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Author Topic: You remove ground loop roll with a 3 prong to 2 prong adaptor what next...  (Read 7055 times)

Mike Sokol

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While I agree that the perfect solution is a VGA iso, I'm still not clear on what running a laptop (if it is a laptop) with no AC ground is an issue.
I have seen plenty of laptops that don't have 3 pin AC plugs.
My all metal Macbook has no ground but my all plastic Lenovo does have a ground. Makes me wonder.
BTW... have the same issue at a local gig and we use an SDI to VGA converter which seems to do the trick.

I assumed that the laptop was also supplying the audio for the sound system, and thus was "grounded" to the sound system's ground. If the laptop is also supplying the audio, then the next question is does the sound system also have hum? If your layout "grounds" the laptop via a shield to the sound system, then lifting the EGC ground on the laptop should be perfectly safe. Perhaps you're accidentally bonding the isolated grounds in your laptop by "grounding"  its power supply EGC to its audio ground plane.

Laptops are their own brand of grounding craziness since they have lots of internal switching power supplies to create all the needed voltages. One thing that would be really interesting to try it to put a clamp-on ammeter around the various VGA cables to look for any ground loop current. I do this all the time for XLR audio cables, but have never tried it for VGA cables. I assume it will be about equivalent with maybe 10 to 100 mA of current causing visible hum bars, and 100 to 1,000 mA ground loop current making the picture unstable. And how does this work for HDMI cables? Gosh, that sounds like an interesting experiment, doesn't it?
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Mike Sokol
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Mike Sokol

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Here's a pretty good discussion on ground-loop video hum bars and how to troubleshoot it. A lot of the answers here are incorrect, but it does show just how complicated and confusing video hum bars can be.

http://www.film-tech.com/ubb/f16/t000377.html
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Jonathan Johnson

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While I agree that the perfect solution is a VGA iso, I'm still not clear on what running a laptop (if it is a laptop) with no AC ground is an issue.
I have seen plenty of laptops that don't have 3 pin AC plugs.
My all metal Macbook has no ground but my all plastic Lenovo does have a ground. Makes me wonder.
BTW... have the same issue at a local gig and we use an SDI to VGA converter which seems to do the trick.

I don't really understand the distinction, either. I think the laptop "ground" often only goes to the external power supply, and may not pass through to the laptop.

Nick hasn't said whether he is using a laptop or a desktop. Based on the report, I am guessing that it is a desktop, which very likely bonds the VGA ground with the mains ground.

Is the VGA signal (-) tied to ground or is it a balanced signal? If it is balanced, not ground referenced, you should be able to break the shield of the VGA cable (on one end) to safely eliminate the problem. (I don't know the VGA cable spec/pinout.)
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Mike Sokol

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Is the VGA signal (-) tied to ground or is it a balanced signal? If it is balanced, not ground referenced, you should be able to break the shield of the VGA cable (on one end) to safely eliminate the problem. (I don't know the VGA cable spec/pinout.)

From what I can see from the pinout diagram, the VGA cable spec is unbalanced with a common return path from each of the signals, that are typically bonded together. Also, there's no real spec for the cable, so some VGA cables are coax shielded while others aren't. If they are indeed unbalanced, then lifting the shield (ground?) on one end of the cable will only make the hum bars much worse. I'll contact one of my video manufacturers to learn more.

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Nicholas Bailey

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Thanks for the reply guys.

As usual to fix it correctly is not cheqp.

I will do some more testing when I install a center projector for the church.

I was running sound the morning the lady running the powerpoint ran in and plugged in the ground lift.


Nick
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Mike Sokol

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I was running sound the morning the lady running the powerpoint ran in and plugged in the ground lift.

While plugging in a ground lift plug on the computer isn't probably immediately dangerous, when something else goes wrong you can get into really big trouble, fast. When that perfect storm of failures occurs, death can be the result. So every safety device we disable puts us one step closer to catastrophe. That's why it's so easy to ignore these safety devices. You don't need it until you really need it, and then you need it badly.
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John Roberts {JR}

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If you must lift a safety ground, a GFCI outlet will still work and protect against hazardous shock currents. One of those GFCI power drops plugged into your 3/2 plug adapter isn't crazy.

JR 
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Mike Sokol

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If you must lift a safety ground, a GFCI outlet will still work and protect against hazardous shock currents. One of those GFCI power drops plugged into your 3/2 plug adapter isn't crazy.

Good call. I've posted a link to this before, but for the newbies here, here's something I wrote about how GFCI's perform this current balancing act that doesn't require a safety ground to operate. It's just flat-out ingenious. http://noshockzone.org/rv-electrical-safety-part-viii-gfci/
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Stephen Kirby

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While I agree that the perfect solution is a VGA iso, I'm still not clear on what running a laptop (if it is a laptop) with no AC ground is an issue.
I have seen plenty of laptops that don't have 3 pin AC plugs.
My all metal Macbook has no ground but my all plastic Lenovo does have a ground. Makes me wonder.
BTW... have the same issue at a local gig and we use an SDI to VGA converter which seems to do the trick.
Mac float all over the place.  The definitely need to be isolated as much as possible.  Fortunately I've not been involved with using one to feed both video and audio.  Just audio, which I fixed with a Radial transformer DI.

I just got back from China and plugging a Mac into the 240VAC mains over there results in leakage on the case you can feel.  This odd buzzing tingle that makes the case feel textured.  One of these days I'm going to get hold of a meter and see what the potential between the case and a real ground is.
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Jonathan Johnson

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Good call. I've posted a link to this before, but for the newbies here, here's something I wrote about how GFCI's perform this current balancing act that doesn't require a safety ground to operate. It's just flat-out ingenious. http://noshockzone.org/rv-electrical-safety-part-viii-gfci/

In the unlikely event that a person contacts both the hot and neutral, and is otherwise insulated from the surrounding environment, a GFCI will not protect against that shock. :o

One scenario where it could happen is if a "grounded" device is plugged into a bootleg ground receptacle, and another "grounded" device is plugged into an RPBG receptacle, and the person contacts the chassis of both devices. A GFCI upstream may not protect from that. Of course, it's likely that the GFCI would trip well before that happens (because of the bootleg grounds), but hey, it's a conceivable problem.
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Stop confusing the issue with facts and logic!

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