Instructables

FET Distortion Pedal

Picture of FET Distortion Pedal
Here's a little project that guitarists might appreciate. It's a  booster/overdrive/distortion pedal, depending on how you configure it.

With a few electronic components and some basic knowledge of breadboarding and/or soldering, it should be a relatively simple build.
 
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Step 1: A Brief Overview of FETs

A FET, or a field-effect transistor, is a transistor (funnily enough) that uses a variable electric field to control the electrical resistance of a semiconductor channel.

The type of FET we'll be using for this project is an N-channel JFET.

The channel in a JFET is somewhat like a garden hose - under normal conditions, water (current) flows through it at a rate determined by its area of cross section. The depletion region is like your hand. As you reverse bias the P-N junction (squeeze the hose), the depletion region grows wider (and the channel narrower), thereby increasing the channel's resistance. Then you hit a point where the depletion region has "pinched off" the channel, effectively making the JFET a large value resistor (around a couple of megaohms from drain to source).

Unlike a diode however, the P-N junction in a FET isn't capable of handling much current, and any forward biased (Vgs > 0) n-channel JFET will likely fail soon. (MOSFETs get around this by adding a thin layer of silicon dioxide at the junction, effectively disallowing current flow).

Okay, so let's get started.

Step 2: Parts

Picture of Parts
Here's a list of what you're going to need:

Resistors
--(3) 2.2k
--(3) 51k
--(3) 100k
--1M

--(3) 50k trimpots
--100k pot, audio taper.
--500k pot, audio taper.

Capacitors
--200nF
--(3) 4.7 uF
--(3) 22 uF
--220 uF

Semiconductors
--(3) J201 JFETs

Miscellaneous
--9V battery
--Clip for said battery
--Breadboard/Stripboard
--An enclosure for the thing, if you choose not to breadboard
--(2) Jacks for input and output
--A guitar?  (you could probably plug in a mic instead and try singing into it, but...)
bissell9411 months ago
Where exactly do the jacks get attached? I'm not sure how the circuit with the jack and the 220uf capacitor are integrated into the rest of the circuit.
tbissell11 months ago
Is the first capacitor after the input a 100nF or a 200nF? I ask because the schematic and the parts list differ in this respect.
tbissell tbissell11 months ago
Nevermind, I'm dumb. It says so right in the instructions. Now, off to find a 100nF capacitor!
reed001121 year ago
Any videos of this working ? Would love to hear it. Especially if someone has added the 4th stage, or even a 5th.
SebaMozo1 year ago
Hi!

Thank you for this great project.

For me, works bether smaller values for the couplin caps between the stages, rolling off bass frequencies.

It's possible someone can draw the PCB?? I can't understad EAGLE.

Thanks a LOT!
prphntm2 years ago
Do you think you could put a pot before the 22uF capacitor to make the sound even more versatile?
andrezfen2 years ago
can i connect speaker directly at the ouput?
Cookie Monster! (author)  andrezfen2 years ago
The output needs to go through a power amp stage before you can run it to speakers. You could try headphones on the output of you're really keen, but I'm not sure even those would work.
andrezfen2 years ago
any video of this pedal? i want to build this circuit. tnx :D
(removed by author or community request)
No, one battery is enough. It is a pity though he does not mention the current draw, if would be a pity if your battery was drained in a few hours of playing.
mdog933 years ago
With a lot of help for instructbales answers i might be able to work it out for myself but i'm still a beginner, so would you be able to give a schematic including a tone control and 4 way selector switch please?

another thing, you mentioned using a stero input jack or putting a switch in, do you mean a stereo jack or do you mean an internally switched mono jack?
Cookie Monster! (author)  mdog933 years ago
I mean an internally switched mono jack, I only said "stereo jack" because they tend to include internal switches - thanks for pointing that out, I'll correct it.

As for the tone control and 4-way selector, sure, I'll post a schematic in a bit.

It's useful to learn about tone controls in depth, though, since you'll be able to customize your tone to your preferences.
Thanks, oh and about the jacks, if it sounds like i know what i'm on aboiut i really don't i just had a lot of confusion when i was tryin to buy parts for a slightly modified version of one of these:
http://www.instructables.com/id/Make-a-Sweet-Portable-Guitar-Amp/
And i needed a switched panel mount 2.5mm dc power jack and i couldn't find any in the style i needed and i needed a normal 1/4" mono input jack in a panel mount style and i couldn't find one in the style i wanted but could find switched ones! Bit ironic. Now i realise that a switched input and switched headphone output would have been better than normal ones with a switch. I don't know why the author used a switch instead.

And yeah i agree trial and error can sometimes give the best results for you, but because i'm only a beginner i need a good baseline to start from, then i can try other things and if i don't like it i can revert back to the tried and tested one.
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