Great advice and well-summarized. Isolation transformers are a work-around
for system grounding problems. However, isolation transformers are useful
when it is not easy or quick to do these two things. I keep a supply of
them handy for quick "fixes" when a ground loop problem arises. Ultimately
fixing the ground loop problem is the better approach.
The same thing is true of ferrites to work-around RFI problems in the shack.
Ed W0YK
Bill, W6WRT, wrote:
> Another approach, if you feel a bit experimental, is to
> eliminate the ground loop in the first place. Do that and no
> transformer is needed at all.
>
> The kind of ground loop that causes hum with soundcard
> digital applications is caused because the chassis of the
> computer and the chassis of the radio are not at the same AC
> potential. It takes only a few millivolts of difference
> between the two chassis to cause 60 Hz AC current to flow
> between the two chassis via the audio cables and the I
> squared R voltage drop in the cable shield causes hum to be
> generated and coupled into the audio.
>
> There are two cures:
>
> 1. Most important, place the computer and radio physically
> close to each other and run a short, heavy wire between the
> two chassis. This shorts out almost all the voltage
> difference. No voltage difference = no ground loop current.
> As a bonus, this wire will often help a lot with EMI problems
> as well. Without this wire, the cables between the computer
> and radio form a small loop antenna. It's usually
> non-resonant and not very efficient of course, but in the
> presence of a strong RF field, the loop can pick up enough RF
> to cause trouble. The ground wire shorts out the "feedpoint"
> of the loop.
>
> 2. The AC power plug for the computer and the one for the
> radio should be plugged into the same AC outlet. This
> connects the "third wire" of the two power supplies together
> at the AC source. Different AC sockets throughout your house
> can have minutely different AC potentials on the third wire
> ground and those differences go directly to the two chassis.
>
> I have used these two methods for nearly 20 years now and I
> have never needed an isolation transformer anywhere.
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