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

[Steve Briggs]

Steve Adeff said:
> phasing shouldn't matter as long as the grounds are shared and 
> proper sizing, etc. Ground loops in proper working equipment are 
> formed when the impedence to ground for the two components are 
> different. Connecting them together then causes one of the 
> components to enter into the ground return of the other component.
Technically, when the potential (voltage) to ground isn't the same
between the two components; then there's a potential difference between
the two components and currents can flow from one to the other.

In addition, when the 2 components are on different outlets, the 
ground wires follow different physical paths and stray 60 Hz magnetic
fields will induce currents on the loops -- if the components are 
plugged into the same outlet, the area enclosed by the ground loop
is much less and the corresponding currents are less.

>  As well, in theory, properly working two prong components should 
> never cause ground loop problems... in theory...
True, but in practice, it's almost impossible to actually
isolate anything, particularly with cheap power supplies grounded
to the case and RCA cables (also generally grounded to the case)
used for interconnects.

IMHO, RCA cables are a horrible design; when you have ground currents,
they run on the shield, which turns the shield (supposed to shield the
inner conductor from stray signals) into a radiator.  Bonding the
components together with a separate ground strap (admittedly a 
band-aid, but often the easiest solution, particularly for those of us
who can't put everything on the same outlet) provides a lower-impedance
path between the components and keeps the currents off the RCA
cable shield.

I also note in industrial environments, millivolt sensor wires are
always shielded, but the shield is only connected at one end, never
both (to prevent currents on the shields).  I've often wondered
(but never done a good test) what would happen with RCA cables if
you disconnected the shield at one end of the cables to prevent
currents on the shields.

Steve


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