RCA Shorting Plugs

While I suppose there may be some poor designs that could be damaged working into a 100 ohm load, eventually if not sooner, in general I would expect the combination of the 100 ohm load and the output impedance of the component itself to limit the resulting current to values that would not cause damage.  Some components providing line-level analog outputs can even withstand a direct short, i.e. zero ohms.

Regards,
-- Al

If you want to learn more try this, then report back:1. Solder a 100 ohm resistor across a working input, not an unused input.2. Listen.3. Post your findings here.

Keep in mind that most source components and most preamps and line stages cannot drive 100 ohms with good or even half-way decent results.  And as you may be aware the manufacturers of many tube-based source components and preamps recommend **minimum** load impedances in the area of 20,000 to 50,000 ohms.

There are some source components and preamps which can do that, and of course 100 ohms would be a reasonable load for many low output moving coil phono cartridges.  But the great majority of components providing line-level outputs can't come close to driving an impedance which is that low, while providing reasonable results.

Regards,
-- Al
  

Elizabeth, I’ve never understood why some people put resistors in shorting plugs for analog inputs. As I mentioned earlier in the thread, from the perspective of the input circuit a plug providing a direct short would be little different than if the input were connected to a powered up component having very low output impedance, that is not providing a signal.

Hal, I’m not particularly familiar with the design of integrated circuit chips which provide USB interfaces, but I suspect that what you read to the effect that USB inputs should not be shorted is correct. First, USB interfaces are bidirectional, with communications occurring in both directions on the same differential pair of signal lines when something is connected that can be communicated with. So a USB "input" is also an output. Second, I believe such chips are designed to pull those lines to certain voltages when nothing is connected to them, via internal resistors, and shorting them may affect those voltage states in adverse and/or unpredictable ways. So I would recommend against doing that.

Best regards,
-- Al

Hi Jaybe,

It would be extremely rare for that issue to arise in the case of a preamp.

As I mentioned that issue would arise in cases such as power amps where the designer anticipated that only one of the two inputs (XLR or RCA) would be connected.  And of course nearly all preamps providing XLR and RCA inputs are designed to accept signals from different components that may be simultaneously connected to those inputs, and therefore the signals on those inputs would be kept separate internally.

I vaguely recall once reading a description of a preamp which might have been a very rare exception to that on some of its inputs, but it was not an ARC product.

And in any event such an occurrence would be immediately noticeable as a 6 db reduction in volume (since only one of the two signals in the balanced signal pair would be processed), and probably also as a degradation in sonics.  With damage (to the source component in that situation, not to the preamp) being very unlikely to occur unless the system is used in that condition over a long period of time.  Perhaps even months or never, depending on the particular design.

Best regards,
-- Al

So perhaps the shorting of inputs effects a particular frequency range more so than others? ... In the case of the M6si I'm hearing the sound from the tweeter, whereas in the T Refs the sound is from the woofer.

Hi Hal,

My instinct would be to not extrapolate any general conclusions from your findings. Per my previous comment, pickup of hum or noise at various frequencies will depend on the specifics of the designs that are involved, and on the environment in which they are used, and will tend to have little if any predictability.

Best regards,
-- Al
 

Might or should there be a theoretical or practical difference when using shorting plugs on an input of different amplifier classes?

Hi Hal,

I'm doubtful that any such generalizations could be made, as I would expect whatever benefit shorting plugs might provide (with or without resistors) to be highly dependent on the specific design of the component they are used on, as well as on the specific characteristics of noise that may be generated by other components in the system, and that may be present in the surrounding environment.

Best regards,
-- Al

Jaybe, regarding the safety of using shorting plugs which do not have a resistor (on inputs only, of course), the only situation I can envision in which that might be a problem is the one about which I stated the following in the thread Gdhal linked to in his first post above:

... 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).

Also, as I said in that thread:

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.

Finally, if which type is used makes any difference at all I would expect the kind that provides a direct short, rather than a resistor, to be more effective.  Everything else being equal, low impedance circuit points tend to be less susceptible to noise pickup than higher impedance points.

Regards,
-- Al