Thursday, January 8, 2015

LeSpork Bass Prototype - Make A Circuit With Me

A sweet romance is not for me
I need electricity
If you wanna make me flip
Hit me with a micro chip

It’s not unusual for guitarists to know a little about electronics.  I fall into the category of knowing just enough to be dangerous when it comes to electronics.  Over the years, I have learned my limitations, and generally stay away from tinkering with anything that has more than an amp of alternating current, or anything that would be considered high-voltage DC.  The end of the patch cord I’m working on has low enough voltage and current to make it almost impossible to do any damage to an instrument, an amplifier or myself.  So, with the encouragement, advice and parts-box resources of my long-time friend, electrical engineer and fellow guitar enthusiast BJ, plus a few more inexpensive electronics bits and pieces, I am adding an on-board effects circuit to This Old Guitar.

My first attempt was a simple, passive, hard-clipping diode fuzz circuit.  The idea is that the diodes, one wired with anode shunted to ground and the other wired with cathode shunted to ground, cut away the peaks of the signal.  This changes a clean-sounding sine wave into a fuzzy-sounding wave that approximates a square wave.  The sound is similar to an overdriven amplifier.

The wiring is simple.  I wired a pair of IN4001 silicon diodes and a single pole switch between the pickup and output jack leads.

The downside to this circuit is that you need a healthy amount of impedance in the pickups for the diodes to clip the signal.  As I found out, the single-wound pickup in this bass does not generate enough of a signal to make it work.  So, it was time to move up to an amplifier circuit.

As this is my first time building a circuit in many years, I kept away from micro chips.  I wanted to go with something that was well documented, has one transistor and a minimal number of components.  After considering numerous options, I decided on the classic Electra Distortion circuit.

At the heart of the circuit is a simple one-transistor amplifier.  This little circuit has been used successfully by many audio professionals as well as amateurs like myself with numerous variations and modifications as far back as the 1960s.  In the 1970s, a distortion circuit was offered as a plug-in module by Electra Guitars, allowing guitarists to customize their own on-board guitar effects.  In more recent years, variations of this circuit have been made available under many names by a variety of boutique pedal manufacturers in the form of an external stompbox.  A DIY stompbox version with demo and a tutorial has also appeared in the October 2014 issue of Premier Guitar in an article written by Joe Gore.  With so much already written and documented on this circuit, I've focused this blog entry to a few aspects of my own build.

I started by building the stand-alone circuit on a breadboard.

While waiting for one of the fixed resistors I bought to arrive, I improvised by using a 100k trim pot dialed in to the appropriate 47k value.

After double checking the layout, I moved things into the house to do a proper sound check on the circuit with an instrument and an amplifier.  My only blunder so far was a soldered lead on the output jack.  After a little troubleshooting, the Electra Distortion circuit worked as expected.  Success!

Next up, I'll do some tweaking with the semiconductor types and values to customize the sound for my LeSpork Bass.

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