Trevor Morton wrote:

I thought the LM386 was an audio amp (not to DC), but had a try anyway
with a 5Hz squarewave. Distortion was pretty bad. Also, regardless of
frequency, I only get 1V swing out with 6V in. It is supposed to have
a built-in gain of 20. This I don't understand.

You have to tie a cap from ground to one of the gain set pins to get that.

Graham