Grounding
Tom Martin -- Mon, 07/31/2006 - 16:07
I have a Nordost Thor power distribution box. It has a "dedicated earthing post" for grounding. What do I connect that post to? The instructions say nothing about this, except the box says "this product must be earthed". Well, I'm about ready to drop it from my roof, but that probably won't fix my hum problem :) .
The instructions should have been more clear. I surmise that the grounding post should be connected to the AC ground, but there's no easy way to do that. One would think that the Thor would be grounded through the three-pin AC plug.
Perhaps some other forum members have experience with this unit.
Robert Harley
The Thor's ground can be connected to the earth ground of any three-prong North American 120V outlet using a single wire with banana plugs. The ground connector is the round hole centered below the two vertical slots in a normal outlet. The hole happes to be the right size for bananas. Also, the screw holding the cover plate on the outlet is a good house ground. Run the screw through a crimp-on ring at the end of a ground wire, and tighten the screw suffiiciently to hold the cover in place without cracking it.
Barry Willis
More:
Sometimes hum is made worse by grounding the system, especially if the system includes a cable termination that's at some distance from its house ground. In this case, isolating the electrical system with an oversized isolation transformer should make a big improvement. Or you could try any of the several cable isolation devices on the market, a cheaper solution than a huge isolation transformer.
BW
Barry's suggestion is great (using a banana plug). I was going to ground to the center screw on the wall plate, but that is a little messy. I'm still mystified about Robert's question: why isn't the third prong of the ac cable into the Thor already doing this grounding?
The issue of grounding is actually more complex than it appears. You have an electrical system house ground, and an AV system ground and they are often not the same. There are only two ways to completely eliminate ground hum: either secure both grounds together, or separate them entirely. Anything else --where there are a couple volts difference between the two grounds -- will cause hum. This ground differential is why pro PA systems often sound noisy.
BW
Whenever two grounds are at different potentials and they are connected together, you'll get hum. The hum is a small amount of current flow through the ground. The solution, as Barry stated, is to connect the grounds together so that no potential difference (voltage) exists between the two ground points, or to completely isolate ("lift") one of the grounds to break the connection and prevent current flow through the ground.
Keep in mind that all equipment should be grounded to prevent the risk of electric shock. If for some reason a product's chassis becomes charged, it immediately goes to ground and opens the breaker.
Robert Harley
Editor-in-Chief
The Absolute Sound
The Pefect Vision
Re: the Thor's ungrounded third prong, I don't know this unit, but it wouldn't be the first piece of gear using a three-wire AC cord with the ground wire connected to nothing inside the chassis.
BW
tmartin wrote: I'm still mystified about Robert's question: why isn't the third prong of the ac cable into the Thor already doing this grounding?
If it was grounded internally, using the 3 pin Mains cord, you would not have the option to 'un-ground' it, in case of a hum problem due to the grounding....
Barry Willis wrote:Re: the Thor's ungrounded third prong, I don't know this unit, but it wouldn't be the first piece of gear using a three-wire AC cord with the ground wire connected to nothing inside the chassis.
Which is just as well seeing as beyond its safety function as a return path back to netural for fault current, it does nothing but cause noise.
If a piece of equipment passes Class II specs (i.e. double insulated), the safety ground is not required. And it's a shame because there is probably a fair amount of equipment out there which does meet Class II specs but the manufacturers just ubituitously use a 3 pin IEC connector and connect the safety ground to the chassis, causing all manner of noise problems for people who just want to hook up their gear and enjoy some music.
I think people need to stop thinking of the third pin as a "ground." Or rather stop thinking of it as a "ground" that's relevant to the equipment's ground. It's not.
While it's tied to an earth ground at the service panel, earth ground has no relevance to one's audio system. It's that it's connected to the neutral line which provides the safety factor, which is the sole purpose of that third pin.
se
Post new comment