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I'm repurposing some Harbor Freight megaphones for amps. Everything is okay as far as switching out the mic for a jack, replacing the speaker, and adding a rotary volume control in place of the slide pot. Only problem I'm having now is an annoying hiss through the speaker. When I touch various spots on the volume pot or circuit board, the hiss gets much louder, which leads me to believe it's a ground problem, but I can't track it down. Any ideas?
Could be a ground loop, although I've seen your wiring, so can't imagine where the loop could be. Obvious first things are: check the soldering on the vol pot to see if you really scraped through to base metal on the back of the pot. Make sure all ground leads converge on the back of the vol pot, before sending a single ground lead to the jack. Or maybe one or more of your wires is unshielded? Amp too close to the speaker magnet (tough to fix in a small box)?...
You could try also shielding the amp with copper foil or tape...