LM386/Bug Zapper Guitar Amp

Pictures coming soon! I don't have a camera atm :( I will get some recordings up on youtube too.

Recently I've been working on some basic guitar amp designs, based off a really simple circuit using the LM386 op-amp. I made my first one in a old computer speaker box, using the original speaker for convenience. Called it the Bug Zapper (this must be pronounced in the most bogan accent you can handle). It's quite a lot of fun to use as a slide on guitars, with it kinda screaming and feeding back as I play. It's great to have a cheap, fairly quiet battery powered amp on hand. It's also quite nice to have a clean amp and a dirty little one going at the same time.

Quick build description:
Take and LM386 and solder it to a small piece of perfboard.

Ignore pins 1 and 8 for now, we'll come back to those. The input jack goes to pin 2 and 3, ground and signal respectively. Connect ground pins 2 and 4 together. The whole thing runs off a 9V battery (actually, the LM386 can take anything from 18V to 5V, which means it'll keep going long after a 9V battery would usually be considered 'dead') so connect the positive end of the battery clip to pin 6. Negative battery lead goes to ground. Pin 7 isn't needed, still not entirely sure what it does. Pin 5 goes through a 220uF cap, then into the speaker. Other side of the speaker goes to ground. Later I came in and put a switched jack on the output so I can run it into other things (this is a rather hot output though so you need to be careful with levels).

My favourite part of the circuit, by far, is what you can do with pins 1 and 8. The LM386, by default, amplifies at a gain of 20. If you jump pins 1 and 8 together though, you can set the gain manually to anything between 20 and 200, which brings in a bunch of epic harmonic distortion while not making it all that much louder. The datasheet says you need a 10uF cap to jump the pins together but when I tried it without it just worked. I wanted to use a pot to do this, and with a bunch of experimentation, I figured out that somehow a 332k ceramic cap across the terminals sounded good. Read up on pots to see how they work in their entirety. I put my 332k cap across lugs 1 and 3, with the 386's pin 1 going to the wiper and pin 8 going to lug 3. Oh and it was a 10k pot. I tried a 100k (too much), a 1k (too little), and the 10k was just right. I really like just mucking around with this stuff with some tones in mind.

That's the entire circuit. It's really simple. If you add an extra resistor and a switch (I used a switched pot from some other old computer speakers) you can make it work like a regular amp, with an on/off switch and a light. It's cool because if you hit it with a fast, hot transient the LED dims, which makes the whole thing feel just a touch more interactive.

The whole build cost is just over $10, plus a box and a speaker that can be easily salvaged from all sorts of broken stuff. The trashier the better.

I copied my original circuit with a few layout modifications (regular switch, better perfboard layout(particularly for the ground)) and dumped it into a Dora the Explorer lunchbox for my mate Sam. I salvaged a red LED from a nerf nitefinder and put it in her right eye. Used a grille from a PC power supply to stop the speaker getting munted. The lunchbox is cool because you can open it up and change/edit stuff really easily.

I know a lot of other people have used the LM386 for overdrives and fuzzes, mostly by doing something like this on the output of the amp. I just really like the sound of the harmonic distortion that the 386 does by itself though, It's closer to the original tone while still being scary. For some reason the some Girl in Stilettos by Annah Mac sounds particularly good through the Bug Zapper.

I ordered a whole bunch of 386s to do experiments with, and I really like how these amps have come out so I'll definitely do some more of them. If you want one, or something specific, email me and I'll give you a (low) price for a comission.

also: Try building one yourself! It's so much fun to muck around with this stuff.

Morgan