Seven-band graphic EQ, 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 GE-7 Equalizer.

Seven sliders from 100 Hz to 6.4 kHz, each lifting or cutting its band by up to about 15 dB, and a Level slider. Six of the bands are op-amp gyrators standing in for coils.

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. Two op-amp stages carry the sliders. Each slider is a 10 kΩ pot from a stage’s plus input to its minus input, and its wiper carries a leg tuned to one band. At the middle the leg pulls on both sides the same, and the stage passes everything at a gain of one. Toward the top it pulls on the feedback and that band comes up, about 15 dB at the end. Toward the bottom it pulls on the signal coming in and the band goes down the same amount. The 6.4 kHz leg is a cap and a resistor, so it lifts everything from about 4 kHz up and peaks near 13 kHz. The input stage lifts the treble and the network at the end cuts it back the same, so the hiss from the stages between is cut too. With every slider at the middle the pedal measures flat within about 0.6 dB. Not measured on a real pedal.

Download.

Parts and what they do

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

C17, 100 µF electrolytic
Across the 9 V supply.
R20, 3.3 kΩ
Top of the Vcc/2 divider.
R23, 3.3 kΩ
Bottom of the Vcc/2 divider. Every op-amp works around the 4.5 V between them.
C18, 47 µF electrolytic
Makes Vcc/2 a quiet AC ground for the gyrators and the two band stages.
R30, 10 kΩ
Input series resistor.
C22, 47 nF
Input coupling cap. Against R29 its corner is near 7 Hz.
R29, 470 kΩ
Biases the input stage at Vcc/2 and sets the pedal’s input impedance, about 470 kΩ.
U5a, NE5532
Input stage, an HA12017 low-noise op-amp on the drawing. An NE5532 stands in, since the bench has no HA12017. Its gain is 1 in the bass and rises in the treble, so the hiss the band stages add later gets cut back with the treble at the end.
R31, 4.7 kΩ
Feedback. With R28 it sets the most treble lift, 1 + 4.7 k / 470 = 11 times, reached far above the guitar’s range.
R28, 470 Ω
With C20, the leg that sets where the treble lift starts.
C20, 15 nF
Blocks the leg in the bass, so the stage is a follower there. Against R31 it starts the lift near 2.3 kHz.
R27, 10 kΩ
Into the Level stage’s plus input.
U4b, JRC4558D
The Level stage. It’s wired like a band stage with the Level slider as its only band, and that band reaches every pitch.
R26, 10 kΩ
Feedback, equal to R27, so with Level at the middle the stage has a gain of one.
Level, 10 kΩ pot, linear taper
Level, 10 kΩ linear, from the plus input to the minus input. Its wiper goes to ground through 2.2 kΩ and 10 µF. Toward the minus input it lifts everything, toward the plus input it cuts everything.
R2, 2.2 kΩ
Sets how far Level reaches either way.
C16, 10 µF electrolytic
Keeps DC off the Level leg. Its corner with 2.2 kΩ is near 7 Hz, so Level acts on every note.
R24, 3.3 kΩ
Into the band stage’s plus input. The seven sliders hang from this side and the minus side.
U4a, JRC4558D
The band stage. With every slider at the middle it has a gain of one. A slider moved toward the minus side puts its band’s leg in the feedback and lifts that band. Toward the plus side it puts the leg on the signal coming in and cuts it.
R25, 3.3 kΩ
Feedback, equal to R24.
100 Hz, 10 kΩ pot, linear taper
The 100 Hz slider, 10 kΩ linear, from the output stage’s plus input to its minus input. Its wiper carries the band’s tuned leg. Toward the minus input the leg pulls on the feedback and the band comes up. Toward the plus input it pulls on the signal coming in and the band goes down. At the middle the two cancel.
C10, 1.5 µF
In series with the gyrator, which acts as a coil of about 1.85 H. The cap and the coil pass most at about 96 Hz, the band’s center.
C13, 56 nF
The gyrator’s cap. With R16 and the 330 Ω it sets the coil it acts like: L = 330 × 100 k × 56 nF.
