[mythtv-users] Slightly OT: receiver input buzzing/humming

[Meatwad]

matthew.garman at gmail.com wrote:
> I've got a Yamaha HTR-5550 receiver that receives input from (among
> other places) my mythtv box. This is an analog input: the cord is a
> male 1/8" stereo plug (PC side) that splits into two RCA connectors
> (amp side).  The motherboard is the Chaintech 7NIF2.
> 
> Recently, I've noticed that when I'm not playing music or watching
> TV (i.e. silence), there is a distinct buzzing/humming sound coming
> from the speakers. The sound is barely noticeable at the "-40.0"
> volume setting, and quite pronounced by "-30.0".
> 
> I've tried messing with the volume levels on the PC; it does not
> make a difference in the buzzing. I've tried re-seating both ends of
> the cord (to no avail).
> 
> Note that when I switch input channels on the amp, the buzzing goes
> away. So I tried putting the RCA plugs into a different input---no
> help.
> 
> For what it's worth, the buzzing sound is the same as if I pull the
> 1/8" plug out, and touch it to my finger---I'm sure this is a
> familiar sound to many people.
> 
> My guesses are that either the cable is bad (it's an el-cheapo,
> don't know where it came from) or the input jacks on my PC are
> crummy or not properly grounded. This sound is the kind of sound I
> would expect when my plug isn't properly seated, but, as I said,
> I've tried seating and re-seating it several times (across the
> different outputs), and still have no success.
> 
> Does anyone have any suggestions on my next steps to further debug
> this problem?

Jumping in a little late here. You clearly have a ground loop or (very 
unlikely) a defect in the signal path.

All the responses so far have very valid points and I certainly don't 
want to drag this on forever but the following info also applies to 
loops that affect video, i.e. rolling horizontal bars on your plasma or 
other display.

The most important thing to do when faced with solving a ground loop is 
to NOT use a band-aid. In other words, solve the source of the problem 
rather than cure the symptoms.

- Bonding chassis to one other is a band-aid. If you absolutely must, 
chassis grounds should all be brought to a single reference ground 
point, i.e. cold water pipe.

- Lifting the grounding lug in a three wire power circuit is a band-aid 
  and a life-safety hazard. Only do this to track down ground loop 
problems and then only with a three-to-two prong adapter. Never chop off 
the lug.

Here are some other things to try:

- Assuming you have cable, temporarily remove ALL cable feeds from ALL 
pieces of equipment in your setup one at a time. If the hum goes away, 
your local cableco has failed to properly ground at the point of 
penetration. This is *the* most prevelant source of ground loop problems.

- Move all HT equipment to the same circuit breaker.

- If same circuit is not possible, try moving all HT equipment to 
circuits fed from the same side of the panel if you have 120Vac mains.

- Use a transformer-based ground isolation device between the two pieces

There is a white paper at 
http://www.jensen-transformers.com/an/an004.pdf by the master of all 
things ground, Bill Whitlock. There is also a wealth of info on this 
very problem at the Custom Installer's Lounge at the Remote Central web 
forums (search "Ground Loop" before posting or you'll soon feel the 
uncomfortable rhinocerous-up-the-arse-feeling). The S/N ratio at AVS 
Forums is just too high to quickly find accurate suggestions and answers.

Keep us informed of your progress and good luck.