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Posted By: NJwirenut VGA ground loop isolation? - 01/15/12 07:14 PM
Is there anyone here who has dealt with ground loops in computer display systems? I recently added a connection between the VGA input of my Sony LCD TV and my PC, and it works, but there is a 60 Hz buzz in the audio and a hum bar rolling through the picture.

Temporarily lifting the AC power ground at the PC clears it up, but obviously that isn't a safe permanent fix. The PC and the TV are already on the same branch circuit. Everything works fine when testing with a laptop (on battery), but plugging the laptop into AC brings the hum back, as well.

The connection between the TV and PC consists of 2 cables, a 15 pin VGA cable and a 1/8" stereo audio cable. An isolation transformer for the audio link is easily available for a few bucks, and has already been ordered. A suitable VGA isolator seems a LOT harder to find. I only seem to find ONE manufacturer of such a device, at a cost of ~$140:

http://www.markertek.com/Video-Equi...tion/Atlona-Technologies/AT-VGA11S.xhtml

Before I drop the money for one of these, I would like to be SURE that it is going to clear things up. Has anyone used one of these gadgets successfully, or found a cheaper solution to this problem?
Posted By: LarryC Re: VGA ground loop isolation? - 01/15/12 09:40 PM
Verify the VGA cable you are using is a true one to one cable. If the cable tied multiple pins together internally, that might be the source of the problem.

pin 1 red video
pin 2 green video
pin 3 blue video
pin 4 monitor ID bit 2
Pin 5 system ground
pin 6 red video ground
pin 7 green video ground
pin 8 blue video ground
pin 9 N/C or missing (Key)
pin 10 Horiz & Vert sync ground
pin 11 if tied to ground, the display is color or Monitor ID bit 0
pin 12 if tied to ground the monitor is monochrome or Monitor ID bit 1
pin 13 Horiz sync or Composite video sync
pin 14 Vert sync
pin 15 Monitor ID bit 3

Can you open the cable shield on the VGA cable? That is probably what is connected between the two chassis.

Edit: Additional questions.

1) Is the computer also tied to shielded network cables?

2) Do the hum bars go away when the audio is disconnected.

3) Does the audio buzz go away when the video cable is disconnected?
Posted By: KJay Re: VGA ground loop isolation? - 01/16/12 12:46 AM
Do you possibly have an available HDMI connector on the TV? If so, I’m wondering if using a VGA to HDMI converter or a combo VGA/audio to HDMI converter would help. I’ve seen these for around $30.00 or $40.00.
Posted By: NJwirenut Re: VGA ground loop isolation? - 01/16/12 12:52 AM
All pins are wired straight through. Tried breaking the shield at one end to no effect. The outer cable shield was tied to the connector shell at either end, but not tied to any of the grounded pins (5-8 or 10).

Network cables not shielded, but computer frame is also grounded via a couple USB cables (to UPS and printer).

Hum bars present with only VGA connected. Audio buzz present with only audio cable connected. Connecting both gives bars and buzz.
Posted By: NJwirenut Re: VGA ground loop isolation? - 01/16/12 01:11 AM
HDMI port available, but trying to keep it open for a bluray player to be added soon.
Posted By: KJay Re: VGA ground loop isolation? - 01/16/12 01:50 AM
If things don’t work out and you want to try it, you can find a decent 3 or 5 port HDMI auto switching hub for around $25.00 these days. This will allow you to connect multiple devices to the one HDMI port on the TV.
Posted By: LarryC Re: VGA ground loop isolation? - 01/16/12 04:32 AM
Did you try disconnecting the USB cables to determine if the groundloop is being generated by a USB device?

Back to basics. You stated the receptacles feeding the TV and the monitor are on the same circuit. Can you verify that the outlets are not miswired?

In my AV experience, hum bars and ground buzzes are most often caused by a slight voltage difference between two "ground" potentials. Possible sources for minor differences include multiple connections between the neutral and ground, leakage to ground from defective equipment and surge suppressors / EMI filters.

1) Is your house an older house were someone before you swapped black and white wires on daisy chained outlets?

2) Could have someone cheated by tieing the ground screw to the neutral to make a 3 prong outlet out of a two wire circuit?

3) Do you have any two wire circuits in the house where perhaps a ground leakage current is trying to get back to the neutral via the UPS / printer / PC / TV / sound system?

4) Did someone add a "safety" ground or bond between a device and the water piping?

5) Do you have cable or satelite hooked up to the TV? DSL for the computer? Are the earth connections for the surge suppressor blocks for the telephone, TV, or satellite securely bonded to the grounding network for the house?


