No transformer is necessary. Simply bond from chassis to chassis of all
the interconnected equipment, and, if possible, get power for all of it
from the same AC outlet.
That doesn't always work, Jim. As a matter of fact it is a generally bad
idea, because if a connection develops any resistance you wind up with hum
and noise. We would have been thrown out by the ear if we were caught
bonding cabinets to stop unbalanced line ground loop hum or noise at a BC
station.
Our radios and equipment have common chassis grounds for power supplies, and
the currents on ground leads can be 20 amperes or more. If the bond lead has
.01 ohms resistance, there is the potential for over .2 volts hum or noise.
A transformer, on the other hand, works flawlessly. The sole drawback is
fidelity, but that is not an issue even with very cheap transformers for our
communications stuff.
My headphone lines, because they serve multiple desks, all have isolation
transformers. I suppose I could run a big copper buss bar across the room,
but it seems more logical and safer to just spend $2 on a transformer at
each radio. :)
73 Tom
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