OTA compressor with a treble-lift input stage, early 1980s 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 Korg CMP-1 Compressor.

An op-amp stage lifts the treble, an OTA sets the gain, and a phase splitter with two transistors pulls the envelope cap down on every peak. The OTA’s bias current follows the cap, so louder notes get less gain.

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. IC2a lifts the treble 21 dB above about 3.4 kHz, and C7 at the OTA’s output takes it back down, so the envelope sees more of the pick attack. Q1 splits the OTA stage’s output into two opposite copies, and Q2 and Q3 each pull C5 down on their half of the wave. Q4 follows C5 and sets the OTA’s bias current through Sensitivity and R4: louder notes, less bias current, less gain. With Sensitivity at full, a steady A2 picked at 20 mV comes out at about 153 mV, and at 500 mV, 25 times louder, about 279 mV. The model leaves out the footswitch and the LED.

Download.

Parts and what they do

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

R20, 10 kΩ
Top of the Vref divider.
R23, 10 kΩ
Bottom of the Vref divider.
C10, 220 µF electrolytic
Filters Vref.
R21, 10 kΩ
Input series resistor.
C8, 22 nF
Input coupling cap.
R18, 1 MΩ
Biases IC2a’s + input from Vref and sets the input impedance near 1 MΩ.
IC2a, JRC4558D
Input stage, non-inverting. Its gain is 1 in the lows and rises to 11 above about 3.4 kHz.
R12, 47 kΩ
IC2a’s feedback resistor.
C9, 10 nF
In series with R22, so only the highs see the gain leg.
R22, 4.7 kΩ
The gain leg. With C9 the lift starts near 3.4 kHz.
C6, 10 µF electrolytic
Coupling cap to the OTA.
R1, 2.2 kΩ
Holds the OTA’s + input at Vref.
R2, 43 Ω
Between the OTA’s two inputs. Against R3 only about 2 % of the signal appears across the inputs, which keeps the OTA clean.
R3, 2.2 kΩ
Holds the OTA’s minus input at Vref.
VR1, 22 kΩ pot, linear taper
Bias trimmer, 22 kΩ across the supply. It trims the OTA’s input offset. Set at its middle here.
R7, 47 kΩ
Carries the trimmer’s offset to the minus input.
IC1a, LM13700 (one half)
Half an NJM13600. Its output is a current, the voltage across its inputs times a gain that follows the current into its bias pin.
R9, 150 kΩ
The OTA’s load, to Vref. The output current through it is the signal.
C7, 3.3 nF
Across R9. It rolls the top off above about 320 Hz, which undoes IC2a’s lift.
IC1a buffer, 2N5088
The NJM13600’s Darlington buffer, first half.
IC1a buffer, 2N5088
The buffer’s second half. Its emitter is the OTA stage’s output.
R5, 10 kΩ
The buffer’s emitter load.
C4, 10 µF electrolytic
Output coupling cap.
R13, 4.7 kΩ
In series ahead of Out Level.
R14, 1 kΩ
Beside R13 with S1 at II, so more level reaches Out Level.
Out Level, 10 kΩ pot, linear taper
Out Level, 10 kΩ linear.
D5, 1N914 / 1N4148
From the buffer’s output to Q1’s base. It carries the signal and the buffer’s DC, which biases Q1.
Q1, 2N5088
Phase splitter. Its collector and emitter swing opposite ways, so the envelope sees both halves of the wave.
R8, 10 kΩ
Q1’s collector load.
R6, 10 kΩ
Q1’s emitter load, equal to R8.
C3, 10 nF
From Q1’s collector to Q3’s base.
D3, 1N914 / 1N4148
Catches the negative half at Q3’s base, so Q3 turns on near each peak.
R10, 1 MΩ
Q3’s base to ground.
Q3, 2N5088
Pulls the envelope cap down on each peak from Q1’s collector.
C2, 10 nF
From Q1’s emitter to Q2’s base.
D4, 1N914 / 1N4148
Catches the negative half at Q2’s base.
R11, 1 MΩ
Q2’s base to ground.
Q2, 2N5088
Pulls the envelope cap down on the other half of the wave.
R15, 330 Ω
Between the rectifiers and the envelope cap. It sets how fast the cap is pulled down, the attack.
C5, 10 µF electrolytic
The envelope cap, 10 µF, low leakage on the drawing.
R17, 150 kΩ
Charges the envelope cap back up between peaks, the release.
R16, 10 kΩ
Through S1b, a faster charge path: to C5 at I, to the rectifier side of R15 at II.
Q4, 2N5088
Follows the envelope cap and drives the bias current.
Sensitivity, 200 kΩ pot, audio taper
Sensitivity, 200 kΩ audio as a variable resistor from Q4 to R4. Turned up it takes resistance out, so more bias current flows and the gain is higher until the envelope pulls it down.
R4, 15 kΩ
In series with the bias pin. With Sensitivity at full it limits the bias current.

About this version

Stock
Negative ground. The OTA is an NJM13600, the LM13700 preset here, with two 2N5088s for its buffer. The transistors are 2SC2785s on the drawing, the 2N5088 preset here.

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.

Product and company names are trademarks of their owners. They are used here only to identify the circuits these models are based on; this site is not affiliated with or endorsed by any of them.

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