[mythtv-users] Eliminating a ground loop/hum

Fedor Pikus fpikus at gmail.com
Thu Jul 6 04:00:13 UTC 2006


On 7/5/06, Robert Johnston <anaerin at gmail.com> wrote:
>
> On 7/5/06, Owen Mehegan <owen at nerdnetworks.org> wrote:
> >
> > cable. Audio is routed as follows:
> >
> > cable box -> myth box -> tv -> Sony home theater receiver/speakers
> >
> > All of that is done via basic RCA cables. My problem is that I have an
> > intermittent 60hz hum in the audio. I hear it whether I'm watching TV or
>
> Using SPDIF would help a lot. But you could also try wiring the case
> of the Amplifier to the case of the PC, which will establish a "Common
> Ground" and prevent the hum (Or blow up the equipment because of the
> voltage differential. Though that's not very likely as the act of
> connecting the PC to the Amp via the audio cable would have made it
> blow up too). Or you can cut the ground line in the RCA's to make a
> "Ground Lift" and isolate the ground points of the two systems.
>
> Personally, Though, I'd put everything through a single UPS, making
> sure you use a nice smoothed output.



All of this will help if the problem is with different grounds of all the
components. However, most A/V components don't have a grounding plug in the
first place (in my A/V stack, the PC is the only device which has it). There
is another ground in the system: the shield of the cable, which is always
grounded. It's easy to check if this is the problem: disconnect the coax and
see if the gound loop is still there. You might be able to see it better
than hear if you make your PC display a uniform medium-tone screen, like
solid grey: it shows up like slowly scrolling bands of slightly different
brightness.

If your problem is not coax, you probably just need to plug all equipment
into the same outlet. If it is the coax, this won't help. You can try
grounding the coax to the outlet ground, but you need a very good connection
(no twisting wires, solder only). Or you can break the DC connection through
the cable. You need to buy or make a balun, basically a transformer which
passes through AC but has no shared ground and thus breaks DC (for TV
frequencies, 60Hz  counts as DC). I could not find one, so I built it from
two 75Ohm-300Ohm converters from Fry's, by soldering together the 300Ohm
ends (some of these converters have shared DC ground and some don't, bring
an ohm-meter with you when you go shopping).
-- 
Fedor G Pikus (fpikus at gmail.com)
http://www.pikus.net
http://wild-light.com
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