RCA or XLR iInput Shorting Plugs or Pins

On many power amplifiers which provide both XLR and RCA inputs the signal pin on the RCA connector is connected directly to the non-inverted signal pin on the XLR connector (usually XLR pin 2). In that situation neither the RCA center pin nor XLR pin 2 should be shorted, or the signal that is being provided to the other of those pins will be shorted to ground. In many such cases, though, if the RCA input is being used, and if the amp does not provide a switch to select between the RCA and XLR inputs, it is necessary to short XLR pin 3 to ground (XLR pin 1) for proper operation. The manual for the amp will usually indicate if that is the case.

Aside from that, shorting unused inputs is good practice IMO, although it may not make any difference in many cases. Non-shorting caps, though, are likely to be useless in most or all cases IMO.

Regards,
-- Al

Thanks for your kind response, Al.  Actually my VAC preamp and amp each have selector switch for preamp inputs and amplifier input. So no shorting plugs needed if I fully understood your remarks. However, my Audio Note Kits Interstage Monoblock amps (with RCA and XLR inputs) do not have such a selector switch. Would you recommend trying shorting RCA plug on RCA inputs as I routinely use transformer coupled XLR inputs?

Hi Audiobrian,

Actually my VAC preamp and amp each have selector switch for preamp inputs and amplifier input. So no shorting plugs needed if I fully understood your remarks.

It’s conceivable, although perhaps unlikely, that putting shorting plugs on unused inputs of the preamp might provide a small benefit. In any event, doing so won’t cause any harm. Without knowing more about the design of the power amp, though, I would not apply a short to its unused inputs.

... my Audio Note Kits Interstage Monoblock amps (with RCA and XLR inputs) do not have such a selector switch. Would you recommend trying shorting RCA plug on RCA inputs as I routinely use transformer coupled XLR inputs?

Again, when it comes to power amps I wouldn’t short any inputs without being familiar with the design of the specific amp. I took a look at the schematics for the Interstage Monoblock SET amps that are shown at the ANK website, but they don’t show the amps as having XLR inputs (yours must have been specially configured to provide them). It seems very possible, though, that the RCA center pin is connected to XLR pin 2, and if that is the case you would definitely not want to short the RCA input.

The distinction between shorting preamp inputs and shorting power amp inputs derives from the fact that a preamp is designed to accept and select between inputs from multiple source components, while a power amp usually is not. A power amp providing both XLR and RCA inputs is usually designed based on the expectation that just one of those inputs will be connected to a preamp or other component.

Regards,
-- Al

Cardas makes covers to be used on all unused inputs (made absolutely no difference in my system)

Those Cardas RCA jack covers are just cosmetic---if the plugs aren’t shorting, they do nothing except keep dust off the RCA’s. Why does Cardas (and others) offer such a useless, ineffective product, if not to simply make money off vain, insecure audiophiles? I found genuine RCA shorting jacks on ebay, about a buck apiece. Anyone with a soldering gun can make his own, of course. The cheapest RCA plugs available are fine---no audiophile version necessary ;-). I’ve never seen an XLR version, however.

I've applied shorting plugs to my VAC Ren Sig II's unused inputs, and I sure believe it made it quieter.  Spent all of a $1 a piece off of Ebay, just as bdp24 notes above, so I haven't sweated it much to make dead sure of it...

Guess I was being too harsh :-(. I still question Cardas’ motives, however. George, why not do the soldering necessary to make the plugs shorting, so they provide a sonic benefit? The user will need to know not to insert them in output jacks, though. Perhaps there is a concern over potential pre-amp damage claims.

 The caps are cheap and keep dust out of the RCA inputs. Seems like a good idea. It is as simple as that.