[mythtv-users] Eliminating a ground loop/hum

[Owen Mehegan]

I confess that I'm just assuming it's 60hz hum; I didn't check it  
with a tuning fork or anything :)

I'll take all of these suggestions (and any others I get!!) and see  
if I can crack this egg with one or a combination of them. I'll  
update the list on my results when I get a chance.

--
owen at nerdnetworks.org (Owen B. Mehegan)
'But I have that within which passes show,
These but the trappings and the suits of woe.'
   --William Shakespeare

On Jul 5, 2006, at 8:56 PM, Brian Wood wrote:

>
> On Jul 5, 2006, at 9:24 PM, Owen Mehegan wrote:
>
>> I have a new Myth system that I've just built. I'm using an MSI
>> PM8M-V motherboard, with its on-board audio (Via VT8233/A/8235/8237
>> AC97) for everything. The system is connected to a Sony 34" CRT TV
>> using DVI/HDMI 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 just in the Myth interface. Playing with various
>> mixer volumes doesn't make it go away, in the sense that I can't
>> null it out without having the whole system's volume down too low
>> to hear anything. I have everything plugged into one power strip/
>> wall outlet, and the outlet has three prongs; whether it's really
>> properly grounded or not is anyone's guess. I tried unplugging
>> various unrelated components (Xbox, cable box, cable modem/router)
>> and got no change.
>>
>> So, any suggestions on what I should do to clean up this audio?
>> Maybe buy a better sound card? (And use spdif out while I'm at it?
>> My receiver can handle it, though it's a pain in the ass from a
>> "management of the available inputs" perspective.) What if I bought
>> a UPS and plugged everything into that? I've seen suggestions of
>> "home theater" magical grounding thingys... if you think one of
>> those would help, a link to a specific product would be sweet.  
>> Thanks!
>
>
> The grounding section of Tremain's "Audio Cyclopedia" is hundreds of
> pages long, and you will get many differing opinions on how to solve
> the myriad of problems that come up with analog audio and hum, or
> other common-mode interference.
>
> Transmission grounds, source-terminated grounding, load-terminated
> grounds, it is an extremely complicated matter.
>
> But you said one thing that caught my attention: "intermittent".
>
> This means that the problem does not exist all the time, so anything
> that does not change like your cabling etc. can likely be ruled out.
>
> *Something* is changing though, and you need to find out what that
> is. Once you get to where you can make the problem come and go at
> will you will be halfway to solving it, because you will know the  
> cause.
>
> Cherchez la change :-)
>
> You also said "60 hz.". If it is truly 60 hz. then you can rule out
> problems with DC power supplies as they are almost always either full-
> wave, which would cause a 120hz. hum, or switchers, which cause much
> higher frequency interference.
>
> Of course going to fishing line for an audio cable (called "optical"
> by those who want to sell them at high prices) can eliminate a lot of
> problems as well.
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