Four-stage JFET phaser with resonance, 1980 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 PH-1R Phaser.

Four op-amp stages turn the guitar’s phase further as the pitch goes up. Mixed back with the dry signal, they cancel at two notches that a slow oscillator sweeps. Resonance feeds some of the wet signal back to the start.

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. Four stages reach 180° at one pitch and 540° at a higher one, and there the wet signal cancels the dry: two notches. IC1A turns the guitar over, R8 and R9 halve it into the stages, and the mixer brings the wet half back up 2.1 times. The LFO’s timing cap ramps up and down between two switch points. IC4B passes that ramp to Depth, and from there it reaches the gates through 2.2 MΩ, while the trimmer sets where they sit. The JFETs’ resistance changes over only about 0.2 V of gate voltage, so the trimmer matters: at the default settings the lower notch sweeps from about 80 Hz to 3 kHz. Resonance feeds the wet signal back in, which raises the peaks between the notches. The model leaves out the spread between real JFETs, which were matched for each pedal.

Download.

Parts and what they do

Stock values for the Stock, JRC4558D version. Tap a part on the schematic to change it.

R41, 1.8 kΩ
From the supply to the zener.
Z1, 5.1 V zener, in breakdown
A 5.1 V zener, 1N5231B. It holds Vb, the stages’ reference, steady as the battery runs down.
C20, 47 µF electrolytic
Vb’s filter cap. It makes Vb a quiet AC ground for the stages.
R1, 10 kΩ
Input series resistor.
C1, 47 nF
Input coupling cap.
R2, 470 kΩ
Biases Q1 from Vb and sets most of the input impedance.
Q1, 2N5088
Input follower. No voltage gain.
R3, 10 kΩ
Q1’s emitter load.
C2, 1 µF
Coupling cap into IC1A.
R4, 47 kΩ
Into IC1A’s minus input. Equal to R5, so IC1A turns the guitar over at a gain of one.
IC1A, JRC4558D
Inverts the guitar and adds in the Resonance feedback, which comes in on its plus input. Its output is the dry signal.
R5, 47 kΩ
IC1A’s feedback resistor.
C3, 47 pF
Across R5. It trims the very top.
R6, 47 kΩ
Holds IC1A’s plus input at Vb.
C4, 100 pF
Across R6.
C5, 33 nF
Carries the Resonance feedback into IC1A’s plus input. Against R7 and R6 it cuts the lows of the feedback.
R7, 12 kΩ
In series with the Resonance feedback.
Resonance, 10 kΩ pot, linear taper
Resonance, 10 kΩ linear. It picks off part of the wet signal and feeds it back to the start, which deepens the peaks between the notches.
C15, 1 µF
Carries the wet signal to the top of the Resonance pot.
R8, 1.5 kΩ
With R9 it halves the dry signal on its way into the stages.
R9, 1.5 kΩ
Bottom of the halving divider, to Vb.
R10, 10 kΩ
Into the minus input. With the 10 kΩ feedback it makes the inverting half of the stage, a gain of one, turned over.
R11, 10 kΩ
Feedback, equal to the input resistor, so every pitch comes out at the same level and only the phase moves.
C6, 10 nF
Feeds the plus input. Against the 100 kΩ and the JFET beside it, it sets where this stage turns the phase 90°.
R12, 100 kΩ
From the plus input to Vb. With the JFET pinched off it’s all there is, and the stage turns 90° at about 160 Hz.
R13, 330 kΩ
With the 10 nF, takes the drain’s swing toward the gate. With the 330 kΩ from the gate to Vc it puts half the swing on the gate, which keeps the JFET clean at higher levels.
C7, 10 nF
Keeps the drain’s DC off the gate, so only the swing goes there.
R14, 330 kΩ
Carries the bias line Vc to the gate.
Q2, 2SK30A-GR
Used as a resistor that the LFO varies. Its source is on Vb. The closer the gate comes to its source, the lower its resistance and the higher this stage’s 90° point.
IC2A, JRC4558D
The first phase-shift stage.
R15, 10 kΩ
Into the minus input, as in the first stage.
R16, 10 kΩ
Feedback, equal to the input resistor.
C8, 10 nF
Feeds the plus input, the same as the first stage’s, so all four stages turn together.
R17, 100 kΩ
From this stage’s plus input to Vb.
R18, 330 kΩ
Half the drain swing toward this gate, as in the first stage.
