Four-stage JFET phaser with a Feedback knob, late 1970s 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 Ibanez PT-909 Phase Tone.

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 triangle-wave oscillator sweeps. Feedback sends some of the wet signal back into the second stage.

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. The mixer adds the two at equal level. IC104a ramps up and down between the points where IC104b flips, a triangle of about 1.4 V peak to peak. Width passes part of it to the bias line, and the Bias trimmer sets where the gates sit. The JFETs’ resistance changes over well under a volt of gate voltage, so the trimmer matters. The model leaves out the spread between real JFETs, which were selected for each pedal, and the footswitch’s flip-flop.

Download.

Parts and what they do

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

R151, 1 kΩ
From the battery to the zener.
D104, 5.1 V zener, in breakdown
A 5.1 V zener, 1N5231B. It holds Vb, the stages’ reference. The drawing calls this line 4.5 V.
C125, 47 µF electrolytic
Vb’s filter cap. It makes Vb a quiet AC ground for the stages.
C101, 100 nF
Input coupling cap.
R101, 1 kΩ
Input series resistor.
R102, 510 kΩ
Biases the follower from Vb and sets the input impedance near 510 kΩ.
IC101a, MC1458
Input follower. Its output is the dry signal, to the stages and to the mixer.
R103, 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.
R107, 10 kΩ
Feedback, equal to the input resistor, so every pitch comes out at the same level and only the phase moves.
C103, 47 nF
Feeds the plus input. Against the 30 kΩ and the JFET beside it, it sets where this stage turns the phase 90°.
R106, 30 kΩ
From the plus input to Vb. With the JFET pinched off it’s all there is, and the stage turns 90° at about 110 Hz.
C102, 47 nF
Keeps the drain’s DC off the gate, so only the swing goes there.
R104, 150 kΩ
With the 100 kΩ from the gate to the bias line it puts 40 % of the drain’s swing on the gate, which keeps the JFET clean at higher levels.
R105, 100 kΩ
Carries the bias line to the gate.
Q101, 2SK30A-GR
A selected 2SK30A, 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.
IC102A, MC1458
The first phase-shift stage.
R108, 10 kΩ
Into the minus input, as in the first stage.
R112, 10 kΩ
Feedback, equal to the input resistor.
C105, 47 nF
Feeds the plus input, the same as the first stage’s, so all four stages turn together.
R111, 30 kΩ
From this stage’s plus input to Vb.
C104, 47 nF
Keeps the drain’s DC off this gate.
R109, 150 kΩ
Part of the drain swing toward this gate, as in the first stage.
R110, 100 kΩ
Bias line to this gate.
Q102, 2SK30A-GR
This stage’s JFET, on the same bias line.
IC102B, MC1458
Another phase-shift stage.
R114, 10 kΩ
Into the minus input, as in the first stage.
R118, 10 kΩ
Feedback, equal to the input resistor.
C108, 47 nF
Feeds the plus input, the same as the first stage’s, so all four stages turn together.
R116, 30 kΩ
From this stage’s plus input to Vb.
C107, 47 nF
Keeps the drain’s DC off this gate.
R115, 150 kΩ
Part of the drain swing toward this gate, as in the first stage.
R117, 100 kΩ
Bias line to this gate.
Q103, 2SK30A-GR
This stage’s JFET, on the same bias line.
IC103A, MC1458
Another phase-shift stage.
R119, 10 kΩ
Into the minus input, as in the first stage.
R123, 10 kΩ
Feedback, equal to the input resistor.
C110, 47 nF
Feeds the plus input, the same as the first stage’s, so all four stages turn together.
R121, 30 kΩ
From this stage’s plus input to Vb.
C109, 47 nF
Keeps the drain’s DC off this gate.
R120, 150 kΩ
Part of the drain swing toward this gate, as in the first stage.
