HDMI Earth Loop Hum

Robert F Jarvis

Well-Known Member
We are projecting animations onto the cyc. The MP4's have audio we want to pipe to the desk. When I plug the laptop output sound into the board the sound is great. But as soon as I plug the HDMI cable in we get a horrendous earth loop hum! I checked the projector and sound board power cables are grounded properly though one is FOH and the back stage. I removed the laptop charger but his made no difference. Ideas' suggestions welcome. I have to go in tomorrow early to fix this and not sure where to start.
 
I had a similar problem once ... turned out it was because the monitor was plugged into a different circuit than the board (two outlets under the table happened to be on different circuits). The only wire in common was the VGA cable, but that was enough. So, I would suggest the first thing to try is to get everything onto the same circuit temporarily to see if that is it. Use extension cords for testing, if that turns out to be the issue then you'll have to figure out a more permanent wiring solution.

-- John
 
The ART Clean Box Pro is your friend. For <$100 you can make the hum go away, get balanced lines into your sound console, and match consumer/pro levels.
 
Tim's suggestion is a good one, for all sorts of ground loops.

Lately, I've been using a Mackie MDB-USB for laptop sound. It becomes the sound device, and eliminates all sorts of headaches. For example, Some Dell laptops sense an audio cable plugged in as a pair of headphones, and it responds by applying horrible sounding reverb effects to its internal sound device. Apparently, they think people using headphones like polluted sound. The USB box works painlessly with every computer I've tried, so far, and it provides stereo or mono-sum with a volume control.

Mackie MDB-USB
 
Thanks guys. I failed to mention that I tried an inexpensive USB/Soundcard (which must have a solid in/out ground connection to still get the hum right?)
Comments about different power sockets are bang on. One is FOH. The other way back stage - hundreds of feet apart. So this morning I'm going to replace the projector 3 pin with a two pin (IE lift the ground). But I'm going to order the ART Clean Box Pro for a more professional solution.
 
Do NOT lift the ground on your power cable. That is an unsafe practice. You need to lift the signal ground on the audio cable to eliminate the AC power ground loop from infiltrating into your audio signal cabling.

If you're interfacing the laptop audio through a DI box, there may be a built-in ground lift on the DI box, otherwise you need to lift the shield on the cable that goes from your laptop to the mixer. I can advise better how to do that if you can detail what your cabling between the laptop and the mixer looks like.
 
Another contender in the USB audio dongle category is the Peavey USB for $50. Alternatively you might have luck with iso-transformers like Sescoms IL-19.

I've had the most success with fixing computer output noise with some flavor of USB/external sound interface gizmo.
 
Another contender in the USB audio dongle category is the Peavey USB for $50. Alternatively you might have luck with iso-transformers like Sescoms IL-19.

I've had the most success with fixing computer output noise with some flavor of USB/external sound interface gizmo.
^^^ This. If you don't need the other stuff the Clean Box Pro does, the Peavey USB-P is a great choice. No special USB drivers (core audio device) and the sound quality is audibly better than the $0.39 headphone amp chip in the computer.

Edit ps: The USB-P is transformer balanced and there is no ground connection between the USB connection and the audio outputs. Also can do mono.
 
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Do NOT lift the ground on your power cable. That is an unsafe practice. You need to lift the signal ground on the audio cable to eliminate the AC power ground loop from infiltrating into your audio signal cabling.

If you're interfacing the laptop audio through a DI box, there may be a built-in ground lift on the DI box, otherwise you need to lift the shield on the cable that goes from your laptop to the mixer. I can advise better how to do that if you can detail what your cabling between the laptop and the mixer looks like.
Agreed. Have PYLE Mini Hum Eliminator with Ground Loop Isolation arriving tomorrow.
 
Other Options
Radial USB DI, Which is what I have and works great
Whirlwind PCDI (not a usb card (thought I think whirldwind has one now as well), but has all the functions you want reagarding input, output, and Grd Lift and Pads)
 

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