Shorted RCA plugs

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  • Dr C
    Member
    • Feb 2003
    • 86

    Shorted RCA plugs

    The recent thread about jumpers had me thinking about something I was considering to do for some time.

    Some folks advise shorting out all the inputs on your pre-amp to lower noise (eg see http://www.tubeaudio.com/page16.html.

    Others say that shorting plugs is not good ... possibly damaging and recommend other ways http://shopping.netsuite.com/s.nl/c....it.A/id.371/.f

    Questions I want to ask are :

    1. Anyone try either ... and what were the results ?
    2. On a technical/electrical level; which is theoretically better ?
    3. Anyone try either on a Rotel 1098 or 1066 ?
    4. Anyone try them on unused inputs on active subwoofers ?

    Thks - Dr C
  • brucek
    HTG Expert
    • Aug 2000
    • 303

    #2
    Dr C

    I'm a fan of shorting plugs on unused analog inputs. Inputs are high impedance, usually in the order of 10K-50K and as such are susceptable to picking up some RFI/EMI noise if left open. I'd rather not use a short, so I usually make my own shorting plugs from an RCA plug with any 10-100 ohm resistor. This presents an input load that will not allow any noise to enter and also matches the range of output impedance that a device would normally have feeding it.

    They're not necessary on 75 ohm digital inputs since they don't have this noise floor concern. The levels of RFI/EMI would never be strong enough to be considered.

    I would recommend this over a metal cap. What could the cap possibly do? Do they really want us to believe that the RFI/EMI "only" enters through the hole provided in the female RCA connectors. I don't really want to put down the company that makes these, but again, the notion that capping your input connectors as being the only path to get to the high input impedance line stage is silly. RF can easily pass through your case vents and be detected on the short signal path from the capped connector to the input element. It's acts a bit like an antenna when left open at its 10K - 50K input impedance. Drawing the input down to near ground with a shorting plug will alleviate this problem. It's actually a very, very minor problem, but every little inch helps..

    I would recommend loading all your unused inputs (not outputs :cry: ).

    brucek

    Comment

    • Dr C
      Member
      • Feb 2003
      • 86

      #3
      Hi Bruce

      Thks for the tips. Could I get you to post a pix of what you did so I could get a better idea of what I have to do ?

      Thks

      Comment

      • brucek
        HTG Expert
        • Aug 2000
        • 303

        #4
        Simply buy some male RCA plugs (of any quality) and solder a small resistor (75 ohms are easy to find) between the center pin and ground.
        I don't really have a good picture - but you get the idea anyway.

        Comment

        • ht_addict
          Senior Member
          • Dec 2002
          • 508

          #5
          With all the inputs on the back of my Yamaha RX-V1400 wouldn't it be cheaper just to use tinfoil and wrap each one?

          ht_addict

          Comment

          • brucek
            HTG Expert
            • Aug 2000
            • 303

            #6
            Re-read what I wrote in my post above..

            What could the cap possibly do? Do they really want us to believe that the RFI/EMI "only" enters through the hole provided in the female RCA connectors. I don't really want to put down the company that makes these, but again, the notion that capping your input connectors as being the only path to get to the high input impedance line stage is silly. RF can easily pass through your case vents and be detected on the short signal path from the capped connector to the input element. It's acts a bit like an antenna when left open at its 10K - 50K input impedance. Drawing the input down to near ground with a shorting plug will alleviate this problem. It's actually a very, very minor problem, but every little inch helps..

            Comment

            • Chris D
              Moderator Emeritus
              • Dec 2000
              • 16877

              #7
              Holy crap... never heard of doing this. I'd like to hear and learn more. Can this REALLY help a system?




              CHRIS
              Luke: "Hey, I'm not such a bad pilot myself, you know"
              CHRIS

              Well, we're safe for now. Thank goodness we're in a bowling alley.
              - Pleasantville

              Comment

              • Hoosier Audio Nut
                Junior Member
                • Aug 2000
                • 2

                #8
                Can this REALLY help a system?
                Yes, it will. In the test lab, analog inputs are shorted before their signal to noise specs are measured. Shorted inputs (or as Bruce does it) can help prevent input crosstalk too (such as FM radio bleeding over into an unused analog CD input. Doing this may not seem like a huge deal but anything to help keep extraneous noise out certainly doesn't hurt...




                HAN
                HAN

                Comment

                • aarsoe
                  Senior Member
                  • May 2004
                  • 795

                  #9
                  OK - I agree that installing a resistor on any un-used inputs can help improve things - but I really would like to know is what happens when my 1 year old daughter gets her hands on my remote and selects one of the inputs that are shortened????
                  Diasater - or what? This have prevented me from doing this so far, but maybe one of you could tell me otherwise...

                  Comment

                  • brucek
                    HTG Expert
                    • Aug 2000
                    • 303

                    #10
                    It's the input that's shorted, not the output.

                    Shorting the input means there will be no signal getting to the input stage.

                    The short answer (pardon the pun) is that nothing bad will happen when you select a shorted input.... :T

                    Comment

                    • JonMarsh
                      Mad Max Moderator
                      • Aug 2000
                      • 15261

                      #11
                      A lot depends on the architecture of the specific preamp as to whether low Z termination or shorting plugs, or caps will be of any benefit.

                      For many preamps, if an input isn't selected, it isn't connected to anything on the signal side. For some preamps, like certain Ayre models, if an input isn't selected, it isn't connected on either the ground or input side.

                      So, if you're moving between inputs you want to select and an unused input, you will have a lower background noise level if an impedance termination connector is used, but I can't see that it will have much impact on the used channels. OTOH, if the input selection is at some distance removed from the input connectors, there's a tiny possibility that those wires or PCB traces could act as an antenna for RFI in severe environments. Being terminated with a low impedance at the plug would be helpful.

                      If the preamp uses a buffer of some kind for all inputs, and then selects on which buffer output to connect, possibly the system noise floor will be low by having unused inputs terminated. That's not a commonplace configuration.

                      The company which created caps for unused inputs is Cardas. I understand anyone's theoretical skepticism regarding this component. One friend I have known a long time, which use a lot of Cardas parts and who OEM some components and manufactured items through Cardas, thinks these caps are helpful in many system configurations, and uses them and recommends them. Among other things, they manufacture preamps, which do not have any "ventallation" slots or other openings into the chasis, excepting input and output connectors. Caps can be used on both inputs and outputs. I don't use them myself; haven't tried them. So I don't have any personal opinion, other than since my friend has found them useful, it's not something I would dismiss out of hand, but I wouldn't recommend you go out and buy a handful as "the next great thing" for your system. :W

                      ~Jon
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