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I am trying to use a LM386 to amplify an audio signal so that I can listen to it using a speaker. I am using the recommended circuit from the data sheet:

LM386 circuit

When I connect my audio source to the input, I am able to hear the sound in the speaker, but there is a low-frequency noise in the background. If I connect the input to the ground, that noise persists, which indicates that the noise does not come from the input.

Also, the circuit is powered by a 9V battery, which eliminates the power supply as a suspect.

Using an oscilloscope, I connected the (grounded) input to channel 1 and the output to channel 2. As can be seen in the picture below, there is a periodic spike with a frequency of about 15 Hz.

enter image description here

What could be causing this periodic signal?

I admit that it is likely that I made a mistake in my connections, but this is a rather simple circuit, and I already triple-checked every connection.

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    \$\begingroup\$ Do you have a fairly large decoupling capacitor (ca. 100uF) on the LM386? That's something that was always advised for undergrad labs. \$\endgroup\$ – user2943160 Jun 17 '16 at 23:56
  • \$\begingroup\$ @user2943160 Isn't that what the 250uF capacitor shown in the schematic is? \$\endgroup\$ – mbmast Jun 17 '16 at 23:59
  • \$\begingroup\$ No, that is the output decoupling. To clarify: I was suggesting power supply decoupling. At least +V to -V, or both +V to ground and -V to ground (check polarity). \$\endgroup\$ – user2943160 Jun 18 '16 at 0:02
  • \$\begingroup\$ What other devices/circuitry are in your circuit i.e. powered off the battery? \$\endgroup\$ – uint128_t Jun 18 '16 at 0:31
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    \$\begingroup\$ I'd agree - definitely add a decoupling capacitor and see if that changes anything. Also try running it off a AC -> DC power adapter if you can. And try moving it somewhere else - I've had op-amps and similar audio circuits pick up transmissions before (sometimes in the unfortunate form of rush limbaugh!). \$\endgroup\$ – Wyatt Ward Jun 18 '16 at 1:13
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Adding a 100uF decoupling capacitor as suggested in the comments significantly reduced the noise. I also found that the noise was being induced by a MCU that was blinking a led and was powered by the same battery.

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TROUBLESHOOTING HINT: When troubleshooting, ALWAYS start with the power supplies. Using an oscilloscope (not a multimeter), observe the power supply voltage at the device under test's (DUT) power pins. (In your case the DUT would be the LM386 audio amplifier IC.) It is VERY IMPORTANT that you test the voltage at the DUT's power pins and not at some other location on the circuit assembly.

Also, the circuit is powered by a 9V battery, which eliminates the power supply as a suspect.

Actually, your 9 Volt battery is your prime suspect. Those batteries are not suitable for driving loads like a microcontroller development board, and SD card hardware, and audio amplifier with a speaker, etc. A 9 Volt alkaline battery has relatively high internal resistance, low amp-hour (energy storage) capacity, and other limitations that make it an undesirable power source for the system you've described.

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