Introduction: 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

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

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

--(3) 2.2k
--(3) 51k
--(3) 100k

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

--(3) 4.7 uF
--(3) 22 uF
--220 uF

--(3) J201 JFETs

--9V battery
--Clip for said battery
--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

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

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

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.