JFET compressor, 1970s 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.

  1. 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.

  2. 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.

  3. 3Press Play

    Press Play through the circuit to hear a picked note or chord. Play the string is the guitar by itself.

  4. 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.

  5. 5Tap a wire

    Tap a wire to put the probe there. The Probe graph and the spectrum show the signal there.

  6. 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

Modeled after the Dan Armstrong Orange Squeezer.

A JFET shunts the guitar to ground through 82 k, and the louder the output gets, the harder a diode detector turns that JFET on. One op-amp stage makes up the gain. No knobs but Volume and an inside trimmer.

Input

Add

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.

How it’s played
Tap a part to change its value, or a wire to probe it. To add or take out parts, open Modify this circuit, below. Red numbers are DC voltages to ground, not warnings. Scroll or drag sideways to see the whole circuit.
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.

Add a part

Tap where each leg goes. Tap both ends of a part to put the new one across that part.

Change a 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.

Change a wire

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.

Keep and share

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.

Test tone for the graphs
The graphs use a steady tone. Play sounds a plucked string that dies away.

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. R2 and Q2 make a divider. With no signal Q2 is off and the guitar passes to the op-amp, which brings it up about 23 times. The op-amp’s output also feeds D1, which charges C7 on the positive peaks. That voltage reaches Q2’s gate through R6, Q2 starts to conduct, and more of the signal is shunted to B, which C3 holds at AC ground. So louder notes are turned down more. R3 and C2 put half the drain’s swing on the gate, which keeps the JFET’s resistance even over the wave. C7 charges through 1.5 kΩ in about 7 ms and lets go through 100 kΩ in about half a second. The JFET model is typical of the part, so where the squeeze starts depends on R7 and on the real JFET.

Download.

Parts and what they do

Stock values. Tap a part on the schematic to change it.

R1, 4.7 MΩ
Input pulldown.
C1, 47 nF
Input coupling cap.
R2, 82 kΩ
The top of the attenuator. Q2 is the bottom.
Q2, 2N5457
The gain control, a JFET used as a variable resistor from the signal to the bias point. As its gate rises toward its source it conducts more and shunts more of the signal away.
C2, 2.2 nF
Feeds the signal on Q2’s drain to its gate, with R3.
R3, 470 kΩ
With R6 it puts half the drain’s signal on the gate. That evens out the JFET’s resistance over the wave, so it distorts less.
R6, 470 kΩ
From the detector to Q2’s gate.
Q1, 2N5457
A current source. Its gate is on B and R5 is in its source, so it pushes a fixed current into B and holds Q2’s source above ground.
R5, 2.4 kΩ
Sets Q1’s current.
C3, 4.7 µF electrolytic
Holds B steady, so for the signal Q2’s source is at ground.
R7, 10 kΩ pot, linear taper
An inside trimmer, 10 kΩ, from B to ground. More resistance raises B, which holds Q2 further off, so it takes a louder note before it compresses. The drawing shows no setting.
C4, 47 nF
Coupling cap into the op-amp.
R4, 390 kΩ
Top of the op-amp’s input bias.
R8, 470 kΩ
Bottom of the op-amp’s input bias.
IC1, JRC4558D
Make-up gain, non-inverting, about 23.
R9, 220 kΩ
Feedback resistor.
R10, 10 kΩ
With R9 it sets the gain, 1 plus 220 k over 10 k.
C5, 4.7 µF electrolytic
In series with R10, so the gain is 1 at DC.
C6, 4.7 µF electrolytic
Output coupling cap. It feeds both the Volume pot and the detector.
R11, 1.5 kΩ
In series with the detector diode. With C7 it sets how fast the compression comes in.
D1, 1N60 germanium
Germanium detector diode. It charges C7 on the positive peaks.
C7, 4.7 µF electrolytic
Holds the detector’s voltage.
R12, 100 kΩ
Lets C7 discharge, about half a second with C7, so the compression lets go.
Volume, 10 kΩ pot, audio taper
Volume, R13 on the drawing, a 10 kΩ pot. The drawing shows no taper. Audio here.

About this version

2N5457 and 4558
Negative ground, 9 V. Two 2N5457 JFETs and half a 4558, as the drawing lists them. The 1N60 germanium preset stands in for the 1N100 detector diode.

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.

© 2026 Swell Amplification LLC. All rights reserved. The simulator, its circuit models, drawings, presets and text are the property of Swell Amplification LLC and may not be copied, reproduced, redistributed or used to build another product without written permission. The original circuits are historical designs documented in public sources, and no claim is made to them. Everything here is provided as is, without warranty of any kind, for study and personal use. Results are simulations of typical parts, not measurements of any real unit. Don’t rely on them for the safety of any build or repair. Tube amplifiers carry lethal voltages.

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