snips:
Ferrites are clip on items to put onto cables. To really make some difference to any Cable, not just Sales Hype nonsense. Use as many as you like wherever you see fit. Audio, Mains & Video, add those clip on Ferrites to every cable that carries a signal, on the end that the signal has travelled along & picked up RF. You can add ones on both ends though this won't add anything if it's a TV to a Preamp as the signal goes one way only. The biggest difference is putting it on the Speaker cables simply as it loses the 1MHz+ RF hash that the cables pick up & your amp wastes power amplifying. If your amp picks up 999-type calls if the response vehicle is nearby, you badly need ferrites. There are smaller size cased ones, but having tried those, they don't seem as effective. RF is everywhere, your TV cables benefit from them on Scart & HDMI of today uses ferrites to keep the picture edges clean from wishboning of RF artifacts.
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We put Ferrites on our short 1m speaker cable as the amps are very near & the huge difference in focus was quite remarkable, in a world where excited words are overused ad nasueum, this was a genuine case of where amazement that such a simple item would make so much difference. It took a few days to get used to how strong & accurate the treble was, as with anything once you're used to it, it fades into normality.