Two-stage JFET phaser, 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 Phase 45.

Two op-amp stages turn the guitar’s phase further as the pitch goes up. Mixed back with the dry signal, the two cancel at one notch. A slow oscillator moves two JFETs, and the notch sweeps 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. Two stages reach 180° at one frequency, and there the wet half cancels the dry: one notch, against the four-stage pedal’s two. The input buffer lifts the guitar 1.5 times and the two 10 kΩ mix resistors halve it, so the dry signal comes out at about 0.75. The LFO swings the bias line about 0.4 V across the JFETs’ pinch-off. At the middle of Speed the notch sweeps from about 170 Hz to 2.3 kHz, 1.2 times a second, and at full speed 7.8 times a second. The zener holds Vref at 4.7 V. 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.

C13, 10 µF electrolytic
Across the 9 V supply.
R25, 10 kΩ
From the battery to the zener. About 0.4 mA flows through it.
Z1, 4.7 V zener, in breakdown
A 4.7 V zener, 1N5230 on the trace. It holds Vref steady as the battery runs down. The JFETs’ sources sit on Vref, so it also sets where their gates have to be.
C12, 10 µF electrolytic
Vref’s filter cap. It makes Vref a quiet AC ground for the stages.
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.
R18, 1 MΩ
From the trimmer to the bias line. With R19 it divides the LFO’s ramp, so about a fifth of it reaches the gates.
C9, 47 nF
Smooths the bias line. With 1 MΩ and 3.9 MΩ its corner is near 4 Hz, so the sweep gets shallower as the speed goes up.
C1, 10 nF
Input coupling cap. Against R2 its corner is near 34 Hz.
R1, 10 kΩ
Input series resistor.
R2, 470 kΩ
Biases the input buffer at Vref, and sets most of the pedal’s input impedance.
U1a, TL072
Input buffer with a gain of 1.5. It drives the two stages and the dry half of the mix.
R3, 10 kΩ
Feedback. With R4 it sets the buffer’s gain, 1 + 10 k / 20 k = 1.5, which makes up for the passive mix halving the signal.
R4, 20 kΩ
From the minus input to Vref, the bottom of the gain divider.
R5, 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.
R6, 10 kΩ
Feedback, equal to the input resistor, so every pitch comes out at the same level and only the phase moves.
C2, 47 nF
Feeds the plus input. Against the 20 kΩ and the JFET beside it, it sets where this stage turns the phase 90°. Marked .05 µF on the trace.
R7, 10 kΩ
Upper half of the 20 kΩ from the plus input to Vref. With the JFET pinched off the 20 kΩ is all there is, and the stage turns 90° at about 170 Hz.
R8, 10 kΩ
Lower half of the 20 kΩ. The point between the two halves carries half the JFET’s drain swing.
C4, 10 nF
Puts half the drain swing on the gate. A JFET used as a resistor bends the signal across it, and this cancels most of the bend, so the stage stays clean at higher levels.
R9, 470 kΩ
Carries the bias line to the gate. With the 10 nF it keeps the audio on the gate apart from the slow bias.
Q1, 2N5952
Used as a resistor that the LFO varies. Its source is on Vref. The closer the gate comes to its source, the lower its resistance and the higher this stage’s 90° point.
U2a, TL072
The first phase-shift stage. Half the signal goes straight, half through the filter, and the two meet here.
R10, 10 kΩ
Into the minus input, as in the first stage.
R11, 10 kΩ
Feedback, equal to the input resistor.
C5, 47 nF
Feeds the plus input, the same as the first stage’s, so the two stages turn together.
R12, 10 kΩ
Upper half of this stage’s 20 kΩ.
R13, 10 kΩ
Lower half of this stage’s 20 kΩ.
C7, 10 nF
Half the drain swing to this JFET’s gate, as in the first stage.
R14, 470 kΩ
Bias line to this gate.
Q2, 2N5952
The second stage’s JFET, on the same bias line.
U2b, TL072
The second phase-shift stage. Its output is the wet signal.
R15, 10 kΩ
The wet signal into the mix. Where the two stages have turned the phase 180°, it cancels the dry signal: that’s the notch.
R16, 10 kΩ
The dry signal into the mix, equal to the wet one. The two resistors average the halves, so each comes out at half its level.
C8, 47 nF
Output coupling cap. Marked .05 µF on the trace.
R17, 150 kΩ
Output pulldown, so the jack sits at 0 V.
R22, 150 kΩ
From 9 V to the LFO op-amp’s plus input. With R23 and R21 it sets the two switch points, about 3.5 V and 5.5 V.
R23, 150 kΩ
From the plus input to ground, the bottom of the divider.
R21, 150 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, TL072
The LFO. Its output is a square wave, high or low.
R20, 150 kΩ
From the timing cap to the minus input.
C11, 10 nF
From the minus input to the output. With R20 it makes each flip slightly late.
R24, 7.5 kΩ
In series with Speed. It sets the top speed.
Speed, 500 kΩ pot, reverse audio taper
Speed, 500 kΩ, a variable resistor from R24 to the timing cap: all of it at 0, the slowest, none at full. Reverse audio is assumed, since the trace doesn’t mark the taper.
C10, 10 µF electrolytic
The timing cap. It ramps up and down between the switch points, and that ramp is the sweep.
R19, 3.9 MΩ
Carries the ramp from the timing cap to the bias line.

About this version

Stock, TL072
As traced: TL072 op-amps, 2N5952 JFETs, a 4.7 V zener. The trimmer is set for a typical JFET.

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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