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