OTA compressor with Release and Compression controls, 1990 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 DOD FX80B Compressor Sustainer.

A CA3080 whose gain follows the current into its bias pin, with an envelope follower that turns the current down as the note gets louder. Release sets how fast the gain comes back, Compression how much current there is to start with.

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 CA3080’s output current is the voltage between its inputs times a gain set by the current into its bias pin, and that current flows into R9. So the gain follows the bias current. Q1 splits the output into two halves turned opposite ways, and Q2 and Q3 pull C13 down on every peak of each. Q5 follows C13, so a loud note leaves less voltage to push current through Compression, D4 and R20 into the bias pin, and the gain drops. Between notes C13 charges back through R17 and Release. 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.

R6, 18 kΩ
Top of the VB divider.
R3, 22 kΩ
Bottom of the VB divider. With R6 it sets VB at about 4.95 V.
C2, 10 µF electrolytic
Filter cap on VB.
C1, 10 nF
Input cap. Against R2 it cuts below about 34 Hz.
R1, 1 kΩ
In series into the input buffer.
R2, 470 kΩ
Biases the buffer at VB and sets the input impedance near 470 kΩ.
U1A, MC1458
Half a 1458 as a follower, the input buffer.
C6, 1 µF electrolytic
Couples the buffer to the OTA’s input network.
R4, 22 kΩ
With R5, divides the signal by about 7.7 before the OTA, so its input pair stays out of clipping.
C3, 10 nF
Across R4. Above about 720 Hz it lets more signal through, so the treble reaches the OTA less divided.
R5, 3.3 kΩ
From the plus input to VB, the bottom of the input divider.
R11, 3.3 kΩ
Holds the minus input at VB, matching R5.
U2, CA3080
The CA3080. Its output is a current, the input voltage times a gain that follows the current into its bias pin. Less bias current, less gain.
R9, 220 kΩ
The OTA’s load, to VB. The output current through it is the signal.
C8, 120 pF
Rolls the top off the OTA’s output, above about 6 kHz against R9.
R10, 33 kΩ
With C7, a treble cut from the OTA’s output to ground.
C7, 1 nF
With R10, cuts the treble above about 4.8 kHz. It undoes C3’s lift at the input, so the OTA’s output stays within about 1 dB from 100 Hz to 5 kHz.
Q9, 2N5088
Emitter follower off the OTA’s output. It drives the Level pot and the envelope.
R12, 2 kΩ
Top of Q9’s emitter load, feeding Q1’s base.
R14, 6.8 kΩ
Bottom of Q9’s emitter load.
Q1, 2N5088
Phase splitter. Its collector and emitter swing opposite ways, so the envelope sees both halves of the wave.
R16, 10 kΩ
Q1’s collector load.
R15, 10 kΩ
Q1’s emitter load, equal to R16.
C11, 10 nF
From Q1’s collector to Q2’s base.
D3, 1N914 / 1N4148
Catches the negative half at Q2’s base, so Q2 turns on near each peak.
R7, 470 kΩ
Q2’s base to ground. With no bias, Q2 stays off until a peak comes through.
Q2, 2N4124
Pulls the envelope cap down on each peak from Q1’s collector.
C10, 10 nF
From Q1’s emitter to Q3’s base.
D2, 1N914 / 1N4148
Catches the negative half at Q3’s base.
R18, 470 kΩ
Q3’s base to ground.
Q3, 2N4124
Pulls the envelope cap down on the other half of the wave.
R17, 33 kΩ
Least resistance charging the envelope cap, so the fastest release is about 0.33 s.
Release, 500 kΩ pot, audio taper
Release, 500 kΩ as a variable resistor in series with R17. Turned up, more resistance, so the envelope cap climbs back slower and the gain comes back later after a note.
C13, 10 µF electrolytic
The envelope cap. Q2 and Q3 pull it down fast on loud peaks, and it climbs back through R17 and Release.
Q5, 2N5088
Follows the envelope and drives the bias current, so the cap isn’t loaded much.
Compression, 500 kΩ pot, reverse audio taper
Compression, 500 kΩ as a variable resistor in the bias current’s path. Turned up, less resistance, more bias current and more gain, so quiet notes come up further and loud ones are pulled down harder.
D4, 1N914 / 1N4148
In series with the bias current, one way only.
R20, 33 kΩ
In series with the bias pin. With Compression at full it limits the bias current to about 200 µA.
R21, 3.9 MΩ
A trickle of bias current from 9 V, about 2 µA, so the OTA never shuts off.
C9, 1 µF electrolytic
Couples Q9 to the Level pot.
R13, 100 kΩ
In series with the Level pot. With the pot it divides the signal by at least 2.
Level, 100 kΩ pot, audio taper
Level, 100 kΩ over VB.
R8, 1 MΩ
Holds the wiper side of C16 at VB.
C16, 100 nF
Couples the Level wiper into the output buffer.
R28, 100 kΩ
From the input buffer to the output buffer, always. With the effect off it carries the dry signal. With it on, the Level wiper drives the same point through C16, and the dry signal still comes through R28 under it.
U1B, MC1458
The other half of the 1458, a follower, the output buffer.
R29, 1 kΩ
Output series resistor.
C17, 1 µF electrolytic
Output coupling cap.
R30, 100 kΩ
Output pulldown.

About this version

Stock, CA3080
As drawn: a CA3080, a 1458, 2N5088s and 2N4124s.

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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