OTA compressor with an Attack control, 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 MXR Super Comp.

The Dyna Comp circuit with an Attack control. Attack sets where along the envelope’s charging resistor the follower takes its voltage, which changes how fast the gain comes back after a note.

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. It works as the Dyna Comp does. The OTA’s gain follows the current into its bias pin, Q2 splits its output, and Q3 and Q4 pull the 10 µF envelope cap down on every peak. The difference is Attack: the cap charges through 75 kΩ and 300 kΩ, and Q5 follows the Attack wiper along the 300 kΩ. With Attack at 0 the gain follows the cap. Turned up, Q5 sits nearer 9 V and the playing pulls the gain down less. With the knobs at their default settings, a steady A2 at 20 mV comes out at about 55 mV and at 200 mV only about 1.6 dB louder.

Download.

Parts and what they do

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

R16, 56 kΩ
Top of the Vr divider.
R17, 22 kΩ
Bottom of the Vr divider. With R16 it sets Vr near 2.5 V.
CVR, 10 µF electrolytic
Filters Vr. The Dyna Comp file has no cap here.
C1, 10 nF
Input cap. Against the 1 MΩ bias resistor it sets a corner near 16 Hz.
R1, 10 kΩ
Base stopper, in series with the input.
R2, 1 MΩ
Biases Q1’s base at Vr, and sets the input impedance at about 1 MΩ.
Q1, MPSA18
Input buffer, an emitter follower. Its emitter sits about 0.6 V below Vr.
R3, 10 kΩ
Q1’s emitter load.
C2, 1 µF electrolytic
Couples the buffer to the OTA’s minus input.
R4, 1 MΩ
From the minus input to ground. With R5 it sets the inputs at about 4.6 V.
Trim, 2 kΩ pot, linear taper
A 2 kΩ trimmer between the two inputs. The signal reaches the plus input through it, so the two inputs see the guitar minus about an eighth. Its wiper carries R5’s bias current, and its position trims the OTA’s offset.
R5, 470 kΩ
Feeds both inputs their bias from 9 V through the trimmer.
R6, 1 MΩ
From the plus input to ground, matching R4.
R7, 16 kΩ
Holds the plus input near signal ground through C4. Against the 2 kΩ trimmer it divides the guitar by about 9 before the OTA.
C3, 10 nF
Across R7. Above about 1 kHz it lowers the plus input’s path to ground, so more treble reaches the OTA: the Dyna Comp’s bright edge.
C4, 1 µF electrolytic
Grounds R7 for signal only, so the plus input keeps its DC bias.
U1, LM13700 (one half)
One half of an LM13700. Its output is a current, the input voltage times a gain set by the current into its bias pin.
R8, 150 kΩ
The OTA’s load, to Vr. Its current times 150 kΩ is the signal, so it sets the gain with the bias current.
C5, 1 nF
Across R8. With 150 kΩ it rolls the treble off above about 1 kHz, undoing most of C3’s lift.
Q2, MPSA18
Phase splitter. Its emitter drives the output and Q4, its collector Q3, turned over, so the envelope catches both halves of the wave.
R9, 10 kΩ
Q2’s collector load.
R10, 10 kΩ
Q2’s emitter load, equal to R9, so the two sides swing the same.
C6, 10 nF
From Q2’s collector to Q3’s base.
R11, 1 MΩ
Q3’s base to ground. With no bias, Q3 stays off until a peak comes through.
D1, 1N914 / 1N4148
Catches the negative half at Q3’s base, so C6 charges up and Q3 turns on near each peak.
Q3, MPSA18
Pulls the envelope cap down on each peak from Q2’s collector.
C7, 10 nF
From Q2’s emitter to Q4’s base.
R12, 1 MΩ
Q4’s base to ground.
D2, 1N914 / 1N4148
Catches the negative half at Q4’s base, as D1 does for Q3.
Q4, MPSA18
Pulls the envelope cap down on the other half of the wave.
R13, 75 kΩ
First part of the envelope cap’s charging path, from 9 V.
RATK, 300 kΩ
Second part of the charging path, with Attack across it.
Attack, 100 kΩ pot, linear taper
Attack, a 100 kΩ linear pot across RATK. Its wiper drives Q5, so it sets how far above the envelope the follower sits. Which way it turns isn’t on the drawing.
C8, 10 µF electrolytic
The envelope cap. Played hard, Q3 and Q4 pull it down fast (the attack). It climbs back through R13 slowly (the release).
Q5, MPSA18
Follows the Attack wiper and drives the bias current.
Sensitivity, 470 kΩ pot, reverse audio taper
Sensitivity, 470 kΩ reverse audio, a variable resistor in the bias current’s path. Turned up, more current flows and quiet notes come up further.
R14, 15 kΩ
In series with the bias pin. It limits the bias current with Sensitivity at full.
C9, 47 nF
Output coupling cap.
R15, 10 kΩ
In series with the output pot. It sets the 10 kΩ output impedance shown on the drawing.
Volume, 47 kΩ pot, audio taper
Volume, 47 kΩ audio.

About this version

LM13700, MPSA18
As the kit drawing has them: one half of an LM13700 and MPSA18s.

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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