I "permanently" installed a soundhole mag pickup in my acoustic tonight. Real simply, just the pickup, a 500K volume pot, and an endpin jack. It works - the pickup picks up, the pot make it louder and softer. The pickup is humbucking, and it's dead silent with the volume wide open. But...
As I roll the volume down, 60-cycle hum becomes audible. The guitar sound is pretty much gone by the halfway point, but the hum is at its loudest. The hum doesn't go away until the pot is nearly at zero. I've never heard a guitar hum like this at mid-volume.
I used the same kind of cable from the pickup to the pot and the pot to the jack. I'm 90% sure that I wired it right; that is, pickup hot on the top of the pot, line to the jack on the wiper. The bottom of the pot and both cables' shields are soldering to the pot case.
However, there's that 10% doubt - I might have gotten the "top" and "wiper" wires crossed since they look identical. So here's the question: Would getting those wires backwards cause the odd humming behavior?
The pot is hard to see, and I'll have to unstring the guitar and remove the pickup in order to check it. So, I'd like to be more sure of the problem before I tear it apart.