IAN LANG ELECTRONICS

The LM386 is sold by every supplier. Either you can go down to the shops
and get them or buy them online. If you live in/ near Sheffield I recommend NR Bardwell, on Abbeydale Road. Often they come in kits, these are well worth buying as you can make them up and use them as a modular part to your projects.

The LM 386 Amplifier IC

The LM386 is a small (0.5W) power amplifier IC which is perfect for small, portable operations such as radios, walkie -talkies etc. It comes in SO (Small outline) and DIP (Dual Inline) packages. The small outline packages are meant for machine tool soldering and I do not recommend you buy these unless you can solder exceptionally accurately and have a good deal of patience. This is the general form of the DIP package:

As you can see it has eight pins and this needs some explaining. The easiest pins to spot are the power supply ones, these are at pins 6, the positive, and pin 4, the negative.
The inputs go to pins 2 and 3. Usually we feed the signal at pin 3, and take a return path to ground from pin 2.
Pin 5 is where we take our output. This is usually capacitively coupled to an 8 ohm loudspeaker or headset jack. Putting less than 8 ohms on the end results in too much power in the amplifier, leading to a degradation of the signal and possible damage to the chip, so it isn't a good idea to do it. I've put 16 ohms on the end and got away with it, but it isn't a good practice. Below is the usual circuit:

This is another one of those that looks fearsome but isn't. X1 and X2 are the signal terminals, and usually consist of PCB mounted terminal blocks. Between pins 1 & 8 we find a 10 uF capacitor in series with a potentiometer acting as a rheostat. This allows a variable gain from 20 to 200. The maximum output should be about 46 dB.
You'll notice that the DC supply voltage is in parallel with a capacitor, C1. This is to alleviate any buzzing or distortion caused by the supply voltage. In addition, if there is any stray AC in the circuit, it finds an easy path to ground through C1 and does not enter the amplifier.
The output is taken from pin 5 via capacitor C3 which is there to match impedance between speaker and amplifier. C6 and R2 provide a bypass and bias which improves the tone quality of the output.
At pin 7 C5 provides an easy path to ground, and this is there to prevent any howl round in the amplifier. It is not always necessary.

Parts and values are:

C1 & C2 electrolytic 10 uF (watch the polarity)
C3 electrolytic 100 uF (experiment with a 220 uF and see what happens)
C4 & C5 100nF
C6 47nF
R1 preset 10k max
R2 10 ohms
and an 8 ohm loudspeaker rated at 0.5W or more. (Don't use 0.25W as it will end badly).

Supply voltage must be 4.5V min, 7.5 is good and 9V a typical. After this it can get a bit twitchy, although in theory you could feed 15V through there. 9V is more than enough to give a loud clear sound from a portable radio. A volume control can be fashioned by placing a potentiometer acting as a voltage divider before point X1. Use a logarithmic rather than a linear one.