Introduction: 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.

Step 1: A Brief Overview of FETs

Picture of 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...)

Step 3: The Schematic

Picture of The Schematic

The circuit consists of several fairly standard JFET common-source amplifier "stages" cascaded one after the other.

JFETs work in depletion mode, in a manner very similar to vacuum tubes. Because of this, by carefully controlling the gain of each stage, asymmetrical, soft clipping can be achieved.

Note that each stage inverts the signal it is fed, so if asymmetrical clipping is what you're going for, you'll want to cascade alternate high gain and low gain stages.

Or you could just max out the gain of each stage and end up with something resembling a square wave at the output.

To each his own =)

Also note that the overall effect depends on the number of stages used. One stage adds a boost with a mild crunch when driven hard, two will give you a bluesy overdrive. Make three stages, and you get varying levels of distortion, depending on the gain settings of the individual stages.Keep adding more, and I'm guessing you'll get fuzz.

The input capacitor also plays a significant role in the overall tone of the effect. Larger values mean more bass, yielding a darker sound, while lower values will give a brighter sound. 100nF seems like a good compromise.

Also worth mentioning is that JFETs are hopelessly inconsistent from one device to the next. If you look at the datasheet for the J201, for instance, the listed minimum and maximum gate to source cutoff voltage (Vth, one of the parameters we are interested in) are -800mV and -4V respectively. That's a 400% margin from minimum to maximum.

This inconsistency is one reason for the three trimpots in the circuit. A useful starting point is to have them adjusted so that you measure around 6 volts at the JFET drain without any input signal. From there, you simply tweak till it sounds the way you want it to.

Step 4: Building It

Picture of Building It

Whip out your breadboard and start sticking your components into it. The way you lay them out is entirely up to you, but try to keep things as neat as possible, and align all components either horizontally or vertically. May be a bit of a pain while starting out, but it makes debugging a lot easier.

Alternatively, if you decide to solder directly onto stripboard, then go ahead and do it. Same rules apply here as well. If you go this route, your finished board'll be around 6 or 7 square inches.

Step 5: The Enclosure

Picture of The Enclosure

My "stompbox" effect is currently sitting on a breadboard, which makes it a rather fragile hum-magnet. You'll probably want to put the circuit inside a metal box, effectively killing two birds with one stone. The metal shields the effect from mains hum (if properly grounded), and makes it relatively solid.

Use your creativity here, and come up with a metal case for the effect.

Here's an article about making enclosures out of steel house framing members.

Step 6: Further Ideas

It shouldn't be too difficult to build three or four stages, tap outputs out of each one, and wire them all to a three or four way selector switch to get a fuller range of sounds, from crunch to distortion, and beyond.

I've also left out the Tone control on this effect, but that should be relatively straightforward to implement.

Try adding  voltage dividers between stages if cascading more than three stages, to get a mellower distortion.

I'm planning on using this circuit as a preamp section for an amplifier I'm building, but more on that later.

Comments

PedalFiend (author)2016-04-09

The JFet Hyperdrive. I put the third stage on a switch and added a tone control. Sounds great and will blow the doors off with the 3rd stage engaged. Very sensitive to pick attack and harmonics.

Good little build!

DominikK32 (author)PedalFiend2017-03-15

Where did you add a tone control? After first stage?

WaltFles (author)2015-05-01

are those all electrolytic (polarized) capacitors, or which ones are ceramic disk?

Thanks

ChrisB20 (author)2015-02-08

Can this circuit be built with MOSFETs? I have a bunch of those but no JFETs.

cbgslide made it! (author)2014-06-07

I made this on my prototyping rig for pedals. You can learn more about it on my channel.

https://www.youtube.com/user/QbidMusicandMore

tbissell (author)2013-05-16

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 (author)tbissell2013-05-16

Nevermind, I'm dumb. It says so right in the instructions. Now, off to find a 100nF capacitor!

cbgslide (author)tbissell2014-06-02

The first (input) capacitor sets the tone response to much of the pedal. You can experiment with many values and suit it to your taste.

cbgslide (author)2014-06-02

I finished this (on breadboard anyway). If anyone else on here plays cigar box guitar (or any other home made instrument with a piezo contact mic as a pickup) this overdrive circuit is the best one that I've used with my CBG. It helps if you have a volume control on your contact mic, but this sounds much better than anything with diode clipping for sure. In fact, most traditional diode clipping overdrive/distortions are simply unusable with a contact mic. I'll post a picture soon, but I'm not done with this by far. I still need to add my own personal touches including an entire EQ section.

bissell94 (author)2013-05-24

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.

reed00112 (author)2013-01-23

Any videos of this working ? Would love to hear it. Especially if someone has added the 4th stage, or even a 5th.

SebaMozo (author)2012-06-17

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!

prphntm (author)2012-04-10

Do you think you could put a pot before the 22uF capacitor to make the sound even more versatile?

andrezfen (author)2011-07-31

can i connect speaker directly at the ouput?

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.

andrezfen (author)2011-07-31

any video of this pedal? i want to build this circuit. tnx :D

jelleAtProtospace (author)2011-07-09

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.

mdog93 (author)2011-03-05

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)mdog932011-03-05

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.

mdog93 (author)Cookie Monster!2011-03-05

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:
https://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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