June 2003

I Got a Bad Hum

Our readers’ mail is interesting stuff. Much of it, at least the mail I get, contains pleas for help with either selection of components or solutions to problems. This past month I received one of the "classic" questions that deserves to be answered and kept in the archives for posterity because it concerns a problem that happens so often. A reader upgraded to higher-quality equipment, added digital cable TV, and ended up with a bad hum in the system.

This person’s hum problem was pathetically addressed by her cable company's response: "We see this a lot in high-end systems." Obviously, no solution was offered. They also charged the lady $56! It takes a real "screw you and your problem, too" service provider to charge for problems they can’t solve. The solution is actually quite simple.

Description of the problem

The problem crops up like this. You connect everything to your system, but whenever you select cable, satellite, or antenna you get a fairly obvious hum, which can be low level or quite loud. When you select DVD, CD, FM, or tape there is no problem with hum and everything sounds just fine.

What causes the problem?

The problem is something called a "ground loop." A ground loop happens when there is more than one electrical ground point in the home-theater system. The electrical system in your house has a single ground point -- usually. Some older homes, or homes with additions, high-rise buildings, or older buildings may have some challenges with locations of ground points. But for simplicity of discussion, let’s assume you are in a relatively "modern" building where the entire electrical system shares a common ground point.

The cable, antenna, or dish is almost always grounded somewhere else. The cable company will usually ground the shield of the cable after it enters your house. Their favorite ground connection point is often a copper water pipe.

The problem is the pipe and the big copper rod that is driven into the ground to form the ground for the building’s electrical system may be some distance apart. Furthermore, the dirt covering them can have varying electrical properties. One area may be damper with more clay or minerals. The other ground point may be in a drier area or there might be a lot of sand (an insulator) in the dirt. Antenna and dish experts recommend a separate ground rod sunk into the earth not too far from the device. This ground rod may be some distance from the one for the electrical system and subject to the same differences in dampness and soil composition as the water pipe.

The difference in distance, dampness, and soil composition around the ground points can result in varying voltage between them. If you were to connect a sensitive electrical meter between the different points, you would find that you have an actual voltage flowing from one ground point to the other -- the voltage isn’t zero as it has to be to avoid ground loops. When there is a current like this between two different grounds, you get a hum in your home-theater system. Or do you?

Why do some home theater systems hum and others don’t?

In some cases you may just get lucky: there’s no ground loop between the cable, antenna, dish, or electrical ground. In other cases you may have a system built around consumer products that all have two-prong AC power cords. These components are connected to the electrical system’s hot and neutral lines; they are not connected directly to the electrical system’s ground, which is the third, round prong in your electrical outlets. But when you upgrade components you may find that one or more of the nicer components come with a three-prong AC power cord. One component in your system with a three-prong power cord means all your components are now connected to the electrical-system ground. Why are all the components connected to the electrical ground when only one component has a three-prong power cord? All of the interconnects will essentially "ground" every component.

Remedies

Move the cable ground to the electrical-system ground. You can trace the electrical-system ground to the ground rod and run a wire to the cable’s ground block, which is usually mounted somewhere near where the cable enters the building. This means disconnecting the cable ground wire from the water pipe and connecting it directly to the copper ground rod for the electrical system.

If you can’t find the copper ground rod, you can get an old three-prong power cord and use a pair of pliers to pull the round ground pin right out of the plug. Solder a 16- or 18-gauge wire to that prong. Connect the other end of the wire to the cable, antenna, or dish grounding point and stick the prong into the ground pin in any nearby unused electrical outlet. Now you will have grounded the cable and electrical system to the same ground point. There will be no more ground current; the ground loop will disappear and so will the hum.

Isolate the cable ground from the rest of your system. The expensive way to do this is to buy a commercial product designed to isolate the cable ground from your audio/video system. These sell for about $15 to $150 depending on who makes them. One impeccable-quality device is manufactured by Jensen Transformers: the ISO-MAX VRD-1FF CATV Ground Isolator, which retails for $59.95. Some home-theater power conditioners may also include a cable ground isolator. These devices actually interrupt the shield/ground in the cable so the cable ground cannot touch your audio/video system. The bandwidth has to be at least 1GHz (1000MHz) for one of these devices to be compatible with digital cable, digital satellite, and digital/high-def broadcast TV. The Jensen device has an upper limit of 1.3GHz, which is quite good and should work well for any signal you want to receive for the foreseeable future. This solution requires an additional length of coax cable as well as the CATV ground-isolator device.

The cheap way to do this is to buy two twin-lead 300-ohm-to-75-ohm matching transformers. Connect the two 300-ohm ends together, plug the cable into one end and a new length of coax into the other end, and you have an isolated cable ground. These matching transformers sell for $1 to $5 each. However, it may be difficult or impossible to find these with a high enough bandwidth for digital cable, satellite, or high-def broadcasts. Conventional analog TV and analog cable require a bandwidth of around 600MHz. This is about half of what you need for digital cable, digital satellite, and digital/high-def broadcast.

One of these remedies is guaranteed to solve this particular type of ground-loop hum. And you’ll know something your cable or satellite company might not know.

 ...Doug Blackburn
db@hometheatersound.com

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