Covers on unused RCA inputs?

It's not clear to me how a plastic cover would block those pesky little RFI who are sneaking around all the time looking for a way to get into our preamps!

A well-designed preamp input selector will short out all inputs other than the one selected. This is to prevent "bleed-through" of signal from sources that are not selected but may still be turned on. For example, a FM tuner while you are listening to a record. (Don't ask me why one would do that).

You're probably thinking of shorting plugs; I use them on the unused phono inputs on my Lamm phono stage and Jadis preamp (the rest of the inputs are currently used). Cardas also makes metal caps to put over the unused inputs. I can't say how much of a difference the shorting plugs make in my system, as I haven't really noticed much of a difference, but they don't hurt.

The original reason for shorting plugs wasn't RFI but because many early preamps would make a popping sound as you rotated the selector swith past unused inputs. I haven't had (heard) that problem for a long time, but I use shorting Camacs in my Levinson 26s pre anyway, because it reduces my OCD ;--)
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Covering the unused inputs does help, at least in my rig. I opted to put poster putty on the unused inputs being it was what I had. Nice and cheap.

Next time I may put a small piece of aluminum foil over the input and then the putty

I had a RFI problem with my preamp I needed to tame, and after coming across a similar thread as this I set out to find come caps. The cardas caps are pricey and I didn't want to spend the dough without knowing they would solve my problem. I decided to make my own.

DIY Shorting Caps:
I have a drawer full of cheapy RCA cables that accumulated over the years (you know - the ones that come packaged with consumer gear), so I chopped off the cable from the RCA connectors, leaving only about 1/2" pigtail on the RCAs. I stripped the insulation off and twisted the ground shield and postive wires together(knotted them up so they're secure). Voila! Free shorting plug. As for the results, I found they did help - much quieter, though they look like hell - good thing they're not visible.

I also found these on ebay:
RCA Caps which I purchased. I just received them but haven't swapped them out for my DIY models yet. These are like the Cardas caps - they don't short out the inputs, only cover them.

I use them on my unused phono input (no turntable presently) to ensure I don't accidentally plug a line level device in there.

I used Cardas RCA caps and there are no audible effects. Scratches on the input rca jacks, but less dust, is about all these things were able to achieve in my system.

And what is it with the bleeding of signal from other sources? I've seen so many preamps having this issue. Is this a common preamplifier design flaw? Is this an indication of something going wrong?

Audphile1...The good old fashoned rotary selector switch used to have a "deck" the function of which was to short out all the inputs except the one selected. We no longer see rotary selector switches, and the solid state circuitry common today runs at much lower impedance than old tube circuitry and is less susceptable to bleed through. So, preamp designers don't bother with shorting the unused inputs. I guess you could call it "a common preamplifier design flaw". Gone, like tone controls and Loudness compensation.