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

[f-myth-users@media.mit.edu]

    > Date: Wed, 11 Jan 2006 21:11:05 -0500
    > From: Steve Adeff <adeffs at gmail.com>

    > you've obvously not deallt much with audio equipment ground problems.

...except, perhaps, in my days doing live audio work for theaters. :)
[Aha!  I think you said this because I was careless in my phrasing;
see a few paragraphs below.  No hard feelings. :) ]

[I note that going into excruciatingly correct theory on audio signal
handling is -way- beyond the scope of this list.  So I'm simplifying. 
(Ditto for how to design the grounds on sub-microvolt lab instruments.)
There are actually some interestingly controversial theories about it
in the audio field, too, some of which seem like total voodoo, and a
lot of religious arguments over it; I prefer the lab-instrument people,
who tend to be less argumentative as a rule.  And I'm especially glad
that I no longer have to nail other peoples' hum problems, particularly
during live events.]

    >				 					  A ground 
    > loop hum/buzz will work its way from the input ground to the output ground to 
    > the amp ground, etc, etc. causing hum in the output. Usually the hum will 
    > increase in volume as the preamp volume is turned up, depending on the 
    > topology, and can be a good way to tell if its a ground hum or powerline hum.

I never said it wouldn't.  ---oh wait, I see (now) how you read my
reply.  I was answering the coax-bit-flipping case for the "shield
missing" hypothesis, and you were thinking along the lines of "intact
coax shield enables ground loops" hypothesis, which I didn't get into
until the -next- paragraph about tying grounds together.  But I got
careless and typed "ground hum" there instead of "signal hum".  Oops.
Total change in meaning due to a braino on my part.  Sorry for the
confusion.

But my suspicion (as I said before) is that all this talk of ground
loops is probably bogus.  I think he's got a bad signal shield
somewhere, or a bad cap, or a bad design.

    > All good home theatre equipment will have at least 1 screw that is an external 
    > ground point for helping to solve ground loop problems, though really they 
    > are (or "were") there to ground record player arms back when people knew what 
    > vinyl was.

Yeah, but I'll bet his TV and his computer don't, unfortunately.
And frankly, they shouldn't need it---the vast majority of stereos
get along just fine with no more attention to grounding than "make
sure you plug all the cables in all the way".

    > Yes, connecting both peices of equipment to a good filtered power strip 
    > (Panamax, Belkin PureAV, NOT Monster) will be the best way to solve the 

Maybe if it's powerline-induced ground hum, which I doubt.  I've
typically found that concentrating on (and chasing) loops in powerline
grounds is counterproductive; it's usually some screwup in the signal
paths.  Everybody instantly leaps to "oh, it's a ground loop!" but it
turns out to be pretty difficult to really get nailed by that sort of
thing in audio work.  (I could tell you stories of truly bizarre
all-analog line-level (thank god, not mic-level) whole-building
distribution systems that, amazingly enough, functioned despite
looking like the entire thing would be one giant hairball of loops,
but nonetheless were fine if signal grounds were done right.  But
I digress... :)

    > problem, but may not always work, if the hum is coming from, say, a cable 
    > line, or failing capacitor...

My current theories.  (Or just a crappy soundcard that -always- hums,
but was only noticed because he's suddenly turning things up really
load to hear unexpectedly faint output---it'd be good to know if he's
really maxed out his soundcard outputs and mixers all along the way.
Sure, they may distort, but he can back off from that and then see
if he still has to turn up the amp so far that he hears hum.)

I really think that the most productive next step in this discussion
is not to have either one of us keep talking, but to have him start
swapping cables (first), then soundcards (second), then maybe what
outlets things are plugged into (distant third), but I think that
without some experimental evidence, we've reached the end of what
anyone can sensibly say might be wrong.  So I'll bow out now. :)