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Hi,Have run into (and solved) CATV ground loop hum problem in the past.
I originally ran into this problem when integrating a Rotel 200W amp with my system. Its the only component in my audio/HT system with a 3-prong plug. I eventually (thanks to posters here) eliminated the hum with a transformer-style ground isolator (Jensen VR-1FF, also tried the Radio Shack dual-balun-for-cheap solution, which likewise worked well.)
A few weeks ago, I got digital cable. This comes via a Scientific Atlanta cable decoder. Was very glad to get this as I can at last do full video switching on my Onkyo 787, thanks to the S-Video out on the cable box.
The problem I had was that I couldn't get the digital portion of the cable signal. None of the cool features (menus, digital channels, Dolby audio, etc) worked.
Finally have traced the problem back to the Jensen transformer. Eliminating it restored the digital features of my cable service. Replace it, and they are gone again. Apparently the transformer either introduces too much noise, or else drops the frequncy at which these digital signals are sent, or something...
So... off this morning to my local radio shack to buy the bits to make a capacitance-type hum reducer thing, a'la the good info at http://www.smr-home-theatre.org/Ground-Loops/Ground-Loops.html
A brief test indicates that my new homemade isolator it works as well as the Jensen in eliminating hum (and for about $2.50!) but sadly, blocks the digital signal just as effectively as the Jensen.
Anyone have any ideas? I'm about to try defeating the ground on the Rotel, but I'd prefer to avoid that if possible. Hate to zap the kids or the dog.
Regards,
Chris Leger
Sorry to hear that. FWIW, my experience;
My cable TV is digital with digital converter. With SS amp, I did not notice the ground loop hum(buzz), but when I changed to tube power amp (cj), it emerged. I tried MAGIC, and MANY other isolation products in my system, but the conclusion is that cheapter plug on the amp plug works the best. Some devices, like you experienced, hurt some channels, but cheater plug never does that. I wish I could deal with it without lifting ground, but so far even in this dry weather in NY no zap or electrocution. Also, my speaker is pretty inefficient (87db) and it may be helping a lot.
Thanks Old Guy.Still not ready to lift ground. There *must* be a solution to this problem...
I'm using a Rotel (SS) 200W amp. My speakers are pretty efficient (102 db.)
I did make a second trip to Radio Shack. Tried 0.475uF and 0.1uF caps, all to no avail. They all suppress the humm... they all suppress the digital signal.
Maybe Jensen Transformer will get back to me with some advice.
Regards to all,
Chris Leger
You can try using a cheater plug, but my experience with CATV induced ground loop hum is that it may not completely solve the problem. I am not familiar with digital cable, but would it be possible somehow to isolate the signal on the downstream side of the cable box (that is, after it has been decoded into the video signal)?I do use cheater plugs routinely in my system, but many people cannot stand the thought of using them. I guess it's a matter of risk tolerance. My feeling is that the risk is not particularly great with typically well built high end equipment.
Todd
Thanks TOdd,I don't really think the cheater plug will help either...
Unfortunately, I can't isolate the ground downstream of the box, because the coax exisits only upstream (downstream is S-video & dig. audio) and even if I could, it wouldn't help, since the ground would be connected with my system by then.
After a little more reading, I now think the problem is that both the transformer and capacictor isolators won't pass the higher frequencies used by the digital portion of the cable signal.
I'm using 0.01uF caps... I wonder if different values might work better? The author of the article I mentioned hints at it... maybe another trip to Radio Shack is in order...
Thanks,
Chris Leger
the ground loop is actually caused by voltage on the shield (ground) side of the coaxial cable. the noise actually shows the ground is not properly made at the entrance of the cable into your home. have the cable company come back (for free) because they did not properly install the cable. they may have installed a copper grounding wire to the entrance block but either it is not screwed down tightly at one end or both, or the wire's gauge is too small for the distance it is going to the ground, or perhaps it is not attached to a real ground at all. no matter what the problem is, the noise indicates there is voltage on the house side of the ground. the voltage may not be a danger to you now, but it does indicate that no matter what you've done to eliminate it with a transformer, or a any other method, the cable is still not properly installed and all you've done is cover up a problem, not repair it.
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