Given Bob's constraints of having the computer on the same third pin outlet
ground as the rig, a transformerless interface can work. However, with the
proliferation of accessories to computers and rigs, you find many RFI
problems caused by users' need to use more than one duplex outlet, and those
being on different legs of the power system.
If there is a chance you will take your rig to locations for emergencies for
example, or demos, where you cannot verify the ground integrity, a
transformer interface to break ground loops or isolate varying grounds is a
cheap insurance policy.
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