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:
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.
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.