i bought a pa speaker today.
as soon as i plug it in and turn it on (no audio plugged in yet, jut power), the speaker emits a hum, like what you would get from a long run of unbalanced cable. I assumed it was a ground loop, so i slowly unplugged 30+ items in my basement one by one to see if it would go away, but no, still the same hum. I then, using a spare power cable, ripped out the ground prong just to see if that would temporarily fix this problem…but still, i had humming. to make things worse, i take the pa upstairs and use another outlet with a good cable, and there is no hum problem. I CANT figure it out and its a loud enough hum that its drives me NUTS… as you can tell…
ive tried 2 or three different plugs, including ones on the same outlet as my s4 (source) as well as one accross the room, so im confuzed as to what it could be…
if you have no audio connected then the inputs are floating and will amplify mains 60Hz for
US and 50Hz for Uk,
Use balance cables where possible and be aware that its a PA speaker not a hifi or studio monitor, so expect the signal to noise ratio to be less than perfect, the gain involved is huge and any slight difference in a transistor pairs matching will give you noise.
If the hum is audible over the speaker at a working volume with music playing, then you may need an isolation transformer, Art make a dual one for about $40 with a 1:1 ratio so you dont lose any level.
That’s a common line tester we use and find in most store in here. Places like Home Depot and the likes should have it. It’s something like 15$
Not always true. Even without anything plugged in the input jack AND with volume set to 0 (minimum), if the outlet is badly wired (Hot wire in place of Netral, no ground and so forth) you may hear some nasty hum on some Amplifier, Amplified Speakers brands, depending how they’re internally designed some are more "sensitive"to that.
If you’re using the same gear on a properly wired outlet and it fixed the problem… then you have a good idea that you got a bad outlet before.
The tester I posted here will let you know what’s wrong… or give some pointer assuming it’s an outlet related problem.
Even if you use balanced wire, you may solve or not the problem. In some case, you want to use a simple Balanced wire/adapter but with the Pin 1 (shield) lifted. Isolation Transformer (Passive DI) are always good solution if they also have a ground lifting switch.
I did touring all 'round the world as PA engineer and damn, it’s sometime a nightmare to solve them… specially when you’re using lots of KVA power distributions.
There’s a load of trick we used, but some are too technical, or not “Kasher” so I don’t suggest them… even if they works
HIgh likely a CMRR problem here, more often leaky input Capacitor (high esr and so forth) or faulty/leaky input op-amp.
