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LM386 Audio Amplifier
 Despite
the LM386 amplifier IC having been around for ages, I have never
featured it on this website, and as I recently used it in another
project, I thought it was a good time to do so. The LM386 is a very
popular hobbyist
chip (also used commercially) that requires only a few
extra
components to make a simple, low powered audio amplifier. It doesn't
pretend to be HiFi, but if you just need a quick, easy to build
amplifier that can deliver a few hundred milliwatts into a small 8 ohm
loudspeaker then this will do the job. Although Texas Instruments have
discontinued
production, this trusty old workhorse is still produced by other
manufacturers, and so it should be readily available for many years to
come.
There are three different versions with power outputs of 325mW, 700mW
and 1000mW with suffixes N-1, N-3 and N-4 respectively, so take your
pick. The circuit is the same for all.
 Schematic
The
schematic above
shows the signal being fed into the inverting input rather than the
non-inverting input as shown in the datasheet. Either will work, you
just
have to ground the other input. Some say that the inverting input is less noisy
than the non inverting input which if true, is good reason for
using it, though the main reason I used it is explained later. There are also schools of thought that
suggest decoupling the unused input with a capacitor instead of
grounding it to obtain a tiny bit more power, but for most purposes a
short to ground is perfectly fine. The gain of the LM386
can be controlled by pins 1 and 8. If left open circuit the gain is 20,
but if a 10uF electrolytic capacitor is connected between them (+ to
pin 1) the gain is 200. If a 5K potentiometer is placed in series with
the
capacitor, the gain can be made variable. Here I've shown it fixed and maxed out!
 Stripboard Layout
I
wanted the supply to enter from the left with the speaker connection
solely on the right, so I used an 8 pin DIL socket with pin 3 snipped
off and pin 4 shorted to the remainder of pin 3 with a blob of solder
(shown in red). This allows the + supply to pass underneath the IC
to pin 6, which is the main reason for using the inverting input,
because using the non-inverting input would require an added wire link
which I try to avoid if possible, to achieve a neater looking layout. As usual,
I've used my favourite standard size stripboard
of 25mm x 64mm (9 holes x 25 holes) which are readily available.
Connection to the board is via JST XH plugs and sockets and the
two mounting holes are 3mm. The volume control isn't shown as it's 'off board'.
 Track Cuts and Solder Blobs
Treble boost:
There are versions of this circuit with bass boost but none with treble boost. If for any reason you want a bit more 'sparkle' at the top end, then simply replace the 10u capacitor between pins 1 and 8 with a 100n capacitor in series with a 5K potentiometer and this will give you a variable treble control. If you just want a fixed treble boost then simply replace the pot with a 1K resistor. |