, 0.7 at noon and 12 at full (the inverter’s own gain of about 30 keeps it under the 18 the resistors ask for). The second inverter has no input resistor, so it runs near its full gain, about 11, and does most of the clipping. R6 and R8 pull each inverter’s input slightly under its switch point, so the output rests about 0.25 V higher than the input and the clipping is uneven top and bottom. The tone control fades between a 240 Hz low-pass and a 1.1 kHz high-pass, and at noon it dips the mids. The make-up inverter has a gain of 6.7. The four working inverters pull about 5.4 mA each, 22 mA in all, so VA sits near 6.7 V behind the 100 Ω. As they clip they pull less and VA rises. The RC4558 and CD4049UB are typical of their data sheets, not measured.
Download.
Parts and what they do
Stock values. Tap a part on the schematic to change it.
- R21, 100 Ω
- Supply resistor.
- C17, 220 µF electrolytic
- Supply filter.
- C16, 47 µF electrolytic
- Supply filter.
- R19, 220 kΩ
- Top of the VB divider.
- R20, 220 kΩ
- Bottom of the VB divider, half of VA for the op-amps.
- C15, 47 µF electrolytic
- VB filter.
- C1, 100 nF
- Input cap.
- R1, 120 kΩ
- Biases the input at VB and sets the input impedance, 120 kΩ.
- C2, 180 pF
- Cuts radio and the very top end at the input.
- IC1A, RC4558
- Input buffer, a follower.
- R3, 20 kΩ
- Into the gain stage’s minus input.
- R2, 20 kΩ
- Carries the DC to the plus input, so the stage follows at DC.
- C3, 4.7 µF electrolytic
- Holds the plus input still for signal, so the stage is inverting there.
- IC1B, RC4558
- Gain stage, a gain of 3.75 (75 k over 20 k), turned over.
- R4, 75 kΩ
- Gain stage feedback.
- C4, 47 pF
- Rolls the gain stage off above about 45 kHz.
- C5, 100 nF
- Coupling cap to the first inverter.
- R5, 8.2 kΩ
- The first inverter’s input resistor with Drive at full. 150 k over 8.2 k is a gain of 18.
- Drive, 2 MΩ pot, reverse audio taper
- Drive, 2 MΩ as a variable resistor in series with R5. Turned up, less resistance and more gain into the first inverter. Reverse audio, as the kit notes say for the original.
- IC2A, CD4049UB
- First inverter, run as an amplifier. R6 to ground pulls its input slightly low of the switch point, so its output rests slightly high and the two halves of the wave clip unevenly.
- R6, 1.8 MΩ
- From the first inverter’s input to ground. With R7 it sets the resting point.
- R7, 150 kΩ
- First inverter’s feedback.
- C6, 27 pF
- Across R7, rolling the stage off above about 40 kHz.
- C7, 150 nF
- Couples the first inverter to the second with no resistor in series, so the second runs near its full gain.
- IC2B, CD4049UB
- Second inverter. Fed from the first one’s output resistance against its own 200 kΩ feedback, it has most of its own gain and does most of the clipping.
- R8, 2.2 MΩ
- From the second inverter’s input to ground, setting its resting point as R6 does for the first.
- R9, 200 kΩ
- Second inverter’s feedback.
- C8, 10 µF electrolytic
- Feeds the bass side of the tone control.
- R10, 5.6 kΩ
- With C9, a low-pass at about 240 Hz: the bass side.
- C9, 120 nF
- Bass side cap to ground.
- C10, 120 nF
- With R11, a high-pass at about 1.1 kHz: the treble side.
- R11, 1.2 kΩ
- Treble side resistor to ground.
- Tone, 10 kΩ pot, linear taper
- Tone, 10 kΩ, fading between the treble side (clockwise) and the bass side. In the middle the two leave a dip in the mids. Linear, as the kit’s 10kB.
- C11, 220 nF
- Couples the tone wiper to the make-up stage.
- R12, 150 kΩ
- The make-up stage’s input resistor, a gain of about 6.7 with R13.
- IC2D, CD4049UB
- Third inverter, making up the level lost in the tone control.
- R13, 1 MΩ
- Make-up stage feedback.
- C12, 4.7 µF electrolytic
- Output cap of the tone path.
- R14, 47 kΩ
- Pulldown on the tone path’s output.
- C13, 220 nF
- Feeds the tone bypass path straight from the second inverter.
- R15, 430 kΩ
- Bypass stage input resistor, a gain of about 2.3 with R16.
- IC2E, CD4049UB
- The tone bypass path’s inverter. It runs all the time, switched in or not.
- R16, 1 MΩ
- Bypass stage feedback.
- C14, 4.7 µF electrolytic
- Output cap of the bypass path.
- R17, 47 kΩ
- Pulldown on the bypass path’s output.
- Volume, 10 kΩ pot, audio taper
- Output level, 10 kΩ audio.
- R18, 150 Ω
- Output resistor.
About this version
- Stock
- The 1978 circuit as the kit redraw shows it, with the original 2 MΩ Drive pot. The RC4558 and CD4049UB are typical of their data sheets, not measured.
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, not measured parts. Caps are ideal, and the battery or HT is a voltage behind a resistance.
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