Projector earth loop?

[TomHoward]

I've got a bit of an ongoing problem with what I think is an earth loop in our venue, just looking for any pointers.

Attached a DVD player up to the projector today - the DVD player with HDMI out is on side of stage, fed via SDI boxes and a patchbay up to Christie LX605 on the gantry.

The projector gets it's power locally, the DVD player gets it's power locally.

 

When connected up solo, the HDMI / SDI link and projector work fine.

 

Then I added audio - we took audio out from analogue outputs on the DVD player. This was fed back via XLR ties to the control room, (near the SDI patchbay), where it feeds into an InterM PP-6214 rack mixer, and on to DSP and amps.

When we attach the analogue audio into the back of the DVD player, the image on the projector is wiped out.

 

I'm assuming there's an earth loop, where the DVD player has a common earth between the HDMI connector and the analogue audio outs, the projector has joined the connector earths to ground and the audio rack mixer has joined the signal earth to ground.

 

We've had this same issue with VGA laptops, where the VGA was connected to the projector, and a minijack from the tie lines into the headphone output on the laptop, the VGA survives - just - but has a lot of interference.

We solved the laptop issue by switching to using these Peavey boxes as standard which balance and provide ground lift isolation.

The issue's raised its head again though with the DVD player.

 

Would anybody be thinking this is the same thing? One factor is both of these (minijack headphone and DVD phono out) are both unbalanced so there's probably a link between pin 1&3 at the stage end in the cable.

Is an earth loop like this enough to warrant further investigation or is it to be expected? What would be your approach to fixing it? Is it normal for projectors to link signal earth and earth if that's what's happening? (A bodge of lifting the projector earth on the gantry fixes the problem but obviously isn't ideal).

[timsabre]

Yes I've had this too.

 

My fix was to put transformer-isolated DI boxes in the L/R audio outputs of the DVD player, breaking the earth connection. I used these cheap ones from CPC

 

http://cpc.farnell.com/img-stage-line/dib-100/di-box/dp/DP26026

[sunray]

I have the previous version of these http://cpc.farnell.com/pulse/pls00548/ground-loop-isolator-3-5mm-p-s/dp/AV25542