108 Nicolas Collins
Chapter 28: A Little Power Amplifier
You will need:
- Something to amplify: a guitar, a cassette or CD player, etc.
- A breadboard.
- Audio Power Amplifier chip, LM386.
- Assorted resistors, capacitors & pots.
- Assorted jacks and plugs.
- A small speaker.
- Some solid hookup wire.
- A 9 volt battery and connector.
- Hand tools.
Whether in pursuit of a self-contained electronic instrument or some form of
sound sculpture, one day you will tire of choosing between the putty-colored
Radio Shack mini-amplifier and a bulkier, more expensive (and potentially more
dangerous) external amplifier and speaker. If you wish to drive a speaker
directly with the output of one of your circuits, consider the lowly LM386. At
less that US$1.00 retail, this chip, combined with a few other components in a
very simple configuration, makes a cheap but decent low-power audio amplifier.
It is the heart of the aforementioned mini-amp, and -- once soldered up -- can be
substituted accordingly.
LM386
The basic configuration (Figure 2) gives a gain of 20. By adding a 10uF capacitor
between pins 1 and 8 the gain rises to 200 (Figure 3.) The 0.05uF capacitor and 10
Ohm resistor shown at pin 5, and the .1uF “bypass” capacitor at pin 7 are
optional parts, to be added if the circuit oscillates and whines by itself. When
you go to solder up this circuit you may have to rearrange the components a few
times to avoid oscillation -- this amplifier is more stable than many, but audio
power amps are tricky sometime. Both circuit below include a pot for input
volume control. You’ll want to add a power switch as well, or disconnect the
battery from its clip when not in use.