Noise problem possibly ground loop with mbox mini

[Kiawa]

Hi everyone, my first post here, and a newbie to computer recording. I am completely new to both macs and pro tools. The last two weeks have been a very steep learning curve with too many problems and many embarressing mistakes in spite of 8 months research before I purchased my new pro tools rig. After installing and uninstalling and reinstalling two versions of PTLE many times I now have everything basically running, but now with an annoying noise problem. I have done several searches of this forum but not turned up anything that I recognised as useful for my problem, although many people seem to have similar isssues.

First my new rig:

17" iMac MA590X/A 2Gb ram 2Ghz 160Gb HD on OSX 10.4.8
External HD - Lacie D2 Extreme 250Gb firewire 3 partitions: 1 Bootable backup 25Gb 2 personal stuff 50Gb 3 audio 175 Gb
Behringer UMX49 midi controller (not hooked up yet new to midi so leaving this alone until my audio is sorted)
Mbox Mini (shipped with 7.1.1). I qualified for the free upgrade to 7.3.1 which i have downloaded and installed. I have also installed the 7.3.1cs2 update. I have not yet installed the ignition pack will do that today sometime.

Old rig (which I have hooked up to the mac/PT rig):

Roland VS-880Ex MTR (will probably take this out once I have transferred all my songs into pro tools)
Yamaha MG10/2 mixer
Boss DR-880 drum machine (this is probably gonna be ditched depending on how i go with BFD)
Line 6 Pod original (upgraded chip to 2.0). A keeper. It just works.
Cheapo portable CD player
Yamaha KX-80 cassette deck
2 x SM58
5 x SM57
Alesis RA-100 power amp
Alesis Monitor One spkrs
2 Strats, 1 Squire bass, 1 DM Martin acoustic, all lefties!

Signal chain is:
Gtrs (via pod), bass, mics, VS880EX, CD, tape and DR880 > MG10/2 > mbox mini > monitors
mbox > mac > mbox via usb

Took me 10 days to finally achieve sound. I am now at the stage where I can do a simple record and playback! yay.
But...... theres a whine in the monitors mostly from the tweeters, its also in the headphones. It is loud enough to be distracting and annoying, not something I am prepared to live with. Yesterday a friend came over, and we pulled my studio apart, put a new desk in, reset my rig initally with just the mac mbox and alesis monitors/amp. Nothing else connected.
Opened PT - theres the whine. Closed Pt
Opened Garageband - with mbox selected as audio - the whine again.
Same with Audacity, but only when I arm a tracK for recording. Havent messed with this programme too much yet. Dont know about iTunes, haven't used it or quick time.

My friend identified the noise as a ground loop. I was convinced it was the mbox. So we started messing. I got my Onkyo stereo amp/reciever from my home stereo and put that in to drive the spkrs, took the alesis out. Why? Because the aslesis ra-100 has a three prong plug and goes to ground (I think). The Onkyo has a two prong double-insulated plug with no ground. Cranked the system up - NO WHINE! I then hooked up all the gear listed above, and still no whine. All of my hardware have two- prong double insulated plugs - the pod, the yamaha, the vs880 and dr880 etc. The pwr amp is my only hardware device with three prongs ie its grounded.

In another post I read this morning after searching the forum, digi said some computers sent noise down the usb cable which contaminated the audio. This was in ref to pc's, but i am getting this whine on a mac.

Is it reasonable to suppose that there is some firmware/hardware design of the mbox that allows ground loops to develop if connected to certain hardware, be it a computer, pwr amp etc.?

Whatever the cause, it seems that an amp with two-prong double insulated plug solves my immediate problem.
The Onkyo however is unreliable, I stopped using it because of audio drop outs usually the right channel would simply drop out into silence. I have had this problem fixed twice, 2nd time didnt fix it and its not worth spending another dollar on it. I have it working at the moment, but it requires me to pwr on/off several times, flick the volume violently several times before both channels scratch their way to sound. Sometimes I just hit the bloody thing. When it is working the sound is good and NO WHINE.

But a permanent solution: new power amp with 2 prong plug (can you get studio pwr amps like this?)
or buy a consumer level amp/reciever to drive the monitors?
or is there some solution to the grounding issue with the Alesis RA-100?
or is there a solution with the mbox?
or is it my brand new imac?!! NOoooo!
Is there some other solution that will address ground loops and audio noise?

If I upgrade my monitor to say self pwrd KRk's, am I going to run into the same grounding issue (if that is what is which does seem likely to me) and the same whine? I am not prepared to live with the whine, on my rig it is too loud, and volume settings anywhere in the system make no difference to the whines presence. It' s simply just there.


Many thanks for any comments or pointers.

[M.Brane]

Ground loops usually produce a hum (60hz in US) not a whine.

A whine is usually due to some kind of EMI/RFI pollution on the signal lines or the mains. This can be aggravated by improper grounding. Anything that uses a wall-wart PSU or has a 2-prong mains connector cannot be properly grounded as there is no direct connection to ground. Unbalanced signal lines only add to the problem.

Gotta love consumer electronics.

The best plan initially is to make sure your mains are wired correctly (polarity, and grounding) there's nothing on the same mains circuit that will pollute the line (like fluorescent lighting, dimmers, electric motors etc.), and that all you gear is all plugged into the same circuit.

If the noise persists then I'd start looking at gear placement, and low-level signal cabling. Use balanced lines wherever possible, and keep cables away from noise sources. If you have to cross signal lines, and power cables do so at right angles not parallel.

BTW computer CPUs are excellent sources of EMI. That's why my G4 lives under the desk away from critical signal lines.

[Kiawa]

Thanks for responding.

All my gear is plugged into the same circuit.
As much as possible I have kept audio and power lines apart and made sure in a couple of places I crossed the lines at right angles.
The only fluro is in the kitchen on a different circuit but I tested it anyway. The whine is still there light on, light off.
No dimmers or electric motors or anything of that nature.
Guess that leaves an incorrectly wired house - I rent so that makes things a bit diffucult, have to speak to the agent about getting that checked out.
Or my imac. Digi in another post (I will see if I can find it again) said they have found on some machines computer noise gets sent down the usb which is I guess you were referring to and is likely why it looked to me as if the mbox was at fault? If its my computer is there a remedy?
Testing the whine again just now in garageband, i noticed if the mix knob on the mbox is turned hard right there is no whine. As soon as I start to bring it back towards centre the whine is there, loudest peak at 12oclock. Is this an indication of where my problem lies?
I will look at where I can use balanced cables.
I'm not sure what low level signal cabling is - can you give me an example?
I will see what I can do with gear placement.
Thanks Mbrane for the suggestions, guess i will just have to keep trouble shooting.

[M.Brane]

Low-level signal lines are anything before your preamp i.e. mic or instrument cables.

One possibility is that the grounding of the power amp is creating a "carrier" current to flow through the other gear from input toward the output, and brings the noise along with it for the ride. You could try lifting the end of the shield on the power amp input, but this is not an ideal solution. You could also try an isolation transformer like the Ebtech hum eliminator.

I had some hum/noise problems here back when I was using a little ungrounded mixer for input. I found that grounding the chassis of the mixer made the noise go away since that caused the ground loop current to flow opposite of the signal.

Some additional reading material here.

[D. Reck]

I had the same problem with my new imac. i was getting a whine whenever i plugged anything into my mbox (keyboards, etc.). I plugged the imac into an ebtech hum x, and the hum x into a furman power conditioner... that solved the problem immediately.