I introduced my version of the common LM386 guitar amp today over at the Ponoko blog. Here’s some more details for those interested in duplicating my work.
I used 4mm plywood and 20mm M4 bolts. The first version shows the ‘wrong side’ of the ply on the outside, the second version uses the ‘right side’ and has one thin coat of boiled linseed oil on it. I glued all the joints with PVA, besides the back piece which needs to be removable to replace the battery. The speaker is from my box of random treasures but the location of the mounting holes is pretty standard for a 4”/100mm speaker. I opted to use a PCB-mounted input jack and potentiometer; these make a strong mechanical connection between the circuit board and enclosure. I also used a PCB-mounted battery holder so there’s nothing rattling around loose inside the box. The circuit is built on some cheap perfboard.
The circuit itself is a very common beginners project, and there are many variations explained online. Beavis Audio’s Noisy Cricket page is a great place to start. I built two versions and recorded samples of each to give you an idea of the sound. The most basic version has just the chip and one capacitor, and a gain-control potentiometer. This 1k pot varies the gain from 20 to 200, but in the sample I had it set to maximum and just used the guitar’s volume control to get different levels of dirt. Here’s the circuit (schematic and layout and photo), and here’s what it sounds like (playing on the neck pickup of a Fender Strat with the tone rolled back a bit).
The deluxe version includes a JFET input buffer to increase the sustain and some filtering to get rid of some of the harsh ‘ripping cardboard’ sound in the distortion. It is an improvement, but in my opinion it probably not worth the added expense and complexity, unless you were going to connect this up to a bigger speaker (which, by the way, is an excellent idea). Schematic, layout, top photo, bottom photo and sample.
Any questions, just ask.
- Speaker (preferably 8 ohm, 4” is a good size) e.g.
- 9V battery plus clip/holder
- LM386 chip
- 100uF – 220uF polarised capacitor
- Stereo 1/4” input jack
- Perfboard to build it all on
- 1k linear pot + knob for gain control
- 8 pin IC socket so you don’t fry the chip when you solder it in
- 100nF – 470nF input capacitor as a safety device in case you connect something weird to the input that’s not a guitar
- Input buffer: MPF102 JFET, 3k3 & 1M resistor
- Output filters on pins 5 and 7: 10R resistor, 47nF & 100nF capacitors