Hello, this is my first post i think. I realize this is a super common problem, but i can't quite figure it out, maybe something with the shielding in the wires? interference from some other electronics? Or just faulty wiring within the house? But anyways the problem is an intermittent low frequency hum heard in the subwoofers of my JBL mr838 and christ its annoying. Any ideas? I can supply more information about my setup if needed. Any help is good help. And i don't think they're trying to sing and just don't know the words, to go ahead and get that out of the way from the start.


sry for the double post, i can't figure out how to delete the first one.

looking into my crystal sphere, I see the problem isn't caused by the subwoofer, but by the groundloop between computer and stereo equipment
To clear up some fog, you might give us additional hardware info....

Ok heres the setup,

PC with creamware's scope project
Creamware A16 Ultra AD/DA converter
Crown XS900 Amp
JBL MR838 Speakers

PC -> AD/DA via firewire, AD/DA -> amp via 1/4", amp -> speakers via banana-1/4"

Does the hum happen repeatably, or intermittently (i.e. non-repeatable)?

For the latter, try shielding your speaker cables, i.e. running them through a grounded metallic conduit (a run-of-the-mill plumbing pipe will do).

If the hum is 50 Hz or 60 Hz (depends on your mains source), then I suspect a leakthrough of Common Mode noise in the amp. Try grounding your amp better e.g. plant a 150m metal rod in the soil, connect a metal screw (or grounding terminal) on the amp to the rod. Or if your house has reinforced concrete, punch a 15cm nail into the concrete leaving 1-2cm, and ground your amp to the nailhead.

The cables should be sheilded. I don't wanna cut them open to see, but if i remember correctly when i bought them, I was sure to get balanced cables. That grounding solution seems like a hell of a lot of work just to simply ground an amplifier which should be grounded already in the first place. Any other solutions? I've been reading about ground lifts, and thinking about trying that out. And i've also just now put everything on to one power strip where as before it was split between two outlets which i think could've been the problem. I haven't heard any hum yet, but then again i haven't needed to dry clothes or anything today.

Grounding your equipment may remove this type of noise. Regardless, grounding your electrical equipment is a good idea anyway.

Be aware that hammering a copper rod into the ground is the way to create a ground connection, but it may also be illegal (depending on your countrys electrical codes). At least in Denmark, the grounding in your house must be common. So you can hammer all the copper rods into the ground that you want (if you're an electrician), but they must be interconnected. Regarding hammering nails into concrete floors, I have my doubts about that idea. The ground resistance will probably be way too high to be of any use (and thus illegal). Normal ground fault interruptors (30 ms / 300 mA) require a ground resistance of max 1666 Ω.

To conclude, unless you're absolutely sure what you're doing (and are willing to ignore the legal issues of DIY jobs in that case), leave it to an electrician to sort out your grounding.

If possible I would hook up the Subwoofer's amp to normal loudspeakers to see if the problem still exists. This way you can find out if it's your amp or the cables.
I had the same problem and fixed it by building DIY cables using RG58 coaxial cable (10m for 3€) with Neutrik NYS cinch interconnects (full metal, good quality only 1€ each).
You could also try to rearrange your 1/4" cables, place them away from any mains cable. Also if possible, connect the amps and the AD/DA converter to the same extension socket.

Edit: Ah you did that already, hope it works now .

I have a lot of grounding issues in my own house. I have not been able to resolve everything, but I have read that a common problem is grounding of coaxial video cable. Depending where you are from, coax is notorious for grounding problems. If you are driving spikes into the ground, you might want to try connecting it to the shield of any coax ground that is connected to your computer/audio rig.

You could also try hum reducers made for car audio, such as rca to rca interconnects. It is pretty much a transformer that isolates the signal from one side to the other. It has worked a little for me.

In the end, I think my problem is related to old wiring and add-on grounding. I am still searching, but good luck.

If your mains supply does not already have decent ground then you've got bigger problems. As long as all your audio equipment is running off the same socket so there is no opportunity for difference in ground levels between them you should be fine.

The second the is that the earth in your house is unlikely to be clean. This really shouldn't matter to much with modern power supplies unless the earth is really really grotty. In theatres you can have problems with dimmers putting crap into the power supply. In a home setting I'd look for large electric motors such as central heating pumps, possibly washing machines.

If you do determine that this is the problem you can buy power conditioners which should rectify the problem.

You can check that your cables are balanced without slicing them, firstly the jacks should be stereo jacks secondly if the plugs aren't moulded you can unscrew them and you'll see two signal wires connected to two of the terminals and a sheaf connected to the third one. You can use a multimeter or cheap cable tester which can be bought from Radio Shack to test continuity of each conductor.

Even though they are balanced your should keep your signal cables away from your power cables, if they have to run near each other arrange for them to cross at 90 degrees to each other.

If you are really unlucky there could be an issue within one of your bits of kit with the audio ground being coupled to the power ground, I've recently had some issues with radio mics which had three separate grounding issues the last one being due components degrading inside them and causing this issue.

Earth lifts are not relevant to you, the are only important were you have two signals with different earth levels going into a third piece of gear. You can then earth lift on one of the signals so you only have one earth level. It's a bodge to get rid of hum when you can't control the power supply of if you have a mix of balanced and unbalanced gear and DI boxes.

If all else fails you find a local pro-audio dealer and ask them if the know anyone who can come in and track your problem down. They are likely to charge a fee but you then won't spend money on doing things which might not solve your problem.