LM386 Amplifier

The LM386 chip is a simple and cheap amplifier chip that is used in countless audio amplifier projects. I used one to make my own 0.5W amplifier. This is not a complete how-to guide, you can find plenty of those elsewhere. It’s just a write-up of some of the things I learned – and modifications I made – along the way.

Instructions

I largely followed the instructions on the Hack A Week site, with a few modifications.

I added a stereo-to-mono summing circuit to convert the stereo signal from my audio source down to a mono signal for the amp. More on these circuits:

http://www.rane.com/note109.html
http://www.psychicorigami.com/2013/04/30/a-small-monobox-speaker-lm386-amp-speaker-in-a-box/

I made a few modifications to deal with interference problems from my power supply, as per this write-up.

And I added a few extra parts, as detailed below.

Parts

As a result of these modifications, my final parts list ended up being quite different to the standard lists for these kinds of projects.

Resistors

From top to bottom:

  • 1 x 10 Ohm resistor (pin 5 to ground)
  • 2 x 470 Ohm resistor (for stereo summing circuit)
  • 1 x 27K Ohm resistor (for stereo summing circuit)
  • 1 x 10K Ohm resistor (for bass boost circuit)

Capacitors

From left to right:

  • 1 x 0.047 uF non-polarised (decoupling capacitor, voltage high to ground)
  • 1 x 0.47 uF non-polarised (pin 5 to ground, original circuit specifies 0.047 uF)
  • 1 x 2.2 uF polarised (bypass capacitor, pin 7 to ground)
  • 1 x 0.1 uF polarised (pin 3, signal in, high)
  • 1 x 0.022 uF non-polarised (pin 1 to pin 5, bass boost circuit)
  • 1 x 100uF polarised (pin 5, just before the speaker in the circuit, original circuit specifies 220uF)

The instructions on Hack A Week call for almost entirely different values. I found that by swapping out different capacitors I could get better results.

Other components

  • LM386(N) chip
  • 8-pin DIP socket
  • 9V battery and connector, or 9V power supply
  • 8Ohm 0.5W speaker
  • Wire
  • 10K potentiometer (I used linear, but a logarithmic potentiometer is probably better) and knob
  • Switch
  • 1 x power and 2 x mini-jack sockets (for audio in and audio out, assuming your speaker is wired separately)
  • LED (optional, for a distortion indicator)
  • Breakaway female headers (not shown, optional, to allow you to swap out some of the components)

Enclosure

I used an old pipe tobacco tin, and some perfboard to mount the components for the final project.

Breadboard prototyping

I built the circuit on a breadboard first, and this where I solved most of the problems.

Interference

90% of the time I spent building this circuit I was dealing with unwanted interference, either a hum when no audio input was plugged in, or distortion to the amplified signal.

Some research turned up a few mods designed to address this problem, and after lots of trial and error, I settled on:

  • 1 x 0.047 uF non-polarised decoupling capacitor, between voltage high and ground, near the power in.
  • 1 x 2.2 uF bypass capacitor, between pin 7 and ground.

I also found that a battery caused far fewer problems than an AC-DC adapter, and that a 9V supply performed better than 5V or less.

Pre-amp gain control

Most LM386 circuits allow you to adjust the gain of the incoming signal. I ditched this part, on the basis that I could do it more easily with the volume control on the audio device I’m plugging in.

Post-amp volume control and indicator

At the same time, I couldn’t resist adding a different unnecessary knob – and LED – to the enclosure, so I put in a potentiometer and LED wired in parallel between pin 5 and the speaker. The LED is completely redundant. It lights up when the signal reaches a certain point, which is also the point when it starts distorting, so you could argue that it’s a useful visual clipping indicator, so that’s what I’ll be doing.

Stereo summing

The circuit amplifies one signal only. To amplify a stereo signal you’d need 2 chips and 2 speakers. While you could wire up the audio input to just one of the left or right channels, for some recordings, this would result in a strange loss of signal.

So it’s tempting to just wire the left and right inputs together, but do that, and you could end up with some nasty things happening:

Here is the rule: Outputs are low impedance and must only be connected to high impedance inputs — never, never tie two outputs directly together — never. If you do, then each output tries to drive the very low impedance of the other, forcing both outputs into current-limit and possible damage. As a minimum, severe signal loss results.

So I added a proper stereo summing circuit before pin 3.

Making the enclosure

My reason for making this circuit was largely so I’d have an excuse to make a box. I used an old tobacco tin, and cut a piece of perfboard to fit inside, leaving room for the 9V battery. I figured out where I wanted the external controls and sockets to go, and then marked on the board where I would put each part of the circuit.

Starting with the DIP socket, I started to wire up all the components.

When I had all the board-mounted components soldered on and wired up, I turned my attention to the case.

I drilled holes in the case for the power switch, audio in socket, speaker out socket, volume knob and LED. I lined the bottom of the case with a piece of plastic to avoid shorts.

Before fixing the positions of all the controls, and while I could still easily remove the circuit board, I wired everything up one more time to test the circuit.

Some of the parts were proper panel-mount components with nuts and washers to secure them to the case. Others I’d stripped from old circuit boards and were designed to be PCB-mounted. These I had to mount to the case with hot glue. Clearly this is a less than ideal solution for a component that is put under strain every time you plug something in.

I’m ordering some panel-mount mini-jack sockets, so until these arrive, I left the audio output wires terminated with a female header, to which I can easily temporarily plug in wires from the speaker.

The LED is also mounted with hot glue. I also wired this up via a header so I could replace it if I burned it out.

Once the circuit was fully fitted in the box, I made a friction-fit holder for the battery out of plastazote foam.

Finally, I made a temporary speaker cabinet out of foamboard, to be replaced when I have a suitable box.

You can see hi-res versions of all these images on Flickr.

  • John

    Sir could you help me to connect a 2.7ohm speaker to this amp?

    • Hi John, I used an 8Ohm speaker. I found that when making this, the best approach was to make it all on the breadboard, and swap out the capacitors and resisters to optimise the output and deal with problems like ground hum, etc. So I would suggest doing the same. I guess that if your speaker is lower power, you may want to reduce either the input to or the output from the amp chip by using different resistors.

      • sunil

        Hi Andrew ,
        I using piezoelectric speaker ,To amplify the piezoelectric speaker I used Lm386 low power amplifier ,input of amplifier is very low in terms of mv …I am trying to build circuit with gain of 20 ..but I am not getting out voltage what I expected . could you please help me on this

  • Afternooncoma

    I’ve built the lm386 amp and I didn’t use the 10 ohm resister b4 the 1000uf cap and for the power I put a 420 uf cap and a 0.01pf poly cap. Then for pin 6 I used a green poly cap,not sure of the size. As for the speakers they are 8ohms. It sounds great for what it is. The circuit I used,took me ages to find but after many diffrent circuit designers I found the one that I mentioned.