Wednesday, February 12, 2014

Introduction: Guitar Pedal Phaser

We are still Sean Lowen and Greg Edelston, two students at Olin College in Needham, MA. For our EE Prototyping class, we were assigned to take a piece of commercial eletronics, and reverse engineer it; that is to say, we were to deconstruct it, figure out how it works, and create a detailed schematic and block diagram of the system.
We opted to take apart a guitar pedal; specifically, an Ibanez Phaser PH5 Soundtank. You can find this pedal all over the internet; it's a pretty standard phaser pedal, which modulates the amplitude of the audio signal, similar to a wah-wah. If you don't want to spend $20 on a pedal (and have a lot of electrical parts sitting around), hopefully you can get something out of our reverse engineering!

Basic Test Results

To show how the guitar phaser works, we took a few videos which demonstrate its behavior.

The yellow signal is the input signal (from a function generator) and the blue is the output signal of the phaser.  We started with the phaser on (you can see the amplitude of the output signal changing), switched it off (the amplitude remained the same as the input amplitude), switched it back on, and changed the input signal to a sawtooth, which caused some strange distortions in the output signal.  On the phaser, we were able to change the speed (how fast the amplitude changes), feedback (which didn't seem to do much), and the depth (how much the amplitude changes).

Block Diagram

Above is a rough block diagram of our system. Essentially, the audio signal is processed through several op-amps, one at a time, to give it the phasing effect; along the way, the variable resistors are able to change the speed and depth of the phasing, as well as the feedback of the system. We are not entirely sure how this part of the circuit works; we think that the potentiometers for the speed and depth control four JFETs which feed into a series of op-amps (which may be filters), causing the amplitude to change at different speeds and depths.  At the end, the system checks whether the switch has been pressed; if so, it will output the processed audio, and if not, it will output the raw input audio.


The following components were found in the circuit:

The data sheet for the dual op-amp can be viewed here.

Circuit Schematic

The following is the circuit schematic for the guitar pedal:

The values for the part identifiers listed on the schematic can be found here: