Phaser Guitar Pedal
From Makerpedia
by Matthew Song
Project Overview
This project walks through building a phaser pedal for an electric guitar. This is a modulation pedal and creates a "whooshing" effect by shifting different frequencies in and out of phase. This is achieved through 4 building blocks: the power supply, input buffer, phase shifting stages, Low Frequency Oscillator (LFO), and the output mixer
The dry signal from the guitar is first sent through the input buffer and then through multiple all pass filters to create a phase shifted wet signal. These two signals are then mixed together via the output mixer to create the effect heard. The speed at which the effect occurs is controlled by the Low Frequency Oscillator (LFO), which is a potentiometer. Decreasing the resistance increases the speed and vice versa.
Tools and Materials
- Resistors: 200 Ω (1), 10 kΩ (9), 24 kΩ (4), 56 kΩ (1), 150 kΩ (6), 470 kΩ (1), 500 kΩ (1), 1 MΩ (1), 3 MΩ (1)
- Potentiometers: 87 kΩ (1), 500 kΩ (1)
- Capacitors: 10 nF (2), 47 nF (6)
- Polarized Capacitors: 10 μF (1)
- Op Amps: LF411 (6)
- Transistors: 2N3906 PNP (1), MPF102 JFET (4)
- Diodes: LED (1)
- 1 DPDT Toggle Switch
- 2 Quarter Inch Jacks
- Board with 5V, +15V, -15V, GND
Project Files
Step-by-Step Instructions
Build the power supply as shown in the diagram. For Vref, use the fixed 5V source from the breadboard. This will set the source voltage of each JFET to 5V. Vbias will vary depending on the JFETs used as they need to be in a spot where the JFETs are not always on or off. Make sure to test what values of Vbias works for the JFETs used. This quality varies by the specific JFET used.
Build the input buffer shown in the circuit diagram. This step is a little boring as it doesn't change the functionality of the phaser. However, it ensures that the quality of the input is not degraded and keeps the sound clear. Now, attach one end of the quarter inch jack to the input buffer and the other end to ground. This will be where the signal starts and goes through the entire circuit.
Build the LFO shown in the circuit diagram. This step can be more varied as this controls how strong the effect is by changing the voltage at the gate, which then varies the resistance between the drain and source. The LFO will create a triangular waveform by integrating the Schmitt Trigger output, allowing the JFET to be varied repeatedly. By changing the potentiometer value, you can change the speed of the effect. Adjusting the range of the potentiometer can also potentially create a wider range of sounds as well. To test this works as intended, probe the Op Amp output on the oscilloscope to ensure it acts as a Schmitt Trigger. Afterwards, probe the negative terminal of the op amp to ensure that a triangular waveform is created.
Build the phase shifting block shown in the circuit diagram. This block is comprised of multiple all pass filters, which allow all frequencies to be passed but phase shifts the signal by 180 degrees. In the diagram shown, there are 4 all pass filters. However, this can be created with any number of filters (an even number is best recommended). This also decides how pronounced the effect is as it decides how many "notches" are created and being moved around. It is important to ensure the JFETs are all of the same type. If not, there will be discrepancies in the proper Vbias needed per JFET, making the circuit act inconsistently. Make sure to connect Vref and Vbias to all sources and gates of the JFETs, respectively. To test this functionality, connect the function generator to the input buffer and probe the end of the filter chain to ensure that a 180 degree shift occurs.
Build the output mixer shown in the circuit diagram. This block recombines the unfiltered signal with the filtered signal in order to hear the effect of waves constantly going in and out of phase. Note that changing the ratio of the 150 kOhm resistors at the end of the phase shifters and input buffer can result in different percentages of the filtered/unfiltered signals being mixed. For example, since both are 150 kOhms right now, they are each mixed at 50%. The PNP common emitter amplifier shown amplifies the signal so it can be heard better. Then, the 47 nF capacitor and 150 kOhm resistor after the amplifer create a high pass filter, removing hum and buzz from the output. Now, finally, probe the output of the mixer to ensure that the output acts as intended. The output should look like it is increasing and decreasing in amplitude periodically. After this is verified, attach one end of the second quarter inch jack to the output of the mixer and the other to ground. Happy phasing now!
OPTIONAL
Create the input/LED indicator shown in the circuit diagram. This is not necessary to the functionality of the circuit. It does, however, add a bypass switch to the circuit. To do so, use a DPDT switch (or a SPDT switch if you want to ignore the LED part). Connect the guitar input to one of the common terminals. Then, wire one of the poles to the beginning of the circuit (input buffer) and the other directly to the output quarter inch jack. This allows for the signal to bypass the circuit when switched off. Additionally, connect 5V to the other common terminal. Then, connect an LED in series with a resistor so that when the signal goes through the circuit, the LED lights up. Connect the other pole to ground so that nothing lights up when the signal is bypassing the circuit. Test the functionality by ensuring the LED lights up when the signal goes through the circuit and is phased.
