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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, this will do the job. Although Texas Instruments have discontinued production, this trusty old workhorse is still produced by other companies as well, so it should be readily available for some time 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 noisey 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 varied between about 25 and 200. Here I've shown it 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