Fixing Cable-TV Hum in Audio Systems.

Has this ever happened to you?

Your hi fi or computer audio setup works great... until you connect it to a tv or vcr that's also connected to cable.

Then you get an awful hum on every source -- even when the video equipment is turned off!

Note: a cable-tv employee saw this page, and suggested yet another solution. Click here for his idea.
Try this: Did the hum go away? Read on.
The problem is caused by a conflict between the cable company's legitimate ground and yours. It's also easily solved:

Get a couple of cheap 75-ohm to 300-ohm antenna transformers. Radio Shack has them for a few bucks each (US). I suggest one #15-1253, and one of either #15-1140 or #15-1230.

Wire the two 300-ohm sides together, so you end up with a double transformer with 75-ohm connections on each side. (If you bought the part numbers suggested, this means just wiring the spade lugs up to the screw terminals. Takes about fifteen seconds with a butter knife, less with a screwdriver.)

Insert the assembly between the cable coming out of your wall, and your stereo system. Make sure the metal band or shielding around the cable or its connector doesn't touch any part of your hifi or video equipment. You can wrap it in tape if you want.

Hum will be gone.

If hum doesn't go away, chances are what you bought aren't really transformers but are autoformers: check with an ohmmeter to make sure there's absolutely no continuity between the either conductor on the 75-ohm side and either conductor on the 300-ohm side.

This is a low-cost, mostly unshielded solution. It'll work in a lot of situations, but in high rf fields may introduce ghosting between a radiated signal and a cable one. It also causes a very slight rf signal loss, which most tvs and vcrs probably won't have trouble with.

However, if the hum goes away but you can't live with ghosting and video noise:

AUTHOR'S NOTE

The Radio Shack name belongs to those folks. Since they do a pretty good job of what they're trying to do -- and there's a store in practically every shopping mall in the solar system -- I don't want to get them upset.

The circuit has been used many times, works, and is safe. However, strange things can happen to vcrs, lightning can strike tv cables, and klutzes can cut themselves with butter knives. So I don't guarantee anything.

The above document is (c) 1996 Jay Rose.

Feel like doing some soldering? Like to wind your own transformers? Here's another solution on the web.
Another Idea. And then there's this way: On 29 May 96, I got this mail from Mike Bird:

Subject: RE: Audoi Hum

Hi there,
        I'm a cable Tech at Continental Cable. For a cheaper and more 
shielded solution for audoi hum, use an inline attenuator 6DB or less. 
The good ones will not pass AC. Most of your better cable companies will 
give them free of charge, ours will. You have to make a service call 
sometimes just so we can verify that that is what is causing the 
problem, which it usually is, although not enough seperation between 
audio and video(min. seperation should be 12db) will cause the same 
thing!! Just a tip!! I appreciate your making these types of tips 
available as it can make it easier on us techs!!
        Mike Bird
I haven't tried Mike's method... but if the attenuator breaks both the outside shield and the inner conductor, it should work. If it's just a resistor network within a shielded case, it won't.
What I love about the web is wonks helping wonks, even on docs that are close to a decade old: in August '04, I got this from Jamie Hanrahan...

> Incidently, if you are ever moved to update or revise that page for any
> reason... while you're at it, you might also note that there are now much
> cheaper solutions than the $100 isolators. Here's one:

>
> Fry's sells a version too, for just $2.95, but of course they're only
> walk-in.

From the description, it should do the job. But there's no spec for rf shielding, so it might not be any more ghost-free than the Radio Shack units.

While we're at it, Radio Shack also has an audio-domain solution for those who don't want to mess with cable. #270-054, $16. It has RCA plugs and connects to the stereo outs of your TV and your hifi's inputs. It's not audiophile quality, but certainly good enough for most TV sound.

Our digital cable box (RCN) has audio outs that are hum-free. No isolation needed.

This doc, and the first reader response, were posted long before video-on-demand became prevalent. Here's a more up-to-date note I got in August 2005: 

>I was reading an article on your website... There was a reply from a technician that suggested
>using an in line attenuator to solve this problem.  Unfortunately, the attenuators he is
>speaking of do not break the continuity of the shielding between the two devices.  There are
>however several companies that manufacture "hum isolators" with -110dB RFI isolation.
>
>This isolation is VERY important in modernized cable systems utilizing return band services,
>simply because the low frequency interference (5-42 MHz) leaking into a non-isolated
>75Ohm-300Ohm-75Ohm radio shack quick fix ground isolator could completely interrupt services to
>all subscribers connected to that node in a Hybrid Fiber Coaxial plant system.
>
>One of the Companies that makes a very reliable device is Viewsonics Inc.  They are under $10
>per piece and can be connected at different points along the cable path... depending on the
>severity of the problem, you may only need one. ...contact the cable operator concerning these
>devices, as most will carry and install them free of charge [if you need one] if necessary.
> 
>Michael Scarpitti
>Headend Technician
>Time Warner Cable
To tutorials index

To special page for DV Magazine readers

To a couple of books full of Audio for Video tips and techniques

To Digital Playroom main page, with links to other media sound topics.

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