Paul McGowan writes:
One of the least intrusive, yet least effective type of filter for the AC line is the parallel filter. There are a number of different types but these filters generally consist of a single or group of capacitors across the hot and neutral. Placing a capacitor across the hot and neutral does not eliminate line noise; it merely reroutes it from the hot to the neutral.
As I mentioned yesterday, the advantage of this filter type is low insertion loss – meaning it isn’t in the direct path of the power sitting between the wall socket and your equipment – but the disadvantage is low effectiveness – meaning they do not reduce the noise by a great amount and the noise that’s reduced never goes away.
The classic series AC filter, found in the vast majority of AC line filters, has higher insertion loss but is quite effective – converting the trapped noise energy into heat.
A Noise Harvester is a parallel filter with no insertion loss, but unlike a conventional parallel filter, it is more effective and actually eliminates trapped noise instead of merely rerouting it.