[mythtv-users] Hum on analog audio to TV

[James Crow]

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.
>         
>         
>         I had this exact same problem and this solved it cold.
>         
>         
>         
>         
>         Allen
>         
>         
>         
>         
> 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