So, I wanted a tiny battery-powered audio amp for listening to a bit of music in the bath, garden or whatever. Enter the not terribly good audio amplifier chip, the LM386! Also enter a wooden coffee jar, some veroboard, some hot glue and some components. Here’s the circuit:
- Pins 4 and 6 are the ground and supply pins.
- Connecting pins 1 and 8 with a 10uF cap turns on some Internal Magic in the chip, raising the gain from 20 to 200.
- Pin 7 is connected to ground via a bypass capacitor – necessary at higher gains, it sounded bloody awful without it.
- Pins 2 and 3 are the input; we get rid of any nasty DC with the 100nF cap, and then the pot acts as a volume control. The 1nF cap filters out excessively high frequencies (if I recall correctly.)
- Pin 5 is the output, which also has some filtering – the big 220uF to filter out DC, and the resistor-capacitor to ground to filter out high frequency noise.
- The resistor and LED are unlabelled and optional – I’ll assume you know how to work out the appropriate resistor for any LEDs you have lying around.
- The power supply is a 9V battery – if you want to run this from a wall wart, add a hefty capacitor (100uF or greater) in series with the power supply to smooth out any AC ripple.
I’m using a cheap 4W 8 ohm speaker; you can get about 750mW out of this circuit so a 4W speaker is about right. And here’s what it looks like, installed in the jar:
And the insides – the circuit’s on a bit of Veroboard, and everything’s stuck in place with hot glue:
And in action:
