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.
73, Bill W6WRT
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