JFET buffer, op-amp boost and two-band tone, 1980 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 FA-1 FET Amplifier.
A JFET source follower drives an inverting op-amp stage whose Volume pot sets the gain from nothing to about 25 dB. A second op-amp carries a Bass and Treble tone circuit in its feedback. A switch cuts the lows.
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, a JFET source follower, takes the guitar at a high impedance. IC1A is an inverting amplifier: R5 sets its input resistance, and R6 with the Volume pot sets its feedback. With Volume at 0 the pot shorts the feedback and nothing comes through. At 10 the gain is about 25 dB. C4 keeps DC off the pot, so it doesn’t crackle. IC1B carries a Bass and Treble tone circuit in its feedback, flat with both knobs at noon. Each knob boosts its band toward 10 and cuts it toward 0. The FLAT/CUT switch picks the coupling cap ahead of IC1A, 470 nF beside 47 nF for Flat or 47 nF alone for Cut.
Download.
Parts and what they do
Stock values. Tap a part on the schematic to change it.
- R16, 100 Ω
- Series resistor in the supply. With C13 it filters the supply.
- C14, 100 nF
- Small cap across the supply for high frequencies.
- C13, 100 µF electrolytic
- Main supply filter cap.
- R13, 22 kΩ
- Top of the divider that makes VB and VC.
- C11, 47 µF electrolytic
- Filters VB, the op-amps’ reference, about 4.4 V.
- R14, 6.8 kΩ
- Middle of the divider.
- R15, 15 kΩ
- Bottom of the divider. VC sits near 3 V.
- C12, 10 µF electrolytic
- Filters VC, the JFET’s gate bias.
- R1, 10 kΩ
- Series resistor at the input.
- C1, 10 nF
- Input coupling cap. With R2 it passes everything above about 5 Hz.
- R2, 3.3 MΩ
- Biases the gate at VC. At 3.3 MΩ the input impedance is high, so the pickup sees almost no load.
- Q1, 2N5457
- Input buffer, a JFET source follower (2N5457). The pedal’s name comes from it.
- R3, 10 kΩ
- Source resistor.
- C2, 470 nF
- The large coupling cap, used in Flat.
- R4, 470 kΩ
- Keeps C2 charged while the switch is open, so it doesn’t pop.
- C3, 47 nF
- The small coupling cap. Alone, with R5, it cuts below about 200 Hz.
- R5, 15 kΩ
- Input resistor of the gain stage. The gain is the feedback resistance over 15 kΩ.
- IC1A, MC1458
- The gain stage, an inverting amplifier.
- R6, 470 kΩ
- Fixed feedback resistor. With the Volume pot at full beside it, the gain is about 25 dB.
- Volume, 1 MΩ pot, audio taper
- Volume, 1 MΩ audio as a variable resistor, in the feedback beside R6 through C4. At 0 it shorts the feedback and the stage has no gain at all.
- C4, 10 µF electrolytic
- Joins the Volume pot to the output for audio while keeping DC off the pot.
- C5, 10 µF electrolytic
- Couples the gain stage to the tone circuit.
- R7, 8.2 kΩ
- Input resistor of the bass half of the tone circuit.
- Bass, 50 kΩ pot, linear taper
- Bass, 50 kΩ linear. The wiper toward the input end boosts the lows, toward the output end cuts them.
- C6, 33 nF
- Bypasses the input half of the Bass pot above the bass range.
- C7, 33 nF
- Bypasses the output half of the Bass pot above the bass range.
- R8, 8.2 kΩ
- Feedback resistor of the bass half.
- R9, 33 kΩ
- From the Bass wiper to IC1B’s − input.
- C8, 5.6 nF
- Feeds the treble to the Treble pot.
- Treble, 50 kΩ pot, linear taper
- Treble, 50 kΩ linear. The wiper toward the input end boosts the highs, toward the output end cuts them.
- C9, 5.6 nF
- Feeds the treble back from the output to the Treble pot.
- R10, 10 kΩ
- From the Treble wiper to IC1B’s − input.
- IC1B, MC1458
- The tone stage. With both knobs at noon its gain is about 1.
- R11, 470 Ω
- Keeps a long cable from upsetting IC1B.
- C10, 10 µF electrolytic
- Output coupling cap.
- R12, 100 kΩ
- Output pulldown.
About this version
- Stock
- Negative ground, 9 V battery. 2N5457 JFET, MC1458 op-amps for the original’s two HA1457W.
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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