Not enough current capability for a power amp.I have tried one and if it gets too hot you can tell it is not right.Does not really purify anything, just isolates and noise still can get through. Most of them have a fairly loud mechanical buzz.
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I'm afarid I'll have to disagree with Don on all counts.
Current: You can get one that will handle just about any current you can imagine. If it gets too hot you are using one that is too small for the job. There are others but Furman used to make one that could handle 30 amps, big enough for most applications.
Filter: The transformer by itself will have some filtering capability. A transformer designed to pass 60Hz probably won't do as well at higher frequencies, thus providing some filtering. It would be very easy to add some capacitance to either or both sides to increase the effectiveness. Not something to take lightly but it can be done.
Noise: A well designed transformer that is used within its design parameters will not buzz. Torroidal transforms can buzz if the AC line is polluted with noise from things like light dimmers.
The original question about low cost? You need to define "low cost" as that is a very relative term.
Go over to AA on the Tweaks forum and search for the Jon Risch isolation transformer & ac filtering projects. Or search on google for Risch's web page. His designs to build either unit(including a surge protector) will be about as inexpensive as you'll get. If you're not into DIY then look for a use isolation transformer/filter from a medical application.
The Risch AC filter can be assembled fairly easily if you buy the complete (assembled) filter from diycable.com and put the rest together yourself (just a cord plus a male & female AC plug). Which cord & AC connectors to use can be a whole discussion in itself. I'm assembling one of these now.
Here is a very affordable isolation transformer that provides balanced power:
The Transcendent Balanced Power Supply
Other affordable power conditioners would be the Running Springs Haley, Blue Circle Music Rings, Balanced Power Technologies (BPT) transformers.