Steve Lampen, Guilty as Charged
It really all began with my second stint as chief engineer. Sadly, the station, the studios and the format for that channel are all gone. There might be a vestige or two of me at their transmitter site.
When I arrived, I found this little peanut-power (1850 Watts ERP) station, with studios that were lovingly dovetailed into an old Victorian building at 1509-1/2 Webster Street in Alameda, Calif. (When we moved to new digs, we had a contest with the prize being the "1/2" on the door.)
The previous owner was a legend in the San Francisco Bay Area broadcast community. He was the only person I have ever heard of, before or since, who bartered for STL transmitters and receivers. He got a hi-fi chain to buy the stuff, and traded them 10 times their value in spots. Amazing!
The two STL transmitters stood in a rack at the end of the building (actually in the engineering "shop"). One day I noticed that they were each plugged in with a 3-to-2 AC adaptor leaving them, essentially, ungrounded. I knew that this was unsafe, to say the least, and swiftly switched between them so I could turn them off and plug their three-pin cords into a three-pin outlet.
The hum on the air that resulted was not to be believed. So I rapidly went back to the "illegal" adaptors and it stayed that way until we moved to new studios a few years later.
That was my first real lesson on "ground loops." Yes, I left them "unsafe." Guilty as charged!
|Steve Lampen |
The real thing
Oh sure, I knew what ground loops were and where they came from. But there's nothing like the real thing to set your brain in motion.
That's why, when I began to design the new facility, I started with two weeks of putting in a real copper-strap, low-resistance "star" ground system. The fact that these new studios were right across the street from an airport was another good reason for a good ground system. And when we finally went on the air from the new location, all my hard work paid off; not a hum, buzz or fizzle anywhere.
There are really two kinds of grounds, a "safety" ground to prevent you from killing someone by being a path to ground, and a "signal" ground, which allows interference inside a cable or outside a cable to exit the cable before the signal itself enters a piece of equipment. (I could here easily segue into a "Pin 1" discussion, but I won't. Just go Google "Pin 1 problem." You'll be impressed.)
So I have recently seen a company pushing these little plastic isolators for lifting the equipment in a rack off of ground. They claim this will solve all those noise and buzz problems caused by bad grounds.
Unless you lift the ground on the power cord, I don't think this is true. Even if you isolate the box itself, it is still tied to ground through the third pin on the power plug and the two boxes attached with grounded signal cable can still potentially (pardon the pun) have a ground loop.
There are three real solutions for ground loops.
The first real solution is a star ground system, where very large copper strap (or very large copper wire) is attached to every rack, and therefore to every piece of equipment, in your facility.
The trick is that you need to pick a central point and then every "arm" of the star has to be exactly the same resistance (i.e., the same length). If one rack is seriously closer than the others, you still need all that cable (or strap) and you must hide it somewhere. And that cable to strap cannot touch anything metal along the way.
The second way is to cut the ground at one end of each cable. This will only work if you are running balanced lines, where the ground is only a ground and not a signal-carrying part of the circuit (like it is inside unbalanced cables or coaxial cables, for instance).
Of course, if the ground is required, such as phantom-powered microphones or intercom systems, then you'll have to hook up both ends and we're back to a star ground system to solve any ground loop problems. If you do lift one side of a shielded balanced-line cable, you must lift the destination end. Lifting the source end sets up a very interesting RC filter that will affect the response of that line.
Of course, lifting one end reduces shield effectiveness by 50 percent (one wire instead of two) so you become even more dependent on the twisted pair to reject noise. In which case, why not just go the whole way to UTP, unshielded twisted pairs, such as Category 5e or 6? No shield, no drain wire, no ground, so no ground problems. Of course, you are totally dependent on the balance ("CMRR") of the source and destination devices, but the pairs in Category cables, especially "bonded" pairs, are the best balanced pairs ever made.
And even if you could solve your ground problem with plastic isolators, this would mean that the safety ground is also isolated. The whole point of a safety ground is that, if something happens to the power wiring inside the box, if there is a "fault," the juice will flow through ground, not through you. If you isolate the box, anyone touching a box with a "fault" will become the next best path.
The point of having electricity flow to ground is to blow the circuit breaker. That protects everyone. However, the amperage necessary to accomplish this is more than enough to kill you.
Steve Lampen has worked for Belden for 16 years and is its multimedia technology manager. His latest book, "The Audio-Video Cable Installer's Pocket Guide," is published by McGraw-Hill. His "Wired for Sound" columns are archived at radioworld.com.