RCA Caps

Good story, Eric (bdp24).  I took a quick look at the SP-10 schematic at arcdb.ws, and sure enough the design provides two pairs of main outputs, both of them RCAs, and on each channel the two RCA connectors are wired directly together!

Best regards,
-- Al
 

Kalali, a shorting plug will have electrical effects, by lowering the input impedance of the unused input to essentially zero ohms. Low impedance circuit points are inherently less susceptible to pickup of electrical noise than high impedance circuit points, everything else being equal. Including pickup of noise that may even originate elsewhere within the same component, that may in turn find its way from the unused input path to the signal path that is being used, or to a circuit point (such as in the power supply) which can affect that signal path.

As was mentioned earlier, and despite some disagreement that was expressed, caps that do not short the input will do nothing other than perhaps reducing oxidation or corrosion of the ground shell that may occur over time, and keeping out a few dust particles.

Regards,
-- Al

Search at eBay for "RCA shorting plugs" (without the quotes) and you'll find numerous sellers offering them, at low prices.

Best regards,
-- Al

Yes, exactly, @gdhal . From the manual for the INT-250:

There are four inputs, two with XLR and RCA and two with RCA only. The XLR and RCA inputs on INPUT 1 (and INPUT 2) are connected so use XLR or RCA but not both. When using INPUT 1 & 2 with RCAs you should have the XLR shorting jumpers in place.

It’s unusual, though, for inputs of an integrated amp to be configured like that. Typically that situation arises with power amplifiers. But I suppose Nelson wanted to provide users with some added flexibility, by making it possible for those inputs to be used with either type of connection. While another designer might have simply made inputs 1 and 2 XLR only.

So with the INT-250 a shorting plug should not be put on the RCA connectors of inputs 1 and 2 if the corresponding XLR connector is being used.

Best regards,
-- Al

Thanks for the follow-up, Hal, as it reminds me of one situation in which a shorting plug on an input could conceivably be harmful.

That would be on some power amplifiers which provide both XLR and RCA inputs, the intent being that only one of those inputs would be used. In some of those cases the signal pin of the RCA connector is wired directly to one of the two signal pins (usually pin 2) of the XLR connector. So if the XLR input is being used in those cases and a shorting plug is put on the RCA connector, the output circuit of the preamp or other component providing the balanced signal pair to the amp would have one of the two signals on its XLR **output** shorted to ground.

And I suppose a similar situation could conceivably arise in the case of some subwoofers, if both RCA and XLR input connectors are provided for a given signal type (i.e., for either an LFE input or an input that is intended to receive a full range signal).

And, yes, I agree with your statement about phono inputs.

Best regards,
-- Al

Hi Hal,

I can't envision a design in which a shorting plug applied to an RCA **input** would cause any problems.  Consider the fact that the essentially zero ohm impedance presented to the input by a shorting plug is not greatly different than the very low output impedances (e.g., 10 ohms or perhaps even less in some cases) of some components that might be used to drive that input.

Best regards,
-- Al
 

IMO, for unused rca INPUT jacks shorting plugs are preferable to caps, although of course in many cases I would not expect either one to make any difference.

Caps will do nothing to reduce coupling of internally generated rfi/emi onto the unused input lines, from where it might couple into the signal path that is in use. The same goes for rfi/emi that may originate externally, but which enters the component via a pathway that bypasses the rca jack and then couples onto those unused input lines. Shorting plugs can help in either of those situations.

Shorting plugs should not be used on output jacks, of course.

Regards,
-- Al