On 30 May, 18:18, Mad Scientist Jr wrote:
I am building an amplifier

(for schematic see page 5 hehttp://makezine.com/09/crackerboxamp/
)

but when I turn up the gain, I get buzz and can actually hear some
radio broadcasts on it.

I assume I need to shield the circuit, or parts of it...

Would lining the inside of the project case (in this case a cardboard
cracker box) with aluminum foil work?

Incidentally, does anyone know if someone makes a type of "shielding"
spray paint or primer, that you could just spray on a plastic or
cardboard project box, that would provide shielding?

Any help appreciated...

I would firstly put a cap on the input to short out radio frequencies
to ground.
220pf from signal in to ground works well for me.
Ground the case too if its metal.