Op-amp sustainer with a low-pass filter stage, 1977 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.
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- 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 Electro-Harmonix Big Muff π (op-amp version).
An inverting op-amp stage, a low-pass filter stage with a gain of about 10, a Sustain pot, then an op-amp stage with three diodes each way in its feedback. The tone stack fades a low-pass against a high-pass, as on the transistor version.
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 has a gain of about 5.9. IC1b is a low-pass filter with a gain of about 10, flat to near 1 kHz and falling fast above, so the clipping stage gets little treble. IC2 has a gain near 50 until three diodes each way conduct, past about 1.8 V, and the note comes out close to a square wave. Turning Sustain down drops IC2’s drive and how hard it clips. Probe O2, then O3, and watch the wave flatten. The model leaves out the footswitch and the supply’s 47 Ω and 220 µF.
Download.
Parts and what they do
Stock values for the 1977 version. Tap a part on the schematic to change it.
- R2, 220 kΩ
- Top of IC1’s reference divider.
- R3, 220 kΩ
- Bottom of the divider. IC1a’s + input sits at half the battery.
- C2, 1 µF
- Filters IC1’s reference.
- C1, 150 nF
- Input coupling cap. With R1 it cuts below about 19 Hz.
- R1, 56 kΩ
- IC1a’s input resistor, and the pedal’s input impedance, 56 kΩ.
- IC1a, RC4558
- First stage, inverting. Its gain is 330 kΩ over 56 kΩ, about 5.9.
- R4, 330 kΩ
- IC1a’s feedback resistor.
- R5, 10 kΩ
- First resistor of the low-pass filter.
- R6, 47 kΩ
- Second resistor of the low-pass filter, into IC1b’s + input.
- C3, 4.7 nF
- From the filter’s middle to IC1b’s output. This feedback lifts the response near the corner, about 1 kHz.
- C4, 10 nF
- The filter’s cap to ground.
- IC1b, RC4558
- Second stage, non-inverting, the filter’s gain stage.
- R8, 560 kΩ
- IC1b’s feedback resistor. Over R7 it sets the gain, 1 plus 560 kΩ over 62 kΩ, about 10.
- R7, 62 kΩ
- IC1b’s gain leg, to C5.
- C5, 10 µF electrolytic
- Grounds the gain leg for audio only, so IC1b’s gain is 1 at DC. The Sustain pot returns here too.
- Sustain, 10 kΩ pot, linear taper
- Sustain, 10 kΩ linear, from IC1b’s output to C5. Its wiper sets how much reaches the clipping stage.
- R9, 47 Ω
- Under the Sustain pot, so a little signal gets through at 0.
- C7, 4.7 µF electrolytic
- Coupling cap into the clipping stage.
- R10, 8.2 kΩ
- IC2’s input resistor. The clipping stage’s gain is 470 kΩ over this plus the Sustain pot’s share, about 45 to 57.
- R11, 820 kΩ
- Top of IC2’s reference divider.
- R12, 1 MΩ
- Bottom of IC2’s divider. IC2’s + input sits near 4.9 V.
- C8, 220 nF
- Filters IC2’s reference.
- IC2, LM741 / µA741
- The clipping stage, a 741, inverting.
- R13, 470 kΩ
- IC2’s feedback resistor.
- D1–D6, 3 × 1N914 / 1N4148, one each way
- Three silicon diodes each way across the feedback. Past about 1.8 V either way they pull the gain down.
- C9, 150 pF
- Across the feedback. It rolls off the top of the clipping stage, from about 2.3 kHz with 470 kΩ.
- C10, 100 nF
- Tone stack, high-pass side.
- R15, 1.2 kΩ
- Tone stack, high-pass side. With C10 the corner is near 1.3 kHz.
- C11, 1 µF electrolytic
- Coupling cap into the low-pass side.
- R14, 5.6 kΩ
- Tone stack, low-pass side.
- C12, 120 nF
- Tone stack, low-pass side. With R14 the corner is near 240 Hz.
- Tone, 10 kΩ pot, linear taper
- Tone, 10 kΩ linear, between the two sides. Up is brighter.
- Volume, 100 kΩ pot, linear taper
- Volume, 100 kΩ, linear in 1977 and audio in 1978.
- R16, 100 kΩ
- From Volume’s wiper to ground.
Versions
- 1977
- The 1977 board: 150 pF across IC2’s feedback and a 100 kΩ linear Volume pot.
- 1978
- The 1978 board: 330 pF across IC2’s feedback, which rolls the fuzz off from about 1 kHz, a 100 kΩ audio Volume pot, and a Tone bypass switch.
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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