Six-band graphic EQ, 1975 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 Six Band Graphic EQ.

Six sliders from 100 Hz to 3.2 kHz on one op-amp stage, each lifting or cutting its band by up to about 18 dB. Five 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. One op-amp stage carries all six sliders. Each is a 50 kΩ pot from the 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 17 to 19 dB at the end. Toward the bottom the band goes down the same amount. The 3.2 kHz leg is a cap and a resistor, so it lifts everything from about 2 kHz up. The input stage doubles the signal and lifts the treble more. The output divider halves it again and C4 cuts the treble back, so with every slider at the middle the pedal measures flat within about 0.5 dB. Vref sits at 5.3 V, above half the supply, so big lifts clip the band stage on the tops first. Not measured on a real pedal.

Download.

Parts and what they do

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

C100, 100 µF electrolytic
Across the 9 V supply.
R101, 330 kΩ
Top of the Vref divider. With 470 kΩ under it Vref sits at about 5.3 V, above half the supply.
R102, 470 kΩ
Bottom of the Vref divider.
C15, 100 nF
Filters the divider. Against about 194 kΩ its corner is near 8 Hz.
IC1.1, TL072
A follower that drives Vref, so the five gyrators and the stages can pull current from it without moving it.
C1, 100 nF
Input coupling cap. Against R2 its corner is near 3 Hz.
R1, 1 kΩ
Input series resistor.
R2, 470 kΩ
Biases the input stage at Vref and sets the input impedance, 470 kΩ as MXR lists it.
IC1.2, TL072
Input stage. Its gain is 2 in the bass and rises in the treble, so the output network can cut the treble back with the hiss from the band stage.
R3, 10 kΩ
Feedback. With R4 it sets the gain in the bass, 1 + 10 k / 10 k = 2.
R4, 10 kΩ
From the minus input to Vref.
R5, 1 kΩ
With C2, a second leg beside R4 that takes over in the treble. It stops the lift at 1 + 10 k / 909 Ω, about 12 times.
C2, 10 nF
Blocks R5’s leg in the bass. The lift starts near 1.5 kHz.
R6, 10 kΩ
Into the band stage’s plus input. The six sliders hang from this line and the minus input’s.
IC1.3, TL072
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.
R7, 10 kΩ
Feedback, equal to R6.
100 Hz, 50 kΩ pot, linear taper
The 100 Hz slider, 50 kΩ linear, from the band 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.
C6, 1 µF
In series with the gyrator, which acts as a coil of about 2.48 H. The cap and the coil pass most at about 101 Hz, the band’s center.
C5, 220 nF
The gyrator’s cap. With R11 and the 470 Ω it sets the coil it acts like: L = 470 × 24 k × 220 nF.
R11, 24 kΩ
From the follower’s plus input to Vref. Above the band the cap passes, the follower copies N, and almost no current flows through the 470 Ω. Below it the follower holds still and the 470 Ω pulls N to Vref. That current lagging the voltage is what makes it act like a coil.
R10, 470 Ω
The gyrator’s series resistor, 470 Ω. It sets the coil’s loss, so with the 50 kΩ slider it sets how narrow the band is.
IC2.4, TL072
A follower, a quarter of a TL074 (the TL072 stands in, the same op-amp). It copies its plus input to the 470 Ω.
200 Hz, 50 kΩ pot, linear taper
The 200 Hz slider, wired as the 100 Hz one.
C8, 470 nF
In series with this band’s gyrator (about 1.35 H). Together they pass most near 200 Hz.
C7, 120 nF
This gyrator’s cap.
R13, 24 kΩ
This gyrator’s resistor to Vref.
R12, 470 Ω
This gyrator’s 470 Ω.
IC2.3, TL072
This gyrator’s follower.
400 Hz, 50 kΩ pot, linear taper
The 400 Hz slider, wired as the 100 Hz one.
C10, 220 nF
In series with this band’s gyrator (about 703 mH). Together they pass most near 405 Hz.
C9, 68 nF
This gyrator’s cap.
R15, 22 kΩ
This gyrator’s resistor to Vref.
R14, 470 Ω
This gyrator’s 470 Ω.
IC1.4, TL072
This gyrator’s follower.
800 Hz, 50 kΩ pot, linear taper
The 800 Hz slider, wired as the 100 Hz one.
C12, 120 nF
In series with this band’s gyrator (about 341 mH). Together they pass most near 787 Hz.
C11, 33 nF
This gyrator’s cap.
R17, 22 kΩ
This gyrator’s resistor to Vref.
R16, 470 Ω
This gyrator’s 470 Ω.
IC2.2, TL072
This gyrator’s follower.
1.6 kHz, 50 kΩ pot, linear taper
The 1.6 kHz slider, wired as the 100 Hz one.
C14, 68 nF
In series with this band’s gyrator (about 141 mH). Together they pass most near 1.6 kHz.
C13, 15 nF
This gyrator’s cap.
R19, 20 kΩ
This gyrator’s resistor to Vref.
R18, 470 Ω
This gyrator’s 470 Ω.
IC2.1, TL072
This gyrator’s follower.
3.2 kHz, 50 kΩ pot, linear taper
The 3.2 kHz slider, wired as the 100 Hz one.
C16, 68 nF
The 3.2 kHz leg is this cap and R20 alone, no gyrator. With 1.2 kΩ it passes from about 2 kHz up, so this band acts more like a treble shelf than a peak.
R20, 1.2 kΩ
Sets how far the 3.2 kHz band reaches, with C16.
C3, 1 µF
Output coupling cap.
R8, 10 kΩ
Output resistor. With R9 it halves the level and sets the 5 kΩ output impedance MXR lists.
R9, 10 kΩ
Bottom of the output divider, and the jack’s path to 0 V.
C4, 10 nF
Cuts the treble back against the 5 kΩ, from about 3.2 kHz, undoing most of the input stage’s lift.

About this version

Stock
As on the kit drawing. TL074 op-amps, with the TL072 standing 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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