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

Steve Adeff adeffs at gmail.com
Thu Jan 12 02:11:05 UTC 2006


On Wednesday 11 January 2006 16:58, f-myth-users at media.mit.edu wrote:
>     > Date: Wed, 11 Jan 2006 12:38:56 -0500
>     > From: Steve Adeff <adeffs at gmail.com>
>     >
>     > On Wednesday 11 January 2006 10:12, matthew.garman at gmail.com wrote:
>     > > After I posted the message, however, I started reading about SPDIF
>     > > and digital audio IO.  Looks like my 7NIF2 lacks the SPDIF header
>     > > on the motherboard.  But I *do* have a Creative SoundBlaster Live
>     > > in my pile of old components.  It's pretty old, but it does have
>     > > digital out.
>     > >
>     > > I'm guessing that a digital cable would not be affected by
>     > > grounding problems, right?  Or at least any grounding effects would
>     > > not be audible?
>     >
>     > optical connection yes, coaxial no, since coax still uses a ground
>     > which would link the two grounds together. I'd also suggest using a
>     > smaller ground wire, 14 or 16, so that if there is a ground fault it
>     > will want to travel down the components natural ground instead of
>     > flowing through the other component. As well, if you receiver does
>     > not have a ground prong then definitely ground it manually with a 12g
>     > to the wall socket ground!
>
> Eh?  If he's sending digital information down the coax, then no amount
> of ground hum will be audible at the receiver, until the hum is so bad
> it starts flipping bits in the data stream (at which point, he'll hear
> either silence or godawful artifacts, depending on the protocol---but
> it sure won't be 60 Hz hum).
>
> Sure, if the problem is ground loops, then anything which can
> spuriously tie grounds together is a problem. But given the
> randomly-interconnected nature of computer hardware (much less stereo
> hardware), trying for a true single-point ground as it done in, e.g.,
> lab instrumentation amps is infeasible.  If the coax is supposedly the
> -only- source of ground interconnection, then going to an optical
> interconnect might help, but that's a fragile solution.
>
> It's ambiguous from the original poster's comment whether the problem
> is new, or he's just started noticing it.  My guess, if the latter, is
> that the shield on some cable got damaged, or something was recently
> changed in the hardware configuration elsewhere (new component? new
> cable?), and certainly replacing cables is the easier & quickest way
> to debug that.  If he has another way of producing audio, I'd also try
> substituting that at the end of the cable, once the cable is known
> good.  (After all, it -could- be that some filter cap in the sound
> board on the computer blew or is getting leaky; such things do
> happen.)
>
> While experimenting with tying chassis grounds together might be
> interesting, not all components use that as their signal-ground
> reference (though most do).  It might make more sense to try, just as
> an experiment, putting all components on a single outlet strip, so you
> know they're on the same phase and circuit.  (I've seen strange currents
> induced on so-called power grounds when different phases meet through
> equipment, especially in older structures with poor wiring [which is
> one reason why certain lab and audio gear keep chassis ("power") and
> signal grounds rigorously separate], and it's always possible that a
> ground or connection somewhere has gotten ohmic and therefore there's
> a voltage difference around that, in theory, can't exist.)

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

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.

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

-- 
Steve


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