Ground loop experts HELP!

[neeel]

Hi all,

I am having a ground loop issue that I know is experienced quite regularly on this forum. Basically the noise is the screen redraws, mouse movement, hard drive and CPU coming out from my monitors.

All inputs into my mixer are 1/4" TRS balanced and I have 3 stereo inputs into it as follows;

MBOX - Balanced cables 1/4" TRS to TRS - MBOX Has Balanced outs TRS
MAudio 2496 - unbalanced RCA to 1/4" TS - 2496 Does not have balanced outs
Creative SB Live - unbalanced stereo 1/8" jack to 1/4" TS - SB Live Does not have Balanced outs

Here is the weird part.

From any of these inputs in isolation I experience no noise. Plug the M-Audio and Creative SBLive sound cards in and again no noise. Plug either one of the two sound cards, or both, with the MBOX I get noise.

How can I isolate the cuplrit if I get this combination? Could it be that the USB bus is not grounded yet the PCI Bus is? Or Vica Versa?

Your help is appreciated
Regards
Neil.

[sw rec]

My first inclination would be to make sure you're deriving all your AC from the same outlet. I experienced unbelievable ground loop problems when trying to power assorted gear from different outlets.
The standard ground-lift practice is to lift the ground at the END of the cable run (i.e. if it's going FROM a mixer to your M-Box, then lift the signal ground at the M-Box end.)
A more expensive alternative is to use BALANCED AC (made most popular by Equi-tech, but I think Furman makes em now.) I have EVERYTHING in my room being powered from an Equitech supply, and have never had to chase a ground loop.
Hope this helps. I know it's frustrating.....

[neeel]

Hey sw rec,

Thanks for the suggestions. I am already running all the hardware in my studio from one outlet.

I was trying to avoid either purchasing an AC provider or lifting grounds. however, at present I am not sure I have any choice?

Neil.

[daeron80]

That's not ground loop, it's RF. The reason you get it only with all three is that together they make a better antenna. Things to try could include moving the mixer, turning your CPU at right angles to its current position, replacing your CPU case with one made of a heavier guage steel, placing a sheet of lead between the CPU and the mixer, using shorter cables between the three outs and the mixer, and so on. Since one of the pairs of wires running to the mixer ins comes from the CPU, it may not be easy to solve. Your antenna runs right up to the broadcast tower, so to speak.

[randyT]

I have been able to stop problems with the noise caused by USB by using isolation transformers. In my case I had to isolate the audio going into my 002R from synth's that use usb. In your case, I think you could put an iso xfrm between the M-Box monitor outputs and your monitor amp inputs (or powered monitors). I suspect the noise is getting on the M-Box audio grounds via a ground loop caused by the power connection that usb provides and not because of the AC power system in your house. Remeber that USB is capable of powering a USB device and there in lies the possibility to pick up the ground loop IMHO. I don't think this is an RF issue because if it were I would not have been able to elliminate the issue with iso xfmrs in my system. However, there is always the possibility that your problem is not related to the problems I have had. I tend to doubt that though. I have suggested that the iso xfmrs might fix this problem before but no one has commented yet on if it fixed the problem. I'm still currious about it.

randy

[neeel]

Hi Guys,

Thanks for your help.

RandyT, by saying iso transformers, do you mean a DI box with a ground lift? Id so I have thought of this option and am going to buy a stereo one and put it in between the Mbox and the mixer to see if this aleviates it.

It would be good to track down the main cause of the issue though. Any other ideas?

Rgds
Neil.

[bb_aus]

why do you need all of them in there anyway?
i can not see any purpose of the creative

[neeel]

Hi bb aus,

I thought someone might ask that.

I have two versions of xp on my pc, one general windows and one for pro tools.

On the protools partition, I use the Maudio 2496 for midi ins and the MBOX as my sound card - obviously. I have the creative card disabled on this partition

On the general windows partition I use avid freedv to do some video editing. This program is about as picky as you can get. So much so that it will really only work effectively with creative cards. Hence the need for the sb live.

I could on either partition take out the offending party/ies. For example connect just the mbox to the mixer on the pro tools partition and just the creative or 2496 on the general partition. Buy this frankly is a pain in the a£$%

As a side issue. Digidesign are owned by Avid, Why can't Avid make freedv work with digidesign products? Seems a no brainer to me and probably not expensive as they do with the higher spec avid products.

rgds

[bb_aus]

Why can't Avid make freedv work with digidesign products? Seems a no brainer to me and probably not expensive as they do with the higher spec avid products.

it would make sence wouldnt it
cant you use the M-Audio card with avid? they work with everything else i ever tried i would of thought they would work with avid software.

[neeel]

No, I can't for some reason! the maudio 2496 doesn't even get recognised by avid freedv.

Hang on maudio, digi and avid are all one company! I know they would argue their flagship products do integrate, but I ain't spending dosh on video editing as 1 I dont do it enough and for thsi reason it is too expensive.

Rgfds

ps any ideas re the source of the ground loop?