Four-stage JFET phaser, 1974 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 Phase 90 (script logo).

Four op-amp stages each turn the guitar’s phase slightly more as the pitch goes up. Mixed back with the dry signal, the two cancel at two notches. A slow oscillator moves four JFETs, and the notches sweep up and down.

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. Each stage passes every pitch at the same level and only turns its phase, a little for low notes and nearly half a turn for high ones. The wet and dry signals meet at the mixer at equal levels. Where the four stages have turned the wet signal 180°, and again at 540°, the two cancel: those are the notches. Where they line up, the output is about 5 dB above the guitar going in. The LFO swings the JFET gates about 0.3 V across their pinch-off. At the middle of Speed the lower notch sweeps from about 110 Hz to 960 Hz, 1.2 times a second. Speed runs from one sweep every 7.5 seconds to 9.7 a second, and at the fast end C4 smooths the gates, so the sweep narrows. The zener holds Vref at 5.1 V for everything. The JFETs are a 2N5952 typical of the data sheet, with the trimmer set to suit. Real ones spread widely, so a real pedal’s trimmer sits somewhere else. Not measured on a real pedal.

Download.

Parts and what they do

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

R15, 10 kΩ
From the battery to the zener. About 0.4 mA flows through it.
D2, 5.1 V zener, in breakdown
A 5.1 V zener. It holds Vref steady as the battery runs down, and everything in the pedal is biased from it. The JFETs’ sources sit on Vref, so it also sets where their gates have to be.
D1, 1N914 / 1N4148
Reverse-polarity protection, doing nothing in normal use. The traced drawing has it the other way round, which would hold Vref at 0.6 V, so it’s read as the usual protection diode.
C8, 15 µF electrolytic
Vref’s filter cap. It makes Vref a quiet AC ground for the four stages and the mixer.
Trimmer, 250 kΩ pot, linear taper
The internal bias trimmer. It sets how far below Vref the JFET gates sit, which places the sweep. The factory set it for each pedal’s JFETs. On the bench it’s set for the data sheet’s typical 2N5952, so the gates swing across pinch-off.
R20, 1 MΩ
From the trimmer to the gate line. With R22 it divides the LFO’s ramp, so about a fifth reaches the gates.
C4, 47 nF
Smooths the gate line. With 1 MΩ and 3.9 MΩ it sets a corner near 4 Hz, so the sweep gets shallower as the speed goes up.
R3, 10 kΩ
Input series resistor.
C5, 10 nF
Input coupling cap. Against R14 it sets a corner near 34 Hz.
R14, 470 kΩ
Biases the input buffer at Vref, and sets most of the pedal’s input impedance.
U1a, TL061 (low power)
Input buffer, a follower. It drives the four stages and the dry half of the mix.
R1, 10 kΩ
Into the minus input. With the 10 kΩ feedback resistor it makes the inverting half of the stage, a gain of one, turned over.
R5, 10 kΩ
Feedback. Equal to the input resistor, so every frequency comes out the same level, and only the phase moves.
C1, 47 nF
Feeds the plus input. Against the 24 kΩ and the JFET beside it, it sets where this stage turns the phase 90°. Low notes reach the plus input weakly and come out turned over. High notes get through and come out as they went in.
R6, 24 kΩ
From the plus input to Vref. With the JFET pinched off it’s all there is, and the stage turns 90° at about 140 Hz. The JFET beside it pulls that frequency up.
Q2, 2N5952
Used as a resistor that the LFO varies. Its source sits on Vref and its gate on the Vbias line. The closer the gate comes to its source, the lower its resistance and the higher this stage’s 90° point. Past pinch-off it’s out of the circuit and the 24 kΩ is left.
U2A, TL061 (low power)
The first phase-shift stage. Half the signal goes straight, half through the filter, and the two meet here.
R11, 10 kΩ
Into the minus input, as in the first stage.
R10, 10 kΩ
Feedback, equal to the input resistor.
C3, 47 nF
Feeds the plus input, as in the first stage. All four caps are the same, so the four stages turn together.
R25, 24 kΩ
From the plus input to Vref, beside this stage’s JFET.
Q3, 2N5952
This stage’s JFET, on the same gate line as the others.
U2D, TL061 (low power)
The second phase-shift stage.
R12, 10 kΩ
Into the minus input, as in the first stage.
R13, 10 kΩ
Feedback, equal to the input resistor.
C6, 47 nF
Feeds the plus input, as in the first stage. All four caps are the same, so the four stages turn together.
R26, 24 kΩ
From the plus input to Vref, beside this stage’s JFET.
Q4, 2N5952
This stage’s JFET, on the same gate line as the others.
U2C, TL061 (low power)
The third phase-shift stage.
R17, 10 kΩ
Into the minus input, as in the first stage.
R18, 10 kΩ
Feedback, equal to the input resistor.
C9, 47 nF
Feeds the plus input, as in the first stage. All four caps are the same, so the four stages turn together.
R23, 24 kΩ
From the plus input to Vref, beside this stage’s JFET.
Q5, 2N5952
This stage’s JFET, on the same gate line as the others.
U2B, TL061 (low power)
The fourth phase-shift stage. Its output is the wet signal.
R16, 150 kΩ
The dry signal into the mixer.
R8, 150 kΩ
The wet signal into the mixer, equal to the dry one. Where the four stages have turned the phase 180°, or 540°, the two cancel, making the notches.
Q1, 2N4125
The mixer, a PNP stage with its emitter on Vref. R7 from collector to base makes the base a near-still point where the two signals add, each passing at a gain of about one, turned over.
R7, 150 kΩ
Feedback from collector to base. Equal to R16 and R8, so the dry and wet halves each come out at about the level they went in.
R4, 56 kΩ
The mixer’s collector load.
C2, 47 nF
Output coupling cap.
R2, 150 kΩ
Output pulldown, so the jack sits at 0 V.
R19, 150 kΩ
From Vref to the LFO op-amp’s plus input. With R21 it sets the two switch points either side of Vref.
R21, 470 kΩ
Positive feedback from the output. It snaps the op-amp to one side and holds it there until the cap reaches the switch point.
U1b, TL061 (low power)
The LFO. Its output is a square wave, high or low.
R24, 150 kΩ
From the timing cap to the minus input.
C7, 10 nF
From the minus input to the output. With R24 it makes each flip slightly late, so at top speed the cap swings past the switch points.
Speed, 470 kΩ pot, reverse audio taper
Speed, 470 kΩ, a variable resistor from the LFO’s output to the timing cap: all of it at 0, the slowest, none at full. Reverse audio is assumed, since the taper isn’t marked.
end, 4.7 kΩ
What’s left of the Speed pot at full turn, not a part on the drawing. 4.7 kΩ is about 1 % of the track, common for a carbon pot, not measured. It sets the top speed.
C10, 15 µF electrolytic
The timing cap. It ramps up and down between the switch points, and that ramp is the sweep.
R22, 3.9 MΩ
Carries the ramp from the timing cap to the JFET gates.

About this version

Script logo, TL061
The early script-logo circuit, as traced. TL061 op-amps, 2N5952 JFETs. The JFETs and the trimmer setting are typical of the part, not measured.

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, not measured parts. Caps are ideal, and the battery or HT is a voltage behind a resistance.

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