Mad Scientist Jr wrote:
If this amp powered by a battery?

A 9V battery, although I plugged a 200 mA 9V AC adapter into it and it
worked also, though I didn't leave it plugged in that long - maybe if
it was in longer it would have fried it. I'm not sure how many mA
would be unsafe for this circuit - I looked up 9V batteries to see
what the current typically is, but it seems this depends on the
circuit. Until I find this out I'll probably just use a 9V "battery
eliminator" AC adapter.

The reason I asked is that a battery eliminator will
probably have some hum in its output, and pin 7 is there to
remove most of that interference from getting into the
amplifier.

See the data sheet for the LM386:
http://web.mit.edu/6.115/www/datasheets/LM386.pdf

The supply voltage enters the input to bias the output at
half of the supply voltage, going past pin 7, which is
usually bypassed to ground by 10 uF or more to smooth the
bias current from the supply. Even with a battery supply,
the voltage will bounce around a little from the current
drawn by the output transistors, and bypassing pin 7 helps
reduce that effect, too.

In this configuration, pin 2 is the most sensitive node, and
anything connected to it should be surrounded with shielding.

I think I'll start with this. Thanks for your detailed input! !

Good luck.