Ground loop, what next ?

[BusyGirl]

Hello everyone,

I will try to explain my problem, being french native my english isn't the best.

My setup consists of an iMac, a duet 2, a pair of hs50m monitors and two expenders (slim phatty and futureretroxs). Some weeks ago, i had a serious problem with getting electric shocks from my slim phatty, it was insane. My slim phatty was new so i called my reseller to ask why it was doing so. My iMac wasn't connected to earth unlike my slimphatty, problem solved since i connected the iMac properly.

But now it is grounded, i can hear digital noise in my headphones and monitors, when i move the mouse or move a window. I removed the monitors from my setup and the expanders, no more problems. Now when i plug the expanders, still no problems, but when i connect my jack from the synth to the audio interface, i can hear that noise again in my headphones. It is a ground loop, and i know how to solve this problem, ground lift the cable between the synth and the audio interface using a DI box like the radial JDI.

My problem now is about the monitors, they are grounded. When i connect them to the audio interface, that same noise occurs, but i don't know what i need on each cable. The audio outputs on the duet 2 are balanced, and i use two balanced jack cables i need to ground lift. I have read somewhere that a DI 20 from behringer will do it, but as i understand, it will convert my line level from the duet 2 to mic level, and i don't have a mixer before my monitors.

Can i plug the output of the DI 20 to my monitors, or should i use something else ? I have found an Ebtech Hum Eliminator but i am not sure since my cables are balanced. There is also that Behringer DI800 that can be handy, if i need 3 DI boxes. Is it possible to remove the ground from the cables directly ?

Can someone please help ?

[Killawatt]

Put a couple of inline audio transformers on the left and right feeding your monitors. Nuetrik makes some that are cheap.

[BusyGirl]

I don't know where i can find that nearby, my main store is thomann and i can't seem to find that kind of stuff.

Can i just break the screens on my TRS balanced cables on the speaker's end ? I mean buying something to ground lift my cables will do the same, no ?

It's something like the number 8 on here :http://www.rane.com/pdf/ranenotes/So...connection.pdf

By the way can i make a TS to TRS for the synth as of 14 on previous link ? TS: tip to hot, sleeve to cold, shield not connected <-----> TRS: tip to hot, ring to cold, sleeve to shield ?

[gullfo]

any option to make sure all the AC polarity is the same and the systems are all grounded properly? balanced lines are good because they don't pick up noise easily... i'd first concentrate on getting all the power and grounds cleaned up as much as possible. next up - video monitors and speakers - magnetic fields in these devices will mess with your instrument level inputs as well as line level and anything un-shielded.

[BusyGirl]

Thanks for your reply. Going to test the AC polarity first. I should have a grounding issue at home, it's a very old place.

What a headache ...

[BusyGirl]

I did a test on the outlet i use, my AC polarity is reversed. I checked my other outlets by the way, some are in the right position, others aren't ... Some dude living here long time ago made everything by himself i heard ! I will try putting my wires on the right position.

My ground seems connected correctly, but i can't test if it is a good one or not, i need someone to do that. Should i replace all the outlets by new ones? they look old.

Thank you !

[jhbrandt]

New outlets will have clean contacts and better spring for superior connection. If you are taking them out anyway, you might as well replace them as put the old ones back in. They're cheap.

Be sure to turn off the electricity when you work. Don't test anything with your fingers. - I'm just joking... I'm sure you will be careful.

Let us know if this solves the problem.

Cheers,
John