OTA compressor with Attack and Threshold controls, 1982 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 Ibanez CP9 Compressor/Limiter.

An LM13600 transconductance amp whose gain follows the current into its bias pin, an envelope follower that turns that current down on loud notes, and a dual Attack Time pot that sets both how fast it clamps and how fast it lets go.

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. The LM13600’s output current is the voltage across its inputs times a gain set by the current into its bias pin, and that current flows into R110. So the gain follows the bias current. Q102 splits the output into two halves turned opposite ways, and Q103 and Q104 pull C111 down through the 1 kΩ half of Attack Time on every peak. Q105 follows C111, so a loud note leaves less voltage to push current through Threshold and R113, and the gain drops. Between notes C111 charges back through R122 and the 100 kΩ half. Probe the envelope to see it dip on each pick and climb back.

Download.

Parts and what they do

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

R111, 10 kΩ
Top of the reference divider.
R112, 10 kΩ
Bottom of the reference divider, 4.5 V.
C106, 47 µF electrolytic
Filter cap on the reference. The OTA’s inputs and load return to it.
R144, 10 kΩ
Top of the second 4.5 V divider, for the output stage.
R145, 10 kΩ
Bottom of the second divider.
C122, 47 µF electrolytic
Filter cap on the second 4.5 V line.
C101, 47 nF
Input cap.
R101, 10 kΩ
In series with the input.
R103, 9.1 kΩ
Top of Q101’s bias divider.
R104, 22 kΩ
Bottom of Q101’s bias divider, about 6.4 V.
C102, 10 µF electrolytic
Filter cap on Q101’s bias.
R102, 510 kΩ
Biases Q101’s base and sets the input impedance near 500 kΩ.
Q101, 2SC536F
Input buffer, an emitter follower.
R105, 5.1 kΩ
Q101’s emitter load.
C103, 2.2 µF electrolytic
Couples the buffer to the OTA.
R107, 10 kΩ
Holds the plus input at the reference.
R106, 180 Ω
Between the two inputs. Against the minus input’s path to the reference it lets only a small part of the signal across the inputs, so the OTA stays clean.
R108, 10 kΩ
Holds the minus input at the reference.
C104, 56 nF
Across R108. Higher up the band it pulls the minus input toward the reference, so more treble gets across the inputs.
VR101, 10 kΩ pot, linear taper
Offset trimmer, set at the factory for no DC across R110. At its middle here, since the model OTA has no offset.
R109, 150 kΩ
Carries the trimmer’s offset current to the minus input.
IC101, LM13700 (one half)
Half an LM13600. Its output is a current, the voltage across its inputs times a gain that follows the current into its bias pin.
R110, 150 kΩ
The OTA’s load, to the reference. The output current through it is the signal.
C105, 3.3 nF
Across R110. It rolls the top off above about 320 Hz, undoing C104’s lift at the input.
IC101 buffer, 2N5088
The LM13600’s Darlington buffer, first half.
IC101 buffer, 2N5088
The buffer’s second half. Its emitter is the OTA stage’s output.
R114, 10 kΩ
The buffer’s emitter load.
C107, 220 nF electrolytic
Couples the buffer to the envelope follower.
R116, 56 kΩ
Top of Q102’s bias divider.
R117, 27 kΩ
Bottom of Q102’s bias divider, about 2.9 V.
C108, 10 µF electrolytic
Filter cap on Q102’s bias.
R115, 100 kΩ
Biases Q102.
Q102, 2SC536F
Phase splitter. Its collector and emitter swing opposite ways, so the envelope sees both halves of the wave.
R120, 10 kΩ
Q102’s collector load.
R118, 10 kΩ
Q102’s emitter load, equal to R120.
C109, 10 nF
From Q102’s collector to Q103’s base.
D101, 1N914 / 1N4148
Catches the negative half at Q103’s base, so Q103 turns on near each peak.
R121, 1 MΩ
Q103’s base to ground.
Q103, 2SC536F
Pulls the envelope cap down on each peak from Q102’s collector.
C110, 10 nF
From Q102’s emitter to Q104’s base.
D102, 1N914 / 1N4148
Catches the negative half at Q104’s base.
R119, 1 MΩ
Q104’s base to ground.
Q104, 2SC536F
Pulls the envelope cap down on the other half of the wave.
Attack time (1 k), 1 kΩ pot, linear taper
The 1 kΩ half of the dual Attack Time pot, between Q103 and Q104 and the envelope cap. Turned up, more resistance, so the cap is pulled down slower and the start of each note gets through louder.
C111, 10 µF electrolytic
The envelope cap.
R122, 24 kΩ
Least resistance charging the envelope cap: a release of at least 0.24 s.
Attack time, 100 kΩ pot, linear taper
Attack time, the 100 kΩ half of a dual pot, in series with R122 charging the envelope cap. Turned up, both halves add resistance, so the clamp comes on slower and lets go slower.
Q105, 2SC536F
Follows the envelope and drives the bias current.
Threshold, 200 kΩ pot, reverse audio taper
Threshold, 200 kΩ reverse audio as a variable resistor in the bias current’s path. Turned up, less resistance, more bias current and more gain, so the compression starts on quieter notes.
R113, 12 kΩ
In series with the bias pin. With Threshold at full it limits the bias current.
C112, 220 nF electrolytic
Couples the OTA stage to the Level pot.
Level, 100 kΩ pot, linear taper
Output Level, 100 kΩ linear.
R123, 1 kΩ
Under the Level pot, so a little signal is left at its bottom.
C115, 100 nF
From the Level wiper to the effect switch (Q107, a wire here).
R128, 510 kΩ
Holds the switch’s side at 4.5 V.
C114, 100 nF
From the switch to the output follower.
R127, 510 kΩ
Biases Q106 from the 4.5 V line.
Q106, 2SC536F
Output buffer, an emitter follower.
R126, 10 kΩ
Q106’s emitter load.
C113, 10 µF electrolytic
Output coupling cap.
R125, 47 Ω
Output series resistor.
R124, 100 kΩ
Output pulldown.

About this version

Stock, LM13600
As drawn: an LM13600 and 2SC536s. The OTA is the LM13700’s typicals and its buffer two 2N5088s.

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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