I'm an nuc engineer, not electrical. Im my discovery, I came to better understand sources of instrument noise/hum. 1) Electro-magnetic fields from near by lighting, etc. This is INDUCED hum that goes away when you turn side to side, getting the magnetic field to stop generating current in the electronics (get parallel to the field, not perpendicular). Builders shield their bass control cavities to help minimize this induced hum, as well. Hum canceling pickups work to cancel out this induced current. The noise is being created inside the instrument, not from a connected power supply. This is the physics of how the power company makes your power. They rotate a large magnet inside a bunch of coiled copper and electrical current is magically produced. Your instrument is a tiny power plant doing the same thing when in an alternating magnetic field. 2) Motor/dimmer noise put into the AC power line by refrigerators, air conditioners, dimmers, iPad chargers (mine creates crazy levels of AC power noise), etc. To control for this, professional installations supply the stage with "tech-power" which is fed to the building from its own transformer connected to the power company separate from the house power transformer, which quickly loads up with noise producing stuff. Don't let folks start plugging motors and dimmers (and iPads!) into such a dedicated tech-power circuit! 3) AC ground loop noise. I think this is your problem as it is in my home. I think this starts with home wiring that is not installed/grounded correctly and small 60 cycle currents circle around in the +/- wiring. They sneak through power wall warts, instrument cables, everything! 4) Instrument grounding. If you touch the strings or bridge and the hum goes away, the instrument parts are properly grounded to itself. Your body is draining the tiny noisy current from the instrument and all the affected parts. If the instrument is not internally grounded, the parts are not electrically connected and touching the strings does not solve the problem.
I had a new (to me) Fender passive P bass that was noisy as heck. I was disappointed. Sent it to the tech who returned it "fixed". Plugged it in at home, same noise, and my set-up is pretty simple. We were both very confused. Researched and learned about ground loops. Took the bass to another building and it was perfectly quiet! I use it at church with tons of lights, radio pack RF signal flying about, AC power running everywhere - perfectly quiet. Bring it home, its a mess. I had a Mesa power amp that exhibited the same issue. I thought it was defective and tried to send it back to Mesa. The Mesa tech told me to try difference AC circuits. Took it to a different house and it was perfect.
), so perhaps at that point.
