Power Conditioners

Not sure if I placed it in the correct topic but here goes. I was just wondering how power conditioners work, as I want to buy one. There are conditioners with only filtered inputs and conditioners with some filtered inputs and some unfiltered. I believe the unfiltered ones are for analogue devices. But why should these go into the unfiltered part? If I buy a power conditioner for example with only filtered inputs, will I not be able to put my class A amp in? Or will it have a negative effect?
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My response does not answer your question but I felt tempted to write.
I was given seriously expensive power conditioners and cables with filters (can't remember the brands) as home demo items long time ago. To me they did not make any difference at all.

May I ask to those who say otherwise, do people around you also hear a difference? An audiophile buddy etc..

Before spending money on a power conditioner I would run a separate spur from the junction box and try passive EMI/RMI suppression. I have great experience with Acoustic Revive powercables and RTP multi-connectors