Ferrite Cores for Power cords

Ferrite rings filter very high frequency RFI/EMI. Most electronics stores carry them or just google and pick the right size. Ultimate Cables uses ferrite rings on both their power and speaker cords, in both cases, on the upstream side. They seem to work very well.
http://www.ultimatecables.com/

Wilfredt - I did not do extensive a/b testing with various types or brands. I read a couple of articles on line. Parts Express had a factory close out sale on Ferrites. 79 cent each for >10. I put them on everything. It seemed like a potential inexpensive, low risk tweak. In general, I put them at the end of the cord or cable closest to the device being powered or driven. (if the length of wire acts as an antenna - it seems to make sense to block the interference just before the "corrupted signal" enters a device; placement upstream did not seem like an effective way to block RF picked up downstream). I thought I heard an improvement in sound quality. Quieter and slightly more detail. It is subtle and I can't swear I'd detect a difference in a "blind test". Eventually, I might remove them and see if things sound worse. Might be worth an experiment for you as well. I will be very interested to follow any other responses you receive.

My experience, works well on a pc for digital, not so good on pcs for preamp or amp. I found when I tried them on the preamp or amp it caused the sound-stage to collapse somewhat. Just seemed to take the life from the music. Just a note here, my preamp and amp are tubes.

For the CDP it helped in two ways. It helped stop RFI from entering the unit through the pc and stopped the RFI generated by the CDP from going back out on the pc. I also found they worked best when installed on the pc where it entered the equipment.

Jea48, I have tried ferrite clamps extensively everywhere and you are indeed correct. Too many and it sucks the life out of a system. I think the best place to use them is on the power cord on the cdp. One on each end. Someone sent me a Analysis Plus Power Oval 2 with a ferrite clamp on each end and WOW this cord is extremely musical this way. Take the clamps off for a few minutes and more grain is added. It was very noticeable. This cord was not good on the amps but great on the cdp. My recommendation is to start at the cdp and work your way down. My system has changed recently so I may go and throw a couple of clamps on the ic's from cdp to pre. If the sound darkens or closes in they will come right off. I have not played around with these for a while except when I received the power cord I am using now.
Find a real good price on a used Power Oval 2 and put two of the box type ferrites clamps on it. I don't think enough can be said about rfi entering and exiting the cdp.
Digi-key has all kinds of ferrite clamps too.

These devices act as a choke (filter out higher frequencies that may be picked up as a cable acts like an antenna).

Generally these should not be required on a power cord or speaker cable as impedances are very low (unless you have a radio operator next door with a big antenna radiating you). The most useful application will be on video monitors or data cables that operate at high frequency with low signal levels and high impedances.