R122, 100 kΩ
Bias line to this gate.
Q104, 2SK30A-GR
This stage’s JFET, on the same bias line.
IC103B, MC1458
The fourth phase-shift stage. Its output is the wet signal.
C106, 100 pF
Across the second stage’s feedback. It trims the very top.
C111, 1 µF
Couples the wet signal on, a 1 µF non-polar electrolytic.
R124, 100 kΩ
Holds the wet line at Vb for DC.
C120, 100 nF
Carries the wet signal to the top of the Feedback pot.
Feedback, 10 kΩ pot, linear taper
Feedback, 10 kΩ linear. It picks off part of the wet signal and sends it back into the second stage, which deepens the peaks between the notches.
C119, 100 nF
Carries the Feedback wiper on toward the second stage.
VR102, 10 kΩ pot, linear taper
Internal Feedback trimmer, 10 kΩ as a variable resistor. Less resistance, more feedback. Set on each unit at the factory. The setting here is typical.
R113, 10 kΩ
Into the second stage’s minus input. That stage adds the feedback to its input, turned over.
C116, 1 µF
After the effect switch Q105 (left out, on), a second 1 µF non-polar cap.
R131, 100 kΩ
Holds this side of the switch at Vb.
R132, 10 kΩ
The wet signal into the mixer.
R135, 10 kΩ
The dry signal into the mixer, equal to R132. Where the stages have turned the phase 180°, the wet cancels it.
R133, 22 kΩ
From the mixer’s + input to Vb. With R132 and R135 it takes each signal down to about 41 %.
IC101b, MC1458
The mixer, non-inverting, a gain of 1.47.
R134, 4.7 kΩ
The mixer’s feedback resistor.
C117, 100 pF
Across R134.
R136, 10 kΩ
From the minus input to Vb. Its gain is 1 plus 4.7 kΩ over 10 kΩ.
R137, 470 Ω
In series at the output.
C118, 10 µF electrolytic
Output coupling cap.
R138, 10 kΩ
Output pulldown.
VR101, 100 kΩ pot, linear taper
The internal Bias trimmer, 100 kΩ across the battery. It sets where the JFET gates sit, which places the sweep. The factory set each unit. The setting here suits this file’s JFETs.
R125, 200 kΩ
From the trimmer to the bias line.
C112, 470 nF electrolytic
Smooths the bias line, a 0.47 µF tantalum.
Width, 100 kΩ pot, linear taper
Width, 100 kΩ linear, from Vb to the LFO’s triangle. Turned up, more of the sweep reaches the gates.
R126, 240 kΩ
From the Width wiper to the bias line.
IC104a, MC1458
The integrator. Its output ramps up and down, a triangle wave, and that’s the sweep.
C113, C114, 23.5 µF
The timing cap: two 47 µF electrolytics back to back, 23.5 µF with no polarity.
C113 leak, 3.3 MΩ
Not on the drawing: the timing cap’s leakage, typical of two small electrolytics. Without some path for DC around the integrator the model has no operating point. The speed is the same with it.
R127, 2.4 kΩ
In series with Speed. It sets the top speed.
Speed, 500 kΩ pot, reverse audio taper
Speed, 500 kΩ reverse audio, a variable resistor from the comparator to R127: all of it at 0, the slowest, none at full.
IC104b, MC1458
The comparator. Its output is a square wave, high or low, and it flips each time the triangle reaches a switch point.
R129, 22 kΩ
From the triangle to the comparator’s + input. With R128 it sets how far the triangle swings, about a quarter of the square’s swing.
R128, 82 kΩ
Positive feedback from the comparator’s output. It holds the comparator on one side until the triangle reaches the switch point.
R130, 100 kΩ
From the comparator’s + input to Vb.
C115, 10 nF
From the comparator’s output to its minus input, on Vb.

About this version

Stock, MC1458
MC1458 op-amps, selected 2SK30A JFETs and a 5.1 V zener, as the drawing lists them. The trimmers are 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.

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