Transistor and op-amp fuzz with diodes to ground, 1975 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 Roland AF-60 Bee Gee.
A transistor stage, then an op-amp with a gain of 220, then two diodes to ground clip the note flat. A Big Muff style Tone control and a transistor stage follow.
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. Q1 brings the guitar up about 34 dB, and the op-amp adds up to about 30 dB more near 400 Hz. Together that clips any note on the op-amp’s rails, and the two diodes then square it off near 0.6 V. C6 brings the op-amp’s gain down above about 290 Hz, so the top end is smoother than the gain suggests. Tone fades between a treble path and a bass path, with a small dip in the mids at noon. Q2 brings the level back up about 2 times into Level. The phaser in the Jet Phaser isn’t modeled. No drawing of it was at hand.
Download.
Parts and what they do
Stock values. Tap a part on the schematic to change it.
- R1, 100 kΩ
- From the input to ground.
- R2, 15 kΩ
- Series resistor into Q1.
- C1, 22 nF
- Input coupling cap. With Q1’s input it trims the low bass.
- R3, 220 kΩ
- From Q1’s base to ground.
- R4, 470 kΩ
- Half of Q1’s bias feed from its collector.
- R5, 470 kΩ
- The other half. With R4 and R3 it holds the collector near 3 V.
- C2, 10 µF electrolytic
- From the middle of R4 and R5 to ground, so the feedback sets the bias only and Q1 keeps its full gain.
- Q1, 2N3904
- First gain stage, emitter grounded.
- R7, 1 kΩ
- Feeds Q1 from the 9 V. With C3 it filters Q1’s supply.
- C3, 10 µF electrolytic
- Filter cap on Q1’s supply.
- R6, 15 kΩ
- Q1’s collector load.
- C4, 10 nF
- Coupling cap into the op-amp stage. With R8 and R9 it trims the bass, from about 800 Hz down.
- R8, 10 kΩ
- Series resistor into the op-amp.
- R20, 22 kΩ
- Top of the Vb divider.
- R21, 22 kΩ
- Bottom of the Vb divider, so Vb sits at half the supply.
- C14, 10 µF electrolytic
- Filters Vb. The kit drawing shows 10 µF and its parts list 47 µF.
- R9, 10 kΩ
- Biases the op-amp’s plus input at Vb.
- IC1, LM741 / µA741
- The op-amp, an LM741 here. With R11 over R10 its gain is about 220, so it clips on the rails with any guitar note.
- R10, 10 kΩ
- Sets the op-amp’s gain with R11.
- C5, 10 µF electrolytic
- In series with R10 to Vb, so the gain applies to the signal and not to the bias.
- R11, 2.2 MΩ
- Feedback resistor, 2.2 MΩ.
- C6, 250 pF
- Across R11. It brings the gain down from about 290 Hz up, toward one at the top.
- R12, 10 kΩ
- Series resistor into the diodes.
- C7, 47 nF
- Coupling cap into the diodes.
- D2, D3, 1N914 / 1N4148, one each way
- Two 1N914s to ground, one each way. They hold the note near 0.6 V either way.
- C8, 2.2 nF
- Tone, treble side: in series to the top of the Tone pot.
- R14, 33 kΩ
- Tone, treble side: to ground. With C8 it passes above about 2.2 kHz.
- R13, 33 kΩ
- Tone, bass side: in series to the bottom of the Tone pot.
- C9, 1 nF
- Tone, bass side: to ground. With R13 it passes below about 4.8 kHz.
- Tone, 100 kΩ pot, linear taper
- Tone, a 100 kΩ linear pot, fading from the bass side to the treble side. The Jet Phaser is said to have a two-way switch in its place, at the pot’s two ends.
- C10, 47 nF
- Coupling cap into Q2.
- R16, 100 kΩ
- Top of Q2’s bias divider.
- R17, 56 kΩ
- Bottom of Q2’s bias divider.
- R15, 1 MΩ
- From the divider to Q2’s base.
- C11, 1 µF
- From Q2’s emitter to the divider. The divider then swings with the base, so R15 hardly loads the Tone control.
- Q2, 2N3904
- Output stage, a gain of about 2.
- R18, 15 kΩ
- Q2’s collector load.
- R19, 6.8 kΩ
- Q2’s emitter resistor, unbypassed. With R18 it sets the gain near 2.
- C12, 47 nF
- Output coupling cap.
- Level, 50 kΩ pot, audio taper
- Level, a 50 kΩ audio pot.
About this version
- LM741, 2N3904
- Negative ground, 9 V. The LM741 stands in for the TA7504, and the 2N3904 for the 2SC1000GR, as the kit drawing has them.
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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