[mythtv-users] Eliminating a ground loop/hum

[Brian Wood]

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.