Hi Igor.
GaryB's suggestion (lift ground) is a good one *but* be very careful... I suspect you're lifting ground on the mic pre? Then do NOT use a mic (or guitar etc) with that mic pre!
(I've learned the hard way to make sure I don't touch anything that isn't grounded.
)
If you have a ground loop there are a few ways of dealing with it. There are a few different "ground" channels for any given audio device:
- Audio ground: this is the reference point against which unbalanced signals' amplitudes are measured.
- Earth ground: this is the 3rd prong in your AC power cord.
- Chassis ground: the metal casing on your device is a ground channel, too!
The signal ground and the earth (AC power) ground usually provide you with the "ground loop":
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Device 1 <----signal ground---> Device 2
| (via audio cable) |
_|_ _|_
||| |||
Wall outlet <-------------> Wall outlet
This loop antenna picks up noise from left, right and centre, and the noise signal is carried by the audio ground channel. When you measure your audio signal against it, your measurement fluctuates with the noise.
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Non-noisy ground channel:
________________
Noisy ground channel:
////////
So if you're getting a ground loop here, Igor, then you need to cut the loop. What GaryB suggested is the following:
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Device 1 <----signal ground---> Device 2
| (via audio cable) |
_|_ _|_
| | |||
Wall outlet Wall outlet
(not earth-grounded) (earth-grounded)
There are a few safer techniques for eliminating ground loops:
- Single-point grounding (common for recording studios)
- Multiple-point grounding
- Floating connections
- Telescoping shields (I think this is the technique GaryB suggested)
The basic idea in single-point grounding is that you only have the audio ground connected to earth (and chassis) ground at one single point. I've never actually done this. If anyone has, I would greatly appreciate hearing how you went about doing it.
If you decide to pursue the ground loop elimination route, be prepared for a slog. It's not simple stuff.
Sound Reinforcement Handbook is a good book for discussion of proper grounding techniques. You should also be able to find online articles that cover the same material. (Do a search with Google.)
Having said all that...
BingoTheClownO's idea of grounding the computer via radiator / plumbing pipes / etc is an interesting one
, but... If you lift the ground on a particular device, and then connect its chassis to a radiator / plumbing pipe, you *may* be grounding the device properly (i.e. avoiding shock) and you certainly will eliminate the ground loop. But on the other hand, are you really sure your device is grounded?...
Attaching all the chassis of your devices together would presumably introduce *more* potential for ground loops. I'm certainly not an expert at this stuff though, so I may be missing the point?
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_________ chassis ground _________
| |
V V
Device 1 <----signal ground---> Device 2
| (via audio cable) |
_|_ _|_
||| |||
Wall outlet <-------------> Wall outlet
I read the bit about the Furman power conditioners. Now traditionally a power conditioner is meant to prevent your gear from frying when you get a power surge (because of lightning or whatever else). Sometimes they'll also regulate voltage, to clamp it to (for example) 105-115 volts, even if the input from the wall outlet drops to 90 volts or pushes up to 130 volts.
The music-specific power conditioners you cited do also apparently eliminate a substantial amount of noise, by using some kind of filtering magic.
Specs are always misleading
, but if those conditioners really do eliminate 10 dB of RFI noise in the normal audio spectrum < 10 KHz (who cares about 1 MHz) then that means they're cutting the noise by about 3 times.
Here are a few things to check:
- Are the chassis of your devices touching? Are they in a metal rack? Try separating them, so that they don't touch each other.
- Try to isolate the problem. Deal with 2 devices at a time (for example, your computer / Luna 2496. and the mic pre) and keep adding devices one-by-one until you get noise. Try starting with a different pair (e.g. Computer and K2000) and see if you get the same results. This may give you extra clues.
- Keep amplifiers and lights well away from your other audio gear. Power supplies (for example, for mixers) can be noisy too. Don't use lights with dimmers! Don't use fluorescent lights!
- Make sure your cable isn't the source of the problem. Behringer makes a cable tester that is cheap but works very well (as long as you keep the batteries fresh...). If you have corrosion at the connections, or a bad solder job, or a slice in the cable, you'll get noise by moving the cable. If the noise is constant even when you're moving the cable, then the cable is definitely *not* the source of the problem.
- Bring in the electrician! (See GaryB and Astroman's posts.)
Now my experience with noise is very particular. I have certainly not dealt with it enough to know exactly what you're encountering here. But if I had to hazard a guess, I would say that the MP3 you posted sounds to me more like interference from a light (such as a fluorescent light or a light with a dimmer switch) than your traditional ground loop.
In fact I may be completely out to lunch here, because I don't think I've ever even heard ground loop hum. (Maybe this is because I'm using the Furman PL-8?!?!? I didn't even know it had any filtering until you pointed it out!) I *have* heard radio stations from Seattle loud and clear -- that is how ground loops have manifested themselves in my studio. Not at all like your MP3.
I have, however, heard lots of other noise sources (monitors, cell phones, lights, fans, power supplies, etc) that produce buzzing very similar to your MP3.
So just make sure you carefully check all the other electronics that might be affecting your audio gear. Turn them all off and see whether the noise goes away. Even if I'm way off base, it'll be worth it, in the end, to be thorough.
Good luck with it Igor! Keep us posted on your progress, eh?
Cheers,
Johann
