[mythtv-users] Hum on analog audio to TV
fsquires at gmail.com
Thu Oct 23 12:14:32 UTC 2008
On Thu, Oct 23, 2008 at 7:44 AM, James Crow <james at ultratans.com> wrote:
> On Wed, 2008-10-22 at 19:51 -0700, Allen Edwards wrote:
>> Don't use an audio isolator as it is much more difficult to
>> isolate audio than RF. Make an RF isolation device. Just
>> take two .01 micro farad capacitors and connect both the
>> center conductor and the shield through a capacitor to the TV
>> set. There are a ton of ways you can make this. I made mine
>> by taking apart an 75-300 ohm adapter and using a piece of
>> coax as the output instead of the 300 ohm lead. Then I
>> wrapped the entire thing in aluminum foil well up the coax but
>> not touching the braid. This gave a little extra capacitance
>> and shielding to the thing without connecting the shields
>> together. This will cost almost nothing to do but will take a
>> little time and you need to know how to solder and have the
>> tools. That is the drawback.
>> It used to be you could take two 75 to 300
>> ohm transformers and connect them back to back to make a
>> 75-300-75 ohm thing that would isolate the ground. But, there
>> are various ways to wire these and some don't isolate the
>> ground so they don't help. If you have a couple of these and
>> a ohm meter this might do the trick, depending on what you
>> If you don't want to make one I think this device will work:
>> It is the one on the right which I think is the more expensive one.
> I will try both ideas. First I will grounding the coax right where it
> exits the wall with a grounding block. I have the block on hand and that
> only takes a couple of minutes. This would essentially be the same thing
> as grounding the braid to the PC chassis right?
> I would also like to try the RF isolation with two capacitors that you
> mentioned. I have a soldering iron on hand and know how to use it. I
> think RatShack will have the caps. Just to be sure I understand you
> basically cut a piece of coax and use one capacitor for the center
> conductor and one for shield.
> There is no center to center connection except through the cap and no
> braid to braid connection except through the cap.
> If I have it right I should be able to do that tonight. Maybe on a small
> piece of breadboard wrapped in foil?
Thanks to this discussion I've finally looked into this problem (I've
been having it for quite a while). I had previously switched to
external speakers (they sound better than the TV speakers anyway), and
I've been putting up with the static.
After searching around for a while I found a cable TV ground loop
isolator at http://www.jensentransformers.com/. Of course, it seems
to be much more expensive than some of the solutions listed here.
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