Pelliott321 wrote:
first thing we did
we have gone through all possible check points for ground loops.
disconnected everything, leaving only power amp and speakers.
no hum in main system but hum in subs
cheated mains on sub amps, no help.
cheated mains on power amp no change.
added each component back in one at a time
no change with either cheating mains or not
we feel there is a problem in sub amp, but want to check out possible DC on mains first
You are not going to have D.C. on the mains. From the power company steam-turbine-driven generators to your receptacles -- there is not going to be a D.C. component in evidence ever!
But noise? You bet. Florescent lighting ballasts, LED power supplies, refrigerator motors, A/C motors, cell phones, Wi-Fi networks, TV power supplies, neighbor's power tools, automobile ignition noise, etc.
Plus to add insult to injury -- you live in a neighborhood where you get your power from pole-mounted 13-kV to 120/240-volt 45-kVA step-down transformers.
Those old neighborhoods typically had eight to ten houses on the same transformer -- all connected to the secondary in parallel.
HUM? Welcome to the wonderful world of chasing ground loops! I finally won that war, but not after many frustrating battles.