Ground Noise Hum in Theater System

[esposiv]

I recently hooked-up my home theater to my television. There was an immediate hum in the system when I turned on the amplifier. The hum would not go away until I disconneced a cable line which went to a digital cable box for a digital TV. The box is only connected to the television. A Cablevison service rep checked the wiring throughout the house and said there was no grounding proble. However, when I disconnected the cable going to the cable box the hum disappeared.

I bought the Klipsch Magic Splitter which stopped the hum. Unfortunately, it also scrambled two of the digital cable channels. This is not a viable solution to the problem.

Does anyone else have this problem with their home theater? Any suggestions on how to get rid of the hum while keeping the cable line connected to the TV?

Thanks

[mas]

You do have a ground loop. And the cable tech is only checking for a house/system ground (as is the scope of his responsibility).

I am not familiar with Klipsch's magic splitter, but if the hum is being transmitted via an RG6 coax cable, you might try using an inline isolation transformer designed to decouple the shield path.

This is a common problem as you are dealing with two ground paths. You have the house system ground on the one hand, and you have the cable ground. If they are not common bonded so that the potential is identical, your system may use the alternative ground path - often the cable system's!

Note: In this case you do NOT want to lift an AC ground for very real safety issues! I have literally experienced AC line voltage using a more efficient house cable ground fed from the utility company's AC distribution feeder that was melting the aluminum siding on a house and only glowed a brighter red upon being 'hit' with a fire extinguisher! Using lineman's isolation gloves the cable ground was cut and the problem 'disappeared' - well, I think you know what I mean! (The real irony is that the addressable cable worked fine and the customer had called in reporting that the cable was "spitting sparks"! Indeed! )

Again, the ultimate solution is to insure that (instead of the cable company using a ground rod which I suspect is more efficient - but which there is a small possibility it is 'less') the two grounds are common bonded per code at the entrance to the house of the cold water pipe - insuring that no joints exist between the entrance and the bond which may insulate the path to ground.

Barring that, I think an isolation transformer such as the following may help with the hum - but you will still have a rather precarious situation during such events as electrical storms!

http://www.partsexpress.com/pe/pshowdetl.cfm?&Partnumber=180-075&CFID=8177644&CFTOKEN=38613428

By the way, you mention audio hum, but do you see any evidence of faint horizontal bars in the picture - either one or two that may be stationary or seem to roll? Or any sort of a zig zag herringbone effect in the picture?

Any additional info might help us pinpoint the problem more specifically...And you might check to see how the cable system is grounded. At the house entrance of the cable where the drop is anchored to the house (most probably at the electrical mast), check to see if the have a ground block near the bottom drip loop and then see how they have grounded the system. It is typically done one of several ways... A wire running from the ground block may be clamped to the mast, it may be run to a copper clad ground rod driven into the ground, or it may be common bonded to a cold water pipe. Or, they may not have done any of the mentioned!

That will give us a good place to start...

[sunburnwilly]

Well , the Magic Box is what you need to control the hum . "Ground loop isolator" . I wonder if you somehow got a bad one . I have been using a Magic Box for over 12 years and have no trouble with it dropping cable channels . Allthough mine is a single in and out , not a splitter .

I've used it with basic cable and now with HD digital cable with no problems . No way I could do without it .

[Guest " "]

Can you draw a picture of this lay out and post it.

There is a missing element not apparrent.

[SilverSport]

speakerfritz...will this help???

http://www.klipsch.com/products/details/magic-box.aspx

Bill

[hwatkins]

What MAS said is a great start

(and probable solution).

[SCOOTERDOG]

This is what you need.. I think www.partsexpress.com has them

VRD-1FF Cable TV Ground Loop Isolator

Analog, Digital and Cable Modem Compatible

[esposiv]

Mark,

Thanks for your reply. I purchased the in line isloation transformer from Parts Express today. I should have it by mid-week. I will let you know how it goes.

Regards,

Vin

[esposiv]

Bill,

Thanks. If the in line isolation transformer doesn't work I will try the item you recommend.

Regards,

Vin

[CECAA850]

mas will definitely help you get to the bottom of your problem. He was extremely helpful to me with a similar problem. I've read that the parts express isolation transformer works great with standard cable but won't pass high def or some digital channels. The Jensen pictured above is supposed to however. Avcable.com was the cheapest that I found it.