Upsetting ground

If you're looking for a good example of how some grounds are lousy, (and who isn't these days?), all you need to do is turn to your cable TV connection. It has to be at least once a week we're called to help fix a nasty buzz in someone's stereo system. Over the years, and hundreds of phone calls later, the first questions we ask concerns cable TV. If there is any connection to a cable TV in the system, chances of a nasty rasping noise blaring through your speakers is high. Disconnect the cable and bingo! no more buzz (no more TV either, but that's another story). The reason cable TVs are so problematic has to do with different ground potentials–a term you've no doubt heard bandied about, but maybe didn't know what it meant. When we speak of a potential difference we don't mean to speculate whether there will or won't be a difference. A difference in potential refers to voltage as measured between two points. If you were to take a voltmeter and connect its two leads between the incoming ground of a cable TV and the ground in your home, you'd measure a small voltage–a difference in potential. It is this small voltage that, when added to your hifi system, gets amplified into the nasties some we're sometimes plagued with. And often the cure is to remove the house ground from the stereo with an AC cheater plug, though the right thing to do is to disconnect the cable instead. I'll cover an interesting solution to this problem tomorrow.