R16, 100 kΩ
From the follower’s plus input to Vcc/2. Above the band the cap passes, the follower copies N, and almost no current flows through the 330 Ω. Below it the follower holds still and the 330 Ω pulls N to Vcc/2. That current lagging the voltage is what makes it act like a coil.
R17, 330 Ω
The gyrator’s series resistor, 330 Ω. It sets the coil’s loss, so with the 10 kΩ slider it sets how narrow the band is.
U3b, TL022
A TL022 follower, half of a low-power dual. It copies its plus input to the 330 Ω.
R47, 470 kΩ
From the follower’s output to ground. It keeps a little current in the TL022’s output stage.
200 Hz, 10 kΩ pot, linear taper
The 200 Hz slider, wired as the 100 Hz one.
C9, 680 nF
In series with this band’s gyrator (about 0.89 H). Together they pass most near 204 Hz.
C8, 33 nF
This gyrator’s cap.
R9, 82 kΩ
This gyrator’s resistor to Vcc/2.
R8, 330 Ω
This gyrator’s 330 Ω.
U3a, TL022
This gyrator’s follower.
R48, 470 kΩ
Load on this follower.
400 Hz, 10 kΩ pot, linear taper
The 400 Hz slider, wired as the 100 Hz one.
C7, 330 nF
In series with this band’s gyrator (about 0.49 H). Together they pass most near 394 Hz.
C14, 15 nF
This gyrator’s cap.
R18, 100 kΩ
This gyrator’s resistor to Vcc/2.
R19, 330 Ω
This gyrator’s 330 Ω.
U2b, TL022
This gyrator’s follower.
R49, 470 kΩ
Load on this follower.
800 Hz, 10 kΩ pot, linear taper
The 800 Hz slider, wired as the 100 Hz one.
C5, 150 nF
In series with this band’s gyrator (about 0.27 H). Together they pass most near 790 Hz.
C6, 8.2 nF
This gyrator’s cap.
R7, 100 kΩ
This gyrator’s resistor to Vcc/2.
R6, 330 Ω
This gyrator’s 330 Ω.
U2a, TL022
This gyrator’s follower.
R50, 470 kΩ
Load on this follower.
1.6 kHz, 10 kΩ pot, linear taper
The 1.6 kHz slider, wired as the 100 Hz one.
C4, 100 nF
In series with this band’s gyrator (about 0.11 H). Together they pass most near 1.5 kHz.
C15, 3.9 nF
This gyrator’s cap.
R21, 82 kΩ
This gyrator’s resistor to Vcc/2.
R22, 330 Ω
This gyrator’s 330 Ω.
U1b, TL022
This gyrator’s follower.
R51, 470 kΩ
Load on this follower.
3.2 kHz, 10 kΩ pot, linear taper
The 3.2 kHz slider, wired as the 100 Hz one.
C2, 39 nF
In series with this band’s gyrator (about 60 mH). Together they pass most near 3.3 kHz.
C3, 2.2 nF
This gyrator’s cap.
R5, 82 kΩ
This gyrator’s resistor to Vcc/2.
R4, 330 Ω
This gyrator’s 330 Ω.
U1a, TL022
This gyrator’s follower.
R52, 470 kΩ
Load on this follower.
6.4 kHz, 10 kΩ pot, linear taper
The 6.4 kHz slider, wired as the 100 Hz one.
C1, 47 nF
The 6.4 kHz leg is this cap and R3 alone, no gyrator. With 820 Ω it passes from about 4 kHz up, so this band is closer to a treble shelf than a peak.
R3, 820 Ω
Sets how far the 6.4 kHz band reaches, with C1.
C19, 1 µF electrolytic
Coupling cap after the band stage.
R32, 1 MΩ
Holds the line after C19 at Vcc/2 for the bypass switch, left out here.
R33, 4.7 kΩ
De-emphasis. With R36 and C27 it cuts back the treble U5a lifted, and the hiss from the stages between goes with it.
C27, 15 nF
With R33, starts the treble cut near 2.3 kHz, the same corner as the lift.
R36, 470 Ω
Stops the treble cut where the lift stopped, so the two cancel.
C26, 47 nF
Into the output follower.
R34, 1 MΩ
Biases the output follower at Vcc/2.
Q1, 2SC1815
Output follower, a 2SC732 on the drawing. A 2SC1815 stands in. It drives the cable from a low impedance.
R15, 10 kΩ
The follower’s emitter load.
R14, 1 kΩ
Keeps a long cable’s capacitance off the emitter.
C11, 1 µF electrolytic
Output coupling cap.
R12, 100 kΩ
Output pulldown, so the jack sits at 0 V.

About this version

Stock
As on the redraw. TL022 gyrators, a JRC4558 for the Level and band stages, an NE5532 standing in for the HA12017 input stage and a 2SC1815 for the 2SC732 output follower.

Sources

  • Jon Halverson, redrawn GE-7 schematic, 2000

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.

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