[slinkelist] Two sound cards and hum

[Mike Macgirvin]

> Another solution for the ground loop problem that I've heard [from this list I think] is to connect the chassis of your
> computer to the chassis of your amp. I haven't tried this yet but I think someone else on this list has. It'd certainly be
> worth a try, and would be cheaper than buying an isolator from RS. Although if your computer is far from you amp it might not
> be worth it if you have to run another wire.
>  
> -Shawn

With all due respects, this is essentially what has been done by
connecting the stereo lines between the systems. This is not a cure for
a ground loop, it is the cause. The computer is *generally* grounded
through the three-prong plug to the electrical box in your house and
presumably, eventually to a water line or fixed ground stake or such.
The stereo is *generally* grounded through the cable line (which must be
grounded at the inlet box) using a path of slightly or dramatically
different length; since most stereos still use 2-prong power mains. If
the ground paths on these systems are different lengths
(impedance/resistance to earth ground), and you connect the stereo
input/output lines together, you have created a ground loop. Same as if
you connect the frames together. You must break the link between the
systems *or* ensure that both are exactly the same electrical distance
to earth reference ground. The former is relatively easy with isolation
transformers, the latter could be very hard and possibly extremely
expensive, as it means that you would have to re-wire the house to make
sure that all "ground prongs" on electrical outlets and cable-TV outlets
are exactly the same electrical length to earth. 

The only way this solution (tie-together) works is if your stereo can be
lifted from ground completely ("floated") and does not have any ties to
ground except through the computer. This generally rules out having a
cable-TV connection in the system unless you isolate it. Cable-TV
isolators are available as well, but most folks prefer to use stereo
audio line isolators instead because isolators always have a wee bit of
loss and this generally affects high-frequency video signals much more
than audio signals. 

The absolute cheapest solution you can try is to plug the computer in
via a "widow-maker" power line which has the ground wire disconnected
(cut off the ground prong). This assumes the computer isn't pulling in a
ground reference somewhere else through one of your peripherals. If this
removes the hum, great but it's called a "widow-maker" for a reason.
Your computer would now be floating and you could possibly be subject to
deadly shock. Rare, but not impossible. If you're reading between the
lines here, the isolator is really the best way out.