Shorting plugs should only be used on inputs
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All unused RCA plugs should have caps, including video Inputs and outputs, even the TV unused inputs and outputs. Apologies for being mysterious. One note: the caps allegedly prevent EMI/RFI from entering through the open plugs but if you do the math the diameter of the opening is too small for almost all electromagnetic waves.
Thanks so much for the responses guys. To be clear: Rello your suggesting that shorting plugs be put only on Inputs and on no Outputs. So "Tape In" would take a shorting plug, but "Tape Out" can only take a non-shorting Cardas cap,same for a second "Preamp Out". Geoffkait you referenced the application of non-shorting caps, but not shorting plugs. Do you agree with Rello and my summary?
At one of the Bill Johnson in-store appearances I attended, he told the story of a problem caused by shorting plugs. ARC had sent a pre-amp (SP-10?) to Harry Pearson for review, and soon heard from him as he was having an issue with it. The problem was traced to Harry's installation of shorting plugs into the pre-amp's output jacks. Harry's lack of basic electronic engineering knowledge had once again caused a designer/manufacturer grief!
I've had subtle improvements using RCA shorting plugs on unused audio inputs. But rather poor results in some, but not all unused video inputs. Maybe the video problems occurred because adjacent non-RCA (S video/component) inputs for the same video input were being used, or maybe because there was an impedance mismatch? Perhaps not a problem with most of todays current DVI/HDMI inputs?
Seconding the comments by Rello and Bdp24, if you were to put a shorting plug on an unused analog output, you would be stressing the circuit which drives that output, by causing it to supply a greater than normal amount of current. Whether or not that would affect long term reliability, as well as functionality and sonics, would depend on the design of the particular circuit, including its output impedance.
Also, if you were to put a shorting plug on an unused second main output of a preamp, since most (but not all) preamps which provide two sets of output jacks simply wire the two sets of jacks directly together inside the rear panel, doing that would kill the signals at the output that is being used. Perhaps that was the reason for HP's problem that Bdp24 referred to.
And depending on the specific design shorting a tape output might also affect the main signal path.
If you want to consider putting something on unused RCA video or digital audio outputs, what should be done is to apply a 75 ohm termination. I don't know if that would make any difference, and I would not expect any difference it may make to be consistent from system to system. 75 ohm terminations are readily available inexpensively in BNC form, since they are widely used in pro applications. But the RCA versions I've seen, which are marketed to audiophiles, tend to be absurdly expensive. If you can't find an RCA version costing say a few dollars, you might consider either making your own, or else using a BNC version (as shown here) together with an adapter (as shown here).