These are touted as being EMI/RFI noise caps, supposedly shielding the jacks from errant signal input thru the jacks. I used them originally and foremost as simple dust caps as we all know how bad that problem can be, especially if we haven't been down on our hands & knees behind our components for a couple years.
I thought it a bit of a stretch for these to be any kind of an effective EMI/RFI firewall. Not because I believed they wouldn't block such things but moreover that those kinds of things are indiscriminate and don't seek out or find cracks or holes to leak into. I was wrong! My old transport started getting noisy after 7-8 years. Took several days to figure out where the noise originated even tho I had transport nailed within a few minutes. And I had some of these caps in-house but had stopped using them years previously as much adieu about nothing. Turns out the cure for me was not to cap my transport out jacks but rather to cap my amp jacks as the transport was leaking apparently from lots of places. Instant gratification! This is simultaneously when I a developed a profound appreciation for TosLink cables. I always preferred coax digital tho I'm not sure exactly why except perhaps TosLink cables were chintzy/cheesy parts over a decade ago with misaligned lenses, broken internal strands and a plethora of other issues and that mentality stuck with me. Today things are quite different. And great quality TosLinks are LOTS cheaper than similar-end coax cables. I can't hear a difference with eyes wide open, or shut!
Today plain, ordinary caps are a part of my prophylactic repertoire with every component that comes here to visit awhile. They're in EVERY vacant RCA/coax and XLR jack/socket! Only difference is today I tweak em a bit. Big surprise right!?! I stand them on their sealed end, open side up then pour 3-4mm of molten lead into the bottom of RCA/coax and 6-8mm in XLR's as an added hedge towards rear access panel damping. On larger components with many jacks I know I've heard differences but due to the Dev rear accessory panel being already on the order of several times more rigid than traditional components access panels I'm less sure. Tho make no mistake... you'd be hard-pressed to swap me a couple new Cardas caps for my old lead-augmented Cardas heavy-weights on my Devialet
Statements in my posts are opinion only, not to be construed as fact. Any projects I engage in are at my own risk! Their outcome cannot be assured and may result in success, small/no change or catastrophic failure. I encourage no one rely on anything I say or do as gospel and to realize your mileage may vary!
