OTA compressor with treble lift around the OTA, 1981 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 Boss CS-2 Compression Sustainer.

An OTA sets the gain, with the treble lifted going in and cut coming out. A phase splitter and two transistors pull the envelope cap down on each peak, and the OTA’s bias current follows the cap.

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. R15 against R8, C8 and C6 puts about 13 % of the signal across the OTA’s inputs in the mids and nearly all of it in the highs, and R10 with C5 after the OTA rolls the highs back off, so the envelope sees more of the pick attack. Q5 splits the OTA stage’s output into two opposite copies, and Q8 and Q9 each pull C1 down on their half of the wave. Q7 follows C1 and sets the OTA’s bias current through Sustain and R7: louder notes, less bias current, less gain. With Sustain at full, a steady A2 picked at 10 mV comes out at about 105 mV, and at 300 mV, 30 times louder, about 139 mV. Against the output table in the service notes (1 kHz and 100 Hz, Level at full) the model reads within 1.5 dB. The model leaves out the footswitch, the FET switches and the LED, so the effect is always on.

Download.

Parts and what they do

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

R19, 10 kΩ
Top of the bias divider.
R12, 3.9 kΩ
Middle of the bias divider, between VB1 and VB2.
R9, 8.2 kΩ
Bottom of the bias divider.
C9, 10 µF electrolytic
Filters VB1.
C7, 10 µF electrolytic
Filters VB2.
R13, 10 kΩ
Input series resistor.
C10, 22 nF
Input coupling cap.
R18, 1 MΩ
Biases Q1’s gate from VB1 and sets the input impedance near 1 MΩ.
Q1, 2SK30A-GR
JFET source follower, the high-impedance input.
R17, 10 kΩ
Q1’s source resistor.
Q4, 2SC1815
Emitter follower. It drives the OTA’s input network from a low impedance.
R16, 10 kΩ
Q4’s emitter resistor.
R14, 2.2 kΩ
Feeds the signal to the OTA’s minus input.
R15, 2.2 kΩ
Feeds the same signal to the OTA’s + input. Against R8 and C6 the + input follows the minus input closely in the mids, so only a small difference reaches the OTA there.
R8, 15 kΩ
From the + input toward ground through C6. In the mids the difference across the inputs is R15’s share, about 13 %.
C8, 10 nF
Across R8. Above about 1 kHz it shorts R8, the + input drops toward ground, and nearly the whole signal appears across the inputs: the treble lift.
C6, 1 µF electrolytic
Blocks DC, so both inputs sit at Q4’s emitter voltage.
IC1, CA3080
The BA662A’s OTA. Its output is a current, the voltage across its inputs times a gain that follows the current into its bias pin.
R11, 150 kΩ
The OTA’s load, to VB2. The output current through it is the signal.
IC1 buffer, 2SC1815
The BA662A’s buffer, first half.
IC1 buffer, 2SC1815
The buffer’s second half. Its emitter is the OTA stage’s output.
RB, 10 kΩ
The buffer’s emitter load, as the service notes wire it for an LM13600 in the BA662’s place.
R10, 15 kΩ
With C5 it rolls off above about 1 kHz, which takes back the lift at the OTA’s input.
C5, 10 nF
With R10, the treble cut after the OTA.
Q5, 2SC1815
Phase splitter. Its collector and emitter swing opposite ways, so the envelope sees both halves of the wave. The emitter also feeds Level.
R2, 10 kΩ
Q5’s collector load.
R4, 10 kΩ
Q5’s emitter load, equal to R2.
C2, 10 nF
From Q5’s collector to Q8’s base.
D3, 1N914 / 1N4148
Catches the negative half at Q8’s base, so Q8 turns on near each peak.
R5, 1 MΩ
Q8’s base to ground.
Q8, 2SC1815
Pulls the envelope cap down on each peak from Q5’s collector.
C3, 10 nF
From Q5’s emitter to Q9’s base.
D4, 1N914 / 1N4148
Catches the negative half at Q9’s base.
R6, 1 MΩ
Q9’s base to ground.
Q9, 2SC1815
Pulls the envelope cap down on the other half of the wave.
C1, 10 µF electrolytic
The envelope cap, a 10 µF tantalum.
R1, 10 kΩ
In series with Attack. With Attack at full it alone sets how fast C1 charges back up.
Attack, 150 kΩ pot, reverse audio taper
Attack, 150 kΩ reverse audio as a variable resistor from 9 V to R1. Turned up it takes resistance out, C1 charges back faster after each note, and the gain is back up for the next pick, so more of the pick comes through.
Q7, 2SC1815
Follows the envelope cap and drives the bias current.
Sustain, 1 MΩ pot, reverse audio taper
Sustain, 1 MΩ reverse audio as a variable resistor from Q7 to R7. Turned up it takes resistance out, so more bias current flows and the gain is higher until the envelope pulls it down.
R7, 27 kΩ
In series with the bias pin. With Sustain at full it limits the bias current.
C4, 47 nF
Coupling cap from Q5’s emitter to Level.
R3, 10 kΩ
In series ahead of Level.
Level, 50 kΩ pot, audio taper
Level, 50 kΩ audio. Its bottom goes to VB1, which is ground for the signal.
C20, 47 nF
Coupling cap to the output buffer.
R29, 1 MΩ
Holds the effect switch’s side of C20 at VB1.
R31, 1 MΩ
Biases Q6’s base from VB1.
Q6, 2SC1815
Output emitter follower.
R33, 10 kΩ
Q6’s emitter resistor.
C22, 1 µF electrolytic
Output coupling cap.
R36, 100 kΩ
Holds the output at ground for DC, so the jack doesn’t pop.
R37, 1 kΩ
Output series resistor.

About this version

Stock
Negative ground. The BA662A is the CA3080 preset here with two 2SC1815s for its buffer. The 2SC732s and 2SC1815s are the 2SC1815 preset, the 2SK30A-Y the 2SK30A-GR preset.

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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