60-Cycle Hum in Your Stereo? Check Your Cable TV Line for Ground Loop

The traditional solution to "mains buzz" or "mains hum" is to make sure that all of the components in your audio/video setup are plugged into the same outlet, thus ensuring that all grounds are at the same electrical potential.  As long as there’s no potential difference between grounds, then by definition there’s no voltage, so via Ohm’s Law there can be no current flow, and thus no unwanted hum caused by the current flow.

I just finally got around to setting up my receiver here, and got some pretty nasty hum right away, which I figured was due to the computers being plugged into different outlets than the TV and receiver.  So I ran an extension cord from the computers’ power outlet over to the TV/receiver, but it made no difference.

After a little bit of searching I found out that the coaxial cable line can cause a ground loop with the other components in the system, because it’s actually only grounded at the cable company (!) and not at your house.  Sure enough, unplugging the coax resulted in beautiful silence through the stereo speakers.

To fix the problem, you need to break the ground loop, which can be done with an old 75-ohm to 300-ohm matching transformer (Radio Shack Cat. No. 15-1140).  Of course, you need a 75-ohm signal for any modern TV equipment, so the solution is to buy 2 of these transformers and hook them together: the second one reverses the transformation done by the first, so the output signal is (theoretically) the same as the input signal, except that the ground loop is broken.

Radio Shack’s Cat. No. 15-1253 is pretty much the same thing except in the opposite gender, so I bought one of each to make it simple to hook the two transformers together.  But note that item 15-1253 does not break continuity between the input and output grounds (outer shield), so using two of those won’t break the ground loop; you need at least one 15-1140.