A Crazy Ground Hum At The Gig

[tonebender]

We played down near the beach in Dunedin Florida this past Friday night. A cool place to play and we were stoked to get in the line up. We are sort of a bare bones type band. Our entire PA is one of the large Bose subwoofer and stick top. We position it back beside the drummer. Our drummer only uses a cocktail kit with the one drum that is a snare on top and kick on the bottom. I have a powered 8" Alto monitor and my bandmate has a smallish QSC powered monitor, I think it's a 10". The board is an 8 channel digital. I have nothing to do with the PA except running my cable from my mic, acoustic and monitor speaker to the board for them to plug in.

They fired the PA up about 30 minutes before start time to do a sound check and there was an incredibly loud ground hum from the Bose. They immediately shut it down and started evaluating the power situation. I stayed out of it, too many cooks in the kitchen can cause friction when there is a problem. They unplugged the main connection and the monitors worked fine and were quiet. Then the checked all the connections and ran a new cable from the board to the Bose. Still got the same hum. They put a power conditioner multi plug outlet in between the house and the Bose. Still got the hum. Now it's 15 minutes to show time. They decide to go to the back up board. They ran out to the car and got it. They powered it up and only plugged the Bose in and voile, quiet as a mouse and working fine. So, they started transferring all the cables to the spare board. As soon as they plugged my monitor cable into the Aux output BAM, ground hum back. The weird part is my monitor was powered off but the ground is still the ground even off. My monitor was plugged into an outlet at the front of the stage and obviously that is where the problem was originating. We quickly ran an extension cord to my monitor from another power source and everything was up and running just in time. Weird how my monitor was just fine with no hum but it would transfer hum to the main through the board Aux output. That is the first time I have ever seen anything like that but now I know. Maybe one of you electrical geniuses can shed some light on how that can happen.

[mozz]

I'm guessing here, the original outlet you used was wired backwards.

[BatUtilityBelt]

I'm not going to hazard a guess as to any house issues, but will suggest grabbing a few direct boxes for playing anywhere... Having a ground lift switch can be a great problem solver.

[tonebender]

The circuit my monitor was plugged into had a house fan on it but so did the circuit on the back of the stage as well. I think maybe that fan was causing it. Ground lift and conditioner was ineffective. It was just weird that my monitor had no hum when on and working but even powered down when connected to the board it was causing the hum in the mains. Once we changed the power to the monitor it went away so it was definitely associated with that power source yet did not manifest itself in the monitor. That was the real question and mystery. I have plugged into bad circuits any number of times over the years but the way the hum only routed itself through the Aux signal cable connection to the board and then out to the mains was just strange.

[andrewsrea]

+1 for @mozz.

I've run into many dangerous venue wiring, especially outside. This could have been a hot-neural reversed, a ground-neutral reverse or that the other outlet was connected to a different sub-panel which had a separate ground path to earth.

Best bet is to carry a basic outlet tester in your gig bag (Amazon, Lowes, etc.).

The most dangerous I've encountered was a little pub which some bone head utilized different legs of 3-phase industrial power, to each side of the stage. This resulted in visible sparks on the sweaty bass player and guitar player's arms as they shook hands, after a song performance they were happy with. Dropped them both to the ground, as they were both touching their strings. Something like 80VAC potential difference between the two sides of the stage grounds. Unfortunately, you'd need to have a DMM with super-long test leads to check for this illegal condition (splitting legs of 3-phase is only allowed for specific uses, under the IFC).

FWIW: the pub owner was nice and I helped him fix it, by consulting with the licensed electrician he ended up hiring.

Since today's musical equipment and lighting is a fraction of power demand in amperes, compared to 20 years ago, I'd recommend using one outlet for all sound gear. Avoid ground lifts in outdoor venues and check the outlet with the tester. A digital PA, a vacuum tube Fender Princeton, a digital Bass amp (like a Trace Elliot ELF), plus small electric accessories, pull about 8 amps running full-out. Easy for a 15A or 20A outlet to handle.

Put your lights, fans, etc., into a separate circuit, if possible.

This thread reminded me of days gone by, where my light system guy would have to climb the pole to clamp onto the 120VAC mains, to run our 4000w light show! Most clubs in the Philly suburbs didn't have the electrical system to run the 20A of sound equipment and the 34A of lights. Some only had 100A service, which also needed to run air conditioning, refrigerators, lights and big old TV's. It is a wonder we didn't cause electrical fires!

[tonebender]

Thanks for everybody weighing in. I have no doubt the hum was coming from the power and probably some type of bad ground but the mystery still remains as to how the monitor speaker worked fine and was quiet plugged in the "bad" outlet but somehow when the aux signal cable was connected to the board suddenly the main had the problem with the monitor still quiet and working fine. It did not matter whether the monitor was powered up or not if it was connected to the board it causes roar in the main. I guess that is just the way the circuitry in the system is wired.

[mozz]

Lake Ariel hotel/bar up here in PA. About 1984, I was running sound and another guy was running lights. Band was called "Lotus". Used to say it stood for "lords of the unknown star, or lords of the underwear stealers" Maiden. , Hendrix, heavier classic stuff. Marshall and Ampeg stacks etc. Wish I had pictures to describe this better. Bar was square in the middle of a giant room. In the center of the bar was a big hole in the creaky hardwood floor and the bartenders would just throw the empty bottles in, went to the basement, dirt floor. Building was 100 years old, few stories high. Light guy goes to the basement to find power for the light show. He's standing in a foot of water next to a giant smelly 5ft pile of empty beer bottles. Pulls out these giant jumper cables and hooks the light show up direct to the old fuse box where the power comes in. We all survived. The place either burned down or was bulldozed years later.

[BatUtilityBelt]

Ground lift and conditioner was ineffective.

Oh. Given that, I'd not plug my gear into that outlet anymore.

[Tonray's Ghost]

Crazy stuff..here in Thailand the streets in Bangkok are a bird's nest of electrical and comm cables, so much so that it is never advised to touch anything metal when you are crossing the road on any of the numerous pedestrian walkways....one never knows when your time is up. Take your chance with a crazy moto taxi driver taking you out on the road or getting fried overhead...ha ha ha ha

Ok.. couldn't resist an update with real pics. Was in Bangkok Friday and walking across the highway note the wires in close proximity to the railings. Yes that is my hand tempting fate. The light fixtures from a local shop are jury rigged from the shop tin roof straight to the overhead lines running up the handrail of the overpass stairs..no I didn't actually touch the rail..get to live another day.

IMG_20240809_125007.jpg

IMG_20240809_124958.jpg

IMG_20240809_125036.jpg