A quick test to determine if there is a voltage difference between the outlet grounds, is to run a bonding wire between the TV outlet ground and the computer outlet ground and see if the hum bars and ground buzz disappear.
Posted By: Scott35 Re: VGA ground loop isolation? - 01/16/12 05:34 AM
You could try Isolated Grounding type Receptacles, with all the Grounding Conductors bonded to a single Star Point.

This is one application, where IG Circuits actually are of some benefit!

Otherwise, go with some type of Modulated A/V signals.

-- Scott
Posted By: gfretwell Re: VGA ground loop isolation? - 01/16/12 07:15 AM
I had an old Daewoo TV that would trip a GFCI if it was hooked to a PC. My bet is they have tied the neutral/hot to the DC ground some way in the TV, maybe in some crude attempt to provide surge protection. (I have seen delta bridges across all three with something that might have been a MOV type device)

In the PC, DC ground and the EGC are rigidly connected together. (via the mounting pads on the system board to the base plate, bolted to the power supply case).
In my wooden machines, this was not the case. (no pun intended). It showed up for me when I was installing them in cars.

Is there a ground terminal on the TV for intersystem connections? Try bonding that to the PC case. If not, try to bond the DC ground in back of the TV to the PC in some other way. (RCA connector shell) Use something like a #12 stranded.
Since you are incidentally bonding these anyway, give it a shunted path with a lower resistance than your signal cables. That is what fixed the car/PC problems. (a 12ga from the PC case to the chassis of the car).
Posted By: JoeTestingEngr Re: VGA ground loop isolation? - 01/17/12 06:37 AM
Well, I'm not particularly worried about being electrocuted by my computer chassis. I wouldn't put up with what I imagine is really a 120Hz hum in the signal which you will have a hard time getting rid of with that ground connected. When you had your ground lifted, you should have measured the AC voltage between the prong and the outlet. Let's say that you see 3 VAC between ground and the prong that should be grounded. That's a deal killer for your video & audio signal but probably not an NJwirenut killer. So what if you clamp your ground somewhere between a signal killer and a wirenut killer??? Consider inserting a bidirectional transorb like a 1.5KE6.8CA or 2, series inverse unidirectional 1.5KE6.8A between chassis ground prong and Earth. That way, you can avoid the nuisance low voltage ground loops while still retaining 1500W of clamping ability to ground should the chassis try to elevate to a dangerous voltage. Just my 6 volts worth.
Joe
PS: The cost of a Littelfuse 1.5KE6.8CA at Digi-Key is $0.77, assuming you bought enough other goodies to satisfy their $25 minimum. I never seem to have a problem doing that.
Posted By: gfretwell Re: VGA ground loop isolation? - 01/17/12 08:30 AM
Is this on a cable TV system?
The hum may actually be coming down the cable. Try disconnecting that. That could be the only ground reference on a TV that has a 2 prong plug.
Posted By: KJay Re: VGA ground loop isolation? - 01/21/12 05:43 PM
Would converting the analog VGA and audio signal to a digital HDMI signal help clean up or eliminate the noise interference in both the audio and picture?
Posted By: LarryC Re: VGA ground loop isolation? - 02/02/12 07:14 PM
NJwirenut,

Any updates?

didja fix it?

LarryC
Posted By: NJwirenut Re: VGA ground loop isolation? - 02/05/12 08:43 PM
Eventually got things fixed. The CATV grounding was good, tied into the electrical grounding system where it enters the house.

The solution involved 3 separate fixes, each of which improved the hum somewhat, but all 3 together were needed to eliminate it completely.

First, the audio ground loop isolator I had ordered was installed in the audio line, and this eliminated the buzz from the audio.

Installing a similar device in the coaxial RF input to the cable box (which connects to the TV via 5 RCA cables for audio and component video) greatly reduced the severity of the hum bars, but didn't completely eliminate them. The device I used is here:

http://www.amazon.com/VSIS-EU-Cable-Ground-Loop-Isolator/dp/B0017I3K9M

Eliminating the last traces of the hum bar was accomplished by running a 10 AWG THHN grounding wire between the PC chassis and the chassis of the TV. Getting a connection to the TV chassis required removing the back cover of the TV, and attaching the wire to an internal metal shield.

Thanks to all for the input!
Posted By: gfretwell Re: VGA ground loop isolation? - 02/05/12 10:08 PM
What happens now if you remove the filters?
Posted By: Wmackay Re: VGA ground loop isolation? - 02/07/14 09:07 PM
best solution is dedicated circuit and or Isolated ground

micro devices such as dimmers and occupancy sensors placed on switch loop circuits use the ground for a neutral when none is available and this puts small current on ground connection.

Also and perhaps I should have led with this is there a woofer nearby these cause havoc when nearby
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