How to Eliminate
So I broke down and bought some monitors to replace my Altec Lansing PC speakers. Under a tight budget, I bought KRK Rockit 5’s ($300 a pair). Love their clarity and definition, though the bass is a little weak, but acceptable for what I do and listen to. Sure brought out the flaws in my old recordings.
Here’s my problem, though. I’ve hooked them up from my PC card using a 1/8" mini plug to an RCA plug into the KRKs. There’s a distracting hum! It is not noticeable when playing back music, but noticeable when no sound is playing.
The monitors also have a TRS and a balanced 1/4" inputs.
What’s the best way to hook these up to eliminate the hum? Are there shielded RCAs? I know I can go PC to mixer, then RCA plug of the mixer to RCA monitor plug (I should test that to see if hum goes away). However, I want to keep the monitors playing off the sound card, as the family uses them to listen to music, and that would require turning on the mixer everytime the PC is on.
One other thing, the hum gets noticeably louder when I start up n-track, versus just running winamp or anything else. Why is that?
If you listen to the soundcard o/p through 'phones is the hum still there?
If so, then the problem lies in your PC.
If not, then it could be your cables, or you may have a ground loop.
Are they close to the computer video monitors - these generate enormous amounts of hum if the speakers aren’t shielded. Is the hum 60hz? Could be a ground loop if the speakers are on a different power circuit than the computer. Do they make the same hum if you move them elsewhere?
“The monitors also have a TRS and a balanced 1/4” inputs.“
Eh? TRS are 1/4” balanced do you mean XLR?
.-=/gp=-.
No hum when listening with headphones.
Hum does not change when I move the speakers around, and I think the monitors are shielded. Moving the wire, however, can affect the intensity of the hum.
Speakers are plugged into a power strip that is plugged into power strip that the PC is plugged into. How do you ID a gound loop hum?
Are there shielded RCA wires?
Oops, meant to say that the monitors have TRS and XLR inputs.
don’t leave them this way, but a quick check to see if it is a ground loop is to get a ground lift adapter for the outlet (one of those 3 prong to 2 prong adapters). If the hum goes away it is a grounding problem.
If this is the case, your best bet is going to be running it through a cheap mixer, and see if that helps, if not, then you’ll have to use some form of transformer.
One thought…is there a ground lift switch on the monitors? if so, have you tried it in both positions?
Val, even of the speakers and PC are plugged into the same power strip, you can still get a ground loop.
All it takes is a few feet of cable that make up a turn of a “coil”, and that will get a tiny mains ripple induced into it, and there’s your hum.
So, as suggested by others, there’s different ways of removing a ground loop; use a lifter, or remove the ground from the speaker PSU, or only have one end of the audio cable screen connected.
But, looking at your cables…why do you ask if there is such a thing as a screened RCA? It should be screened anyway.
Is this a cable you bought or made up?
Best way of making up the sort of cable you need is to use small gauge high quality co-ax, (two pieces, you can still fit them into the mini-jack if you remove the strain-relief spring).
But I still suspect a ground loop is the problem, especially as the level of the hum changes when you move the wires.
Anyway, if the hum is there on a recording, and it’s a keeper, all is not lost.
I don’t know about the US, but in the UK most audible hum is 150Hz. (It’s the third harmonic generated by the 3 phase Delta/Star arrangement of local transformers).
So a narrow steep notch at 150Hz and another at 50 Hz will get rid of it, (and just tell the bass player he’s not tight enough, and he has to come back for overdubs; he won’t mind, honest! 
)
Why do they hum? Because they don’t know the words! I thought everone knew that!
Seriously: if moving the cables makes a difference, try higher quality cables.
In the US, the main thing we hear is the 60 cycle hum, but yes heavily laden in harmonics. Sometimes only the highest harmonics, making it more of a buzz than a hum.
To help minimize ground loops, assuming they’re powered monitors, plug the monitors and computer into the same power strip. But if moving the cables matters, I doubt that this is the main problem. (It could be a contributing factor, though.)
Recently I noticed a big difference in buzz depending on whether I plugged my powered mixer into the wall at the same socket as my power strip versus into the power strip. This was a live playing situation, thus the powered mixer. However, that power strip is getting old in the tooth, and I suspect it might not have a good ground conductor (time to chuck it, frankly).
First of all, ignore my comment re recorded hum as it’s patently obvious it’s only a monitoring problem; brain freeze I’m afraid. 
But the secret to avoiding hum, is one good ground, and one only.
Everything else ought to be star grounded to that one point, (i.e., well, like a star really. ). And no multiple ground paths.
When we did studio installation, we never trusted the power company’s ground, but always made our own, a nice big copper plate buried deep into permanently damp soil, and the incoming power earth clamped tightly to that. And then careful checks that all equipment PSU’s, all unbalanced cables, were routed to that.
Thanks everyone for your help.
After trial and error, I discovered that when the PC screen was plugged in, hum, unplug the tube, no hum. I plugged the screen into a three prong to 2 prong plug and into the power strip, and viola, no hum.
What’s the danger in keeping this setup (screen plugged into a 3 prong to 2 prong into the power strip)?
Also, why would opening n-track increase the hum? The hum is also gone with the adapter plug.
No ground lift switch on monitors.
Last year I had a hum that was caused by my cheap monitor. I ran my input cables from the mixer into the computer but the wires went around the back of the monitor to get to the sound card. I just moved the computer case so the input cables didn’t go near the monitor on the way to the sound card and that fixed things for me.
The third prong is an important safety device. If the unit happens to fail in such a way that raw AC current is applied to any external parts, it’s potentially lethal. A single layer of plastic shielding isn’t considered sufficient; if a manufacturer wants to avoid the 3rd prong they generally have to double-shield with plastic.
However, there may be an alternative to consider: replace the wall outlet with a GFCI outlet (Ground Fault Circuit Interrupt, a.k.a. GFI). This is something any reasonably handy person can do if they’re smart enough to know how to kill the circuit they’re working on and read the instructions. Before relying on this I suggest you consult someone who knows more than I do, to make sure it’s at least as safe as the the 3-prong cord. The GFCI outlet will make your workstation safer in any event and won’t do any harm. But I’m not sure whether it overcomes the danger of a lifted ground.
BTW, I suspect that the danger here is less than any vintage guitar amp with a 2-prong outlet.