C9, 10 nF
Keeps the drain’s DC off this gate.
R19, 330 kΩ
Bias line to this gate.
Q3, 2SK30A-GR
This stage’s JFET, on the same bias line.
IC2B, JRC4558D
Another phase-shift stage.
R20, 10 kΩ
Into the minus input, as in the first stage.
R21, 10 kΩ
Feedback, equal to the input resistor.
C10, 10 nF
Feeds the plus input, the same as the first stage’s, so all four stages turn together.
R22, 100 kΩ
From this stage’s plus input to Vb.
R23, 330 kΩ
Half the drain swing toward this gate, as in the first stage.
C11, 10 nF
Keeps the drain’s DC off this gate.
R24, 330 kΩ
Bias line to this gate.
Q4, 2SK30A-GR
This stage’s JFET, on the same bias line.
IC3A, JRC4558D
Another phase-shift stage.
R25, 10 kΩ
Into the minus input, as in the first stage.
R26, 10 kΩ
Feedback, equal to the input resistor.
C12, 10 nF
Feeds the plus input, the same as the first stage’s, so all four stages turn together.
R27, 100 kΩ
From this stage’s plus input to Vb.
R28, 330 kΩ
Half the drain swing toward this gate, as in the first stage.
C13, 10 nF
Keeps the drain’s DC off this gate.
R29, 330 kΩ
Bias line to this gate.
Q5, 2SK30A-GR
This stage’s JFET, on the same bias line.
IC3B, JRC4558D
The fourth phase-shift stage. Its output is the wet signal.
C14, 1 µF
Couples the wet signal to the mixer.
R30, 22 kΩ
The wet signal into the mixer. Against R32 its gain is about 2.1, which makes up for the halving ahead of the stages.
R31, 47 kΩ
The dry signal into the mixer, at a gain of one. Where the stages have turned the phase 180°, the wet cancels it.
IC1B, JRC4558D
The mixer, an inverting summing stage.
R32, 47 kΩ
The mixer’s feedback resistor.
C16, 47 pF
Across R32.
R33, 470 Ω
In series at the output.
C17, 1 µF
Output coupling cap.
R42, 47 kΩ
From Vb to the top of the Bias trimmer.
Bias, 100 kΩ pot, linear taper
The internal Bias trimmer, 100 kΩ. It sets where the JFET gates sit, which places the sweep. The factory set each unit. The setting here suits this file’s JFETs.
D2, 1N914 / 1N4148
From the bottom of the trimmer to ground. It lifts the trimmer’s range about 0.6 V and tracks the JFETs a little with temperature.
R43, 1 MΩ
From the trimmer to the bias line.
C21, 100 nF
Smooths the bias line.
R38, 68 kΩ
From the LFO buffer to the Depth pot.
Depth, 100 kΩ pot, linear taper
Depth, 100 kΩ linear. Toward the LFO end more of the sweep reaches the gates, toward the Vb end less.
R39, 10 kΩ
From the Depth pot’s other end to Vb.
R40, 2.2 MΩ
From the Depth wiper to the bias line. With R43 it divides the sweep, so about a third reaches the gates.
R34, 150 kΩ
From Vb to the LFO’s plus input. With R35 it sets the two switch points.
R35, 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.
IC4A, TL022
The LFO. Its output is a square wave, high or low.
R37, 150 kΩ
From the timing cap to the minus input.
C18, 10 nF
From the minus input to the output.
R36, 4.7 kΩ
In series with Rate. It sets the top speed.
Rate, 1 MΩ pot, reverse audio taper
Rate, 1 MΩ reverse audio, a variable resistor from R36 to the timing cap: all of it at 0, the slowest, none at full.
C19, 15 µF electrolytic
The timing cap, 15 µF tantalum. It ramps up and down between the switch points, and that ramp is the sweep.
IC4B, TL022
A follower, so the Depth network doesn’t load the timing cap.

Versions

Stock, JRC4558D
JRC4558D op-amps, a TL022 for the LFO, 2SK30A-GR JFETs and a 5.1 V zener, as Aion lists them. The trimmer is set for a typical JFET.
Guyatone PS-007, modeled after the Guyatone PS-007 Phaser
G. Giblet’s drawing of the PS-007. The PH-1R’s circuit with 51 kΩ in place of 47 kΩ, a 50 kΩ Resonance pot, no input or output series resistor, a 50 kΩ Depth pot straight across the LFO, no diode under the bias trimmer, a 500 kΩ Speed pot and a 22 µF timing cap. The trimmer setting is typical.

Sources

  • G. Giblet, drawing of the Guyatone PS-007 Phaser, 2025 (the PS-007 build)

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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