Seven-band graphic EQ on one op-amp stage, 1983 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 GE9 Graphic Equalizer.

A transistor buffer and one op-amp stage with seven sliders and a Level slider across its inputs, six bands from 63 Hz to 5 kHz on op-amp gyrators, then a transistor output buffer.

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. Every slider is a 20 kΩ pot from IC1’s plus input to its minus input, and its wiper carries a leg tuned to one band: a cap in series with an op-amp gyrator standing in for a coil. At the middle a leg pulls on both inputs the same and the stage passes everything at a gain of about one. Toward the minus input it lifts its band, toward the plus input it cuts it. Level has no gyrator, so it moves every pitch. The gyrators sit on their own 4.5 V reference. The model leaves out the electronic bypass.

Download.

Parts and what they do

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

R42, 10 kΩ
Top of the 4.5 V reference divider.
R43, 10 kΩ
Bottom of the 4.5 V reference divider.
C30, 47 µF electrolytic
Filters the 4.5 V reference.
R12, 10 kΩ
Top of the band reference divider.
R13, 10 kΩ
Bottom of the band reference divider.
C8, 10 µF electrolytic
Makes the band reference a quiet AC ground for the gyrators and Level.
C1, 100 nF
Input coupling cap.
R1, 1 kΩ
Input series resistor.
R44, 510 kΩ
Biases Q1 from its divider and sets most of the input impedance.
R47, 9.1 kΩ
Top of Q1’s bias divider.
R46, 22 kΩ
Bottom of Q1’s bias divider, about 6.4 V.
C32, 10 µF electrolytic
Filters Q1’s bias.
Q1, 2SC1815
Input buffer, an emitter follower.
R2, 8.2 kΩ
Q1’s emitter resistor. The drawing reads 8 k.
C2, 10 µF electrolytic
Coupling cap into the EQ stage.
R3, 100 kΩ
Holds the EQ stage’s input at 4.5 V.
R4, 3 kΩ
Into IC1’s plus input. The sliders pull against it.
IC1A, JRC4558D
The EQ stage. With every slider at the middle its gain is about one.
R5, 3 kΩ
Feedback, equal to R4.
C3, 100 pF
Across R5, against oscillation.
63 Hz, 20 kΩ pot, linear taper
The 63 Hz slider, 20 kΩ, from IC1’s plus input to its minus input. Its wiper carries the band’s tuned leg. Toward the minus input the leg lifts the band, toward the plus input it cuts it.
C10, 4.7 µF
In series with the gyrator, which acts as a coil of about 1.31 H. The two pass most near 64 Hz, the band’s center.
C9, 33 nF
The gyrator’s cap. With R14 and R15 it sets the coil: L = 330 × 120 k × 33 nF.
R14, 120 kΩ
From the follower’s plus input to the band reference.
R15, 330 Ω
The gyrator’s series resistor, 330 Ω. With the 20 kΩ slider it sets how narrow the band is.
IC2A, JRC4558D
Half of an NJM4558D as a follower.
125 Hz, 20 kΩ pot, linear taper
The 125 Hz slider.
C12, 2.2 µF
In series with this band’s gyrator, near 127 Hz.
C11, 18 nF
This gyrator’s cap.
R16, 120 kΩ
This gyrator’s 120 kΩ.
R17, 330 Ω
This gyrator’s 330 Ω.
IC2B, JRC4558D
This gyrator’s follower.
250 Hz, 20 kΩ pot, linear taper
The 250 Hz slider.
C14, 1.5 µF
In series with this band’s gyrator, near 250 Hz.
C13, 6.8 nF
This gyrator’s cap.
R18, 120 kΩ
This gyrator’s 120 kΩ.
R19, 330 Ω
This gyrator’s 330 Ω.
IC3A, JRC4558D
This gyrator’s follower.
500 Hz, 20 kΩ pot, linear taper
The 500 Hz slider.
C16, 680 nF
In series with this band’s gyrator, near 491 Hz.
C15, 3.9 nF
This gyrator’s cap.
R20, 120 kΩ
This gyrator’s 120 kΩ.
R21, 330 Ω
This gyrator’s 330 Ω.
IC3B, JRC4558D
This gyrator’s follower.
1 kHz, 20 kΩ pot, linear taper
The 1 kHz slider.
C18, 330 nF
In series with this band’s gyrator, near 1038 Hz.
C17, 1.8 nF
This gyrator’s cap.
R22, 120 kΩ
This gyrator’s 120 kΩ.
R23, 330 Ω
This gyrator’s 330 Ω.
IC1B, JRC4558D
This gyrator’s follower.
2.2 kHz, 20 kΩ pot, linear taper
The 2.2 kHz slider.
C20, 150 nF
In series with this band’s gyrator, near 2280 Hz.
C19, 820 pF
This gyrator’s cap.
R24, 120 kΩ
This gyrator’s 120 kΩ.
R25, 330 Ω
This gyrator’s 330 Ω.
IC4A, JRC4558D
This gyrator’s follower.
5 kHz, 20 kΩ pot, linear taper
The 5 kHz slider.
C22, 68 nF
In series with this band’s gyrator, near 4911 Hz.
C21, 390 pF
This gyrator’s cap.
R26, 120 kΩ
This gyrator’s 120 kΩ.
R27, 330 Ω
This gyrator’s 330 Ω.
IC4B, JRC4558D
This gyrator’s follower.
Level, 20 kΩ pot, linear taper
Level, a 20 kΩ slider between IC1’s inputs like the bands, its wiper to the band reference through R28 and C23 with no gyrator, so it lifts or cuts every pitch.
R28, 470 Ω
Sets how far Level reaches either way.
C23, 22 µF
Keeps DC off the Level leg, a 22 µF non-polar.
C4, 100 nF
Coupling cap after the EQ stage.
R11, 510 kΩ
Holds the effect switch at 4.5 V. The switch, closed with the effect on, is a wire here.
R10, 510 kΩ
The switch’s other side, also to 4.5 V.
C7, 100 nF
Coupling cap into the output buffer.
R9, 510 kΩ
Biases the output buffer at 4.5 V.
Q4, 2SC1815
Output buffer, an emitter follower.
R8, 10 kΩ
Output buffer emitter resistor.
R7, 470 Ω
Output series resistor.
C6, 10 µF electrolytic
Output coupling cap.
R45, 100 kΩ
Output pulldown.

About this version

GE9
As on the factory drawing. NJM4558D op-amps, 2SC2458GR transistors (the 2SC1815 stands in).

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.

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