Swept band-pass boost blended with the dry signal, 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.
- 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 Boss SP-1 Spectrum.
A buffer, then two paths: an op-amp band-pass with a Wien network tuned by the Spectrum pot, and an inverted dry copy. The Balance pot pans between them before a summing stage.
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. IC1C has a Wien network on its plus input and enough gain to sit just short of oscillating, so it passes a narrow band about 22 dB up. Spectrum moves the band from about 550 Hz to 5 kHz. Balance fades from the dry signal alone to the band alone, so in between the band rides on top of the full signal. The gain is set by a trimmer, set here as the service manual says: 22.5 dB at 5 kHz with both knobs full up. The model leaves out the bypass switching.
Download.
Parts and what they do
Stock values. Tap a part on the schematic to change it.
- R14, 33 kΩ
- Top of the 4.5 V reference divider.
- R15, 33 kΩ
- Bottom of the reference divider.
- C11, 47 µF electrolytic
- Filters the reference.
- R1, 1 kΩ
- Input series resistor.
- C1, 47 nF
- Input coupling cap.
- R2, 470 kΩ
- Biases the input buffer and sets the input impedance near 470 kΩ.
- IC1A, RC3403 / NJM3403
- Input buffer, a follower. A quarter of an RC3403A.
- C2, 4.7 µF
- Coupling cap to both paths.
- R3, 10 kΩ
- Input resistor of the dry path’s inverter.
- IC1D, RC3403 / NJM3403
- Inverts the dry signal, a gain of 1, so it adds to the band in the right phase.
- R4, 10 kΩ
- IC1D’s feedback.
- C3, 4.7 µF
- Coupling cap to Balance’s dry gang.
- Balance (dry gang), 10 kΩ pot, audio taper
- The dry gang of Balance, 10 kΩ audio. Turned up it fades the dry signal out.
- R10, 10 kΩ
- The dry signal into the summing point.
- Balance, 10 kΩ pot, reverse audio taper
- Balance, a dual pot. This gang, 10 kΩ reverse audio, feeds the band-pass. Turned up it fades the band in as the other gang fades the dry signal out.
- R5, 1 kΩ
- Under Balance’s wet gang.
- C4, 100 nF
- Coupling cap into the band-pass.
- R6, 10 kΩ
- Input resistor of the band-pass stage.
- Gain trim, 1 kΩ pot, linear taper
- The 1 kΩ gain trimmer, inside the pedal. It sets how close IC1C runs to oscillating, so how tall and narrow the band is. Set here for the service manual’s 22.5 dB at 5 kHz.
- R7, 18 kΩ
- IC1C’s feedback.
- IC1C, RC3403 / NJM3403
- The band-pass stage. Its plus input on the Wien network turns it into a peak at one frequency.
- C6, 3.3 nF
- The Wien network’s series cap.
- R8, 10 kΩ
- Least series resistance in the Wien network, so the band tops out near 5 kHz.
- Spectrum (gang), 100 kΩ pot, linear taper
- The Spectrum pot’s second gang, a variable resistor in the network’s series arm.
- C5, 3.3 nF
- The Wien network’s shunt cap.
- Spectrum, 100 kΩ pot, linear taper
- Spectrum, a 100 kΩ linear dual pot, each gang a variable resistor in the Wien network. Turned up the band moves from about 550 Hz to 5 kHz.
- R11, 10 kΩ
- Least shunt resistance in the network.
- C7, 1 µF
- Coupling cap from the band-pass.
- R9, 22 kΩ
- The band into the summing point.
- C8, 4.7 µF
- Coupling cap into the output stage.
- IC1B, RC3403 / NJM3403
- The output stage, inverting, adding the two paths.
- R12, 10 kΩ
- Output stage feedback.
- C9, 47 pF
- Across R12, against oscillation.
- R13, 470 Ω
- Output series resistor.
- C10, 1 µF
- Output coupling cap.
About this version
- SP-1
- The pedal’s values from Aion FX’s notes: 4.7 µF couplings, 10 kΩ feedback in the output stage, no Volume pot. No factory drawing was at hand.
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.
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