[mythtv-users] Hum on analog audio to TV

David Brodbeck gull at gull.us
Wed Oct 22 20:28:12 UTC 2008


On Wed, October 22, 2008 10:03 am, James Crow wrote:
> Would connecting the coax braid to house ground
> accomplish the same thing as chassis ground to coax? I have an unused
> ground plug in the outlet I could wire to the coax braid.

It's worth trying, but I never had much luck with that approach.

> Any thoughts on this, or am I pretty much at building an isolation
> transformer to remedy the problem? If anyone has any suggestions for
> isolation transformers that are in the $10-$20 range I would consider
> it. The Jensen ones mentioned earlier are over $100.

You can get cheap audio isolation blocks at any car stereo installer.  Be
forewarned, though, that you get what you pay for -- the cheap ones will
severely roll off your bass.  If you're using dinky built-in speakers on a
TV you might not notice, though.  If you have analog video between the
computer and the TV you'll still get hum bars on the video, too.

I once made an RF isolation transformer by wiring two 300 ohm to 75 ohm
baluns back-to-back.  It worked but was pretty lossy, as evidenced by a
degraded picture on a few (analog) channels.

You can also break the ground path for the PC's case with a 3-prong to
2-prong adapter, but I don't recommend that approach.  It creates a
potential shock hazard and also defeats some of the surge protection in
the PC's power supply.  It's a good way to confirm whether or not the
power ground is the problem, though.




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