[mythtv-users] Hum on analog audio to TV

Fred Squires 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
>>          have.
>> If  you don't want to make one I think this device will work:
>> https://www.tselectronic.com/gc/video/surge_block.html
>> It is the one on the right which I think is the more expensive one.
> Allen,
>  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.
> Shield
> ---------|(---------
> ---------|(---------
> Center
> 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,
> James

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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