AVS Special Member
Join Date: Mar 2008
Location: Australia; now run by adults.
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An isolation tx can help, if it is the right type and used in the correct way. Large EI cores with good electrostatic shielding are the best, and the best and cheapest places to get them are from the transformer manufacturers, of which there will be many in the US. Bigger is better and toroids are useless due the the close coupling of the primary and secondary giving too high a bandwidth.
Many years ago when I was involved in the development of a mobile test lab, in one instance we ran into the worst noise I've ever seen on the mains, bad enough to effect some very sensitive measurements. After a lot of testing, we used a very large 1:1 isolation tx, preceded by large common mode filter. Most of it we got surplus very cheaply, but it took a lot of testing and setting up. We ended up with a 3dB point at about 500Hz from memory which cleaned it up considerably. The two cores weighed about 120kg.
Most of the filters seen in commercial audio products are designed to work from several MHz and above. Placing one of these in a powerboard and then connecting equipment to it is pointless as the RF they blocked from the lead into the powerboard will likely still be picked up in the power leads from the powerboard to the audio gear. The best place for this type of filter in at the input to the chassis for the power lead. This is how they are designed to be used and they are typically made with the male IEC connector as part of them, so when correctly installed they become part of the full shield of the chassis.
For protection against a true HV surge from outside, whole house protection in the switchboard is the best. I'd still suggest good, inexpensive powerboards at the equipment location for second tier protection.