This is a very simple and very effective distortion pedal.
Step 1: Materials
-1, 9 volt battery
-1, 9 volt battery connector
-1, 4.7k ohm resistor
-1, 10k ohm potentiometer
-2, 1/4" audio jacks
-1, LM324 Quad op amp
All materials can be found at radio shack besides the LMN324 which I've only found online.
Step 2: Making the Circuit
Connect pin 3 of the LM324 to ground (pin 11) with your 4.7k ohm resistor. Connect the input jack to pin 2 and ground (pin 11). For your output, connect one wire to ground going to one leg of the jack. Then connect the jack to the left leg of the 10k ohm potentiometer to on the other leg of the jack with the right leg of the potentiometer connected to the center leg. Then connect the center leg to pin 1 on the LM324. Connect the positive wire(red) of the battery connector and connect the negative(black) to pin 11.
Step 3: Play
Plug in your instrument to the input jack and the amp into the output jack. Now if you play clean, it should sound distorted. Your done!
It may take a little bit of messing around to get it to work, but just make sure all of your connections are good and it should work fine. If there are any questions please ask.