Isolating Home Theater Hum
Hum lurks there, in the background, omnipresent. As long as the music or movie is playing, you can forget about it, at least until a quiet passage occurs, then there it is again: HUMMMMMMM! Be gone, bad hum, you think. But, like a bad odor at the back of the fridge, it takes some dogged persistence to track it down and eliminate it.
First, let’s define it: Hum is a steady low-frequency noise, usually at about 60 Hz or 120 Hz, that results from voltage differences between true “ground” (what you’d get shoving a copper pipe into the ground) and the “ground” of your receiver’s chassis, or any components connected to it. When this situation occurs, it’s called a “ground loop,” and it’s darned annoying. Here are some tips to banish hum.
First, isolate it. Power up all the components in your system. Do you hear hum from every source (DVD, VCR, CD, TV, etc.) or only from one source? Say you hear it on the VCR input. Try reversing the VCR’s AC plug in the wall outlet. (Sometimes the polarized prongs don't let you do that). Did the hum go away?
While you’re at it, check your powered subwoofer. Turn it on (with no cables attached) and listen for hum. Reverse the AC plug in the wall if you hear any. Now reconnect the sub to your receiver or amp. If the hum is gone, you’ve triumphed! If not, disconnect the RCA jacks of all the components connected to your receiver or preamp, including the line-level cable from the “Subwoofer Output.” Is the hum still present? If it is, reverse the AC plug for your receiver or amp in the AC outlet. If the hum is gone, but only recurs when you reconnect one particular component, then reverse that component’s AC plug.
Finally, if all else fails, your cable-TV system (or satellite dish and decoder) may be the culprit. Disconnect the cables feeding your TV, VCR(s), a set-top cable box, or a satellite decoder. If the hum disappears (and you don’t use a dish) complain to the cable-TV company. They may have to run a copper rod into the ground outside your house or run a ground wire to a metal cold-water pipe.
If nothing else works, and you’ve isolated the hum to the cable-TV feed, build an isolating transformer. Buy two 75-ohm to 300-ohm cable transformers for a few dollars each, and connect the two pairs of 300-ohm “pigtails” together. Now insert it between the incoming cable-TV feed and the 75-ohm input terminal on your cable box or your TV. That should, finally, put all hum to rest, blocking it from entering your system.
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