ground-loop problems

It's impractical, because my component-video cables aren't long enough to allow me to connect to the monitor while the Blu-ray player remains connected to the audio system.

I have some other tests to run, and some people to talk with about ground loops in general, and my system in particular. I will eventually report back.

It's almost impossible to find cable isolation transformers. PartsExpress sells one, but specifically states it's not suitable for digital signals (presumably because it doesn't work well at the frequencies digital is transmitted on). Comcast doesn't stock any, as they state very few customers claim to have problems. I might buy two baluns, wire them together, and see what happens.

Has the Blue-ray player and or the TV - you're not quite clear if it's only when both of these are used - got a three pin mains lead with a connected ground? Have you tried unplugging the TV aerial (and perhaps cable etc) connections - and then play a DVD?

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    Dave Plowman        dave@davenoise.co.uk           London SW
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The buzz seems to occur only when the BD player and the TV are connected with the HDMI cable. As I explained in the original post, (1) putting an isolation transformer on the BD player, (2) lifting the player's ground & reversing the plug, and (3) disconnecting the Motorola's power cord and HDMI cable produces the lowest level of buzz.

What I have not checked if whether disconnecting /only/ the cable equipment removes the buzz. I should have checked this earlier.

In case you have not already found this document, get this pdf (don't try to open it in a browser, download it):

It has quite an informative collection of electrical grounding advice and horror stories, and data sheets on some products including the "iso-max" isolation transformer. It's a long read, but worth plowing through (apologies to Mr. Plowman).

Michael

A difficult read - but seems to have a reasonable deinition of a grounding loop:- "A ground loop is caused when two pieces of equipment are each hooked to ground (through their power plugs) and are also connected together by a shielded audio or video cable. All grounds are not created equal and there will be a difference in voltage between the two ends of the shielded cable. This induces large currents that get into all the equipment. The trick is to break one of the ground connections, so that the cable shield does not carry any ground loop current."

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Geo

Indeed. Nothing magical at all. Of course not all hum is caused by a earth loop.

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    Dave Plowman        dave@davenoise.co.uk           London SW
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Well I was just kinda curious if it was a problem specific to the HDMI cable itself. I don't know what freq Comcast nodes uses but I'd guess they are still in the 500mhz range. RR here uses 500, some others use

700 and the newest design by Cisco is 1 ghz. Check the freq range of the isolators and remember that even an F connector has a 4db drop in signal across it.

Any ideas what they might be? (I'm not an HDMI expert.) It's a Belkin. I have a No-Name-O I could try.

William, the way broadcast & recording studios in the US eliminate ground loops is with 4" to 6" wide copper run between racks & consoles. This is either silver soldered or brazed at the junctions to reduce the ground system impedance. Soldering is the preferred method, but most people can't solder 16 square inches of copper without burning a hole in it. With a proper grounding system, it doesn't matter which phase or breaker a piece of equipment is on.

is an example of what is used. Be advised that this method isn't cheap, but it is effective, and doesn't degrade like other systems.

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This arrangement assumes the buss has such a low resistance that it forces the equiment rounds to be at essentially the same AC potential, thus reducing or eliminating ground loops. I have no doubt it works -- in a studio.

I'm certain that if I made such a connection between my video+cable equipment and my audio equipment, the problem would go away. But I'm not sure exactly how to do it, especially as it's likely to cause my system to look like Laocoon (et fils) and the sea serpents.

The closest I ever came to such a arrangement occurred in 1993, when I ran cables across the floor from a JVC hall synthesizer to the power amp driving the "side" speakers. There was quite a bit of hum. After some experimenting I decided that the synthesizer's RCA output jacks weren't grounded sufficiently to the chassis. So I ran a copper buss bar across the jacks and soldered it solidly to the jacks and chassis ground. The hum went away.

The UK has very different wiring than the US.

Here, a standard circuit is 120v/20A or 2400 watts [at unity PF...]

In the UK; ring wiring supplies 240v/30A, or 7200 watts. Plugs have internal fuses based on the device.

(The US may well have a 240V/20A outlet for an air conditioner; where it's 120-N-120.)

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Not really, just tossing some suggestions out there. Only HDMI I have is on a Sony upscaling DVD/VCR combo in the bedroom connected to a 32" TV where both the audio and video are sent via the HDMI port.

Yes - I understand that.

Indeed. So one circuit should be adequate for feeding any 'portable' appliances in a room? Excluding heating etc. Not many domestic audio systems will need a 2400 watt supply. Not even mine. ;-)

32 amp is the standard these days.

So a local centre tap transformer gives you the 110v from a 220 ish supply? To me it would make sense to not have both sides of that transformer output in one room - why present an additional hazard if not necessary?

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    Dave Plowman        dave@davenoise.co.uk           London SW
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Why would it matter? When is there a situation when the hot from one outlet is connected to the hot from another outlet?1

Are you talking about a power company transformer, typically sitting atop a telephone pole? There certainly isn't any CT transformer inside any residence as both legs of the 220 two phase are fed to the household.

Try 120-0-120 for 240 VAC. You usually only see 220 in and older area that needs upgrading. 130 V light bulbs are common, too. My line voltage runs around 125-0-125 most of the time.

What hazard? The lighting circuits are supposed to be on different circuits than the outlets. Some family rooms, or 'Great Rooms' can have three or four 20 amp circuits for outlets. Even if they are all on the same phase, the different loads and wire lengths vary the voltage drop in the return leg, no matter if it is a 120 or 240 volt circuit. Take your meter and measure the difference between the neutral and ground on your outlets. If nothing is in use, they are equal. A moderate load will cause the voltage on the neutral to rise. Depending on where you are on that circuit, you will see different voltages. Even with your ring circuits.

The US philosophy is to only trip breaker in a fault condition and that means extra capacity.

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On 6/15/2009 12:13 PM Michael A. Terrell spake thus:

So what is the UK philosophy in this regard?

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Fault condition - you could have 220v between two surfaces. And silly because there's no need.

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    Dave Plowman        dave@davenoise.co.uk           London SW
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You're totally missing the point. 110/120 v is generally thought to be rather safer electrocution wise than 240 - it's used on UK building sites. Although via an isolating transformer so floating and therefore no reference to earth. If you have two phase 120v in one room under certain fault conditions you could have the full 240v risk. In the UK make that

415v. And not allowed, on sockets. Heating - hard wired - would be ok. Why would you want 'three or four' 20 amp circuits in a 'family' room?

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    Dave Plowman        dave@davenoise.co.uk           London SW
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You clearly don't know the folks I do...

Thanks to the EU...

The local transformer is on a pole outside, or a pad. The breaker panel is split; half the loads on each leg. Big loads: stove, water heater, whole house airconditioning, dryer are fed with 240V.

Why would anyone drive down the wrong side of the road, or use Whitworth threads? It's not usual for a large room to have multiple circuits. If we had a 7500 watt scheme, one would do, I suspect.

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A host is a host from coast to coast.................wb8foz@nrk.com
& no one will talk to a host that\'s close........[v].(301) 56-LINUX
Unless the host (that isn\'t close).........................pob 1433
is busy, hung or dead....................................20915-1433