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

[Steve Adeff]

On Wednesday 11 January 2006 22:18, f-myth-users at media.mit.edu wrote:
>     > 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. :)

and thats how flame wars start...
in the end we agree, we just don't type the same langauge ;-)

I've actually seen ground loops with modern 2prong televisions. I never was 
able to figure out the problem exactly, but I got a case of beer for fixing 
the overall issue, so I was happy...

I've also had ground hum through coax digital on my own stereo before I 
purchased a line filter power strip. I could not connect a peice of equipment 
to my receiver in any fashion, the coax digital cable as well. in the end it 
was due to a mismatch in ground potential due to a bad powerstrip, go 
figure...

oh, I like the lab geek method of solving ground hum over the audiophile geek 
version as well. They can postulate a theory as to why their idea works, the 
audiophile geek can't, they're just happy it does, and then charge way too 
much money for it...

-- 
Steve