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Ground lift on DI question

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  • Ground lift on DI question

    Where does the ground get lifted? As I see it there are 3 connection, 2 have hot/ground, one has hot/cold/ground:

    1/4 input/output unbalanced (or balanced)

    1/4 input /output

    XLR output (assuming pin 1 not tied to chassis at all)



    Ground lift is between... Both 1/4 I/O? XLR and 1/4? XLR and chassis?



    I guess I can't believe I went this long without knowing. For ease of discussion assume 1/4 inputs are unbalanced and the DI is passive, no switches for pad.
    www.rock-bot.com
    Live-Band-Karaoke

    bassist and sound reinforcement

  • #2
    My Business: Media Production in the Texas Hill Country

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    • #3
      Ground lift opens the shield connection (pin 1) on the balanced line. The shield is not used for signal, only for shielding. In practice, there are really 2 grounds present, low frequency ground and RF ground. Often the RF ground will remain and only the LF ground will be open.



      Opening the ground on the balanced connection between units prevents current from flowing in the shield due to potential difference beween equipment at 2 different locations (or poorly designed ground current steering in a piece).









      Quote Originally Posted by Unalaska
      View Post

      Where does the ground get lifted? As I see it there are 3 connection, 2 have hot/ground, one has hot/cold/ground:

      1/4 input/output unbalanced (or balanced)

      1/4 input /output

      XLR output (assuming pin 1 not tied to chassis at all)



      Ground lift is between... Both 1/4 I/O? XLR and 1/4? XLR and chassis?



      I guess I can't believe I went this long without knowing. For ease of discussion assume 1/4 inputs are unbalanced and the DI is passive, no switches for pad.




      -------------------------------------------------------------------------------------------------------------------------------------------------------------------
      Former product development engineer: Genz Benz, a KMC Music/Fender Musical Instruments Company, continuing factory level product support and service for Genz Benz

      Currently product development engineer: Mesa Boogie

      Comment


      • #4
        Ground lift is between the unbalanced side "sleeve" and the balanced side pin 1 (ground AKA shield). BTW the XLR does not have hot/cold/ground but +/-/shield - pins 2 and 3 are both "hot" but of opposite phase, usually referenced to pin 1.

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        • #5






          Quote Originally Posted by RoadRanger
          View Post

          Ground lift is between the unbalanced side "sleeve" and the balanced side pin 1 (ground AKA shield). BTW the XLR does not have hot/cold/ground but +/-/shield - pins 2 and 3 are both "hot" but of opposite phase, usually referenced to pin 1.




          Not on actively driven outputs. Input shield can remain ground referenced but the pin one becomes unreferenced to the DI's ground. Phantom power, or some of the AC powered active DI's (think preamp-DI) retain ground reference throughout the unit but break pin 1 between circuit ground and console ground. The unbalanced input sleeve will always (except for a couple of unusual architectures) renain referenced and connected directly to circuit ground. On a passive DI, there is no circuit ground, so the only connection that is common (sleeve and pin 1) is broken.
          -------------------------------------------------------------------------------------------------------------------------------------------------------------------
          Former product development engineer: Genz Benz, a KMC Music/Fender Musical Instruments Company, continuing factory level product support and service for Genz Benz

          Currently product development engineer: Mesa Boogie

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          • #6
            I've got a signalflex DI from the 90s, it has no ground lift on it so I want to install one. I put the switch on the balanced output side between pin1 and the chassis.
            www.rock-bot.com
            Live-Band-Karaoke

            bassist and sound reinforcement

            Comment


            • #7






              Quote Originally Posted by Unalaska
              View Post

              I've got a signalflex DI from the 90s, it has no ground lift on it so I want to install one. I put the switch on the balanced output side between pin1 and the chassis.




              Yep - but note on the schematic above that if your transformer has dual electrostatic shields you want the balanced side one to remain connected to pin one and NOT the DI's chassis when you ground lift.

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              • #8
                AFAIK there is only 3 wires on the 1/4 side of the transformer (middle is shield?) and only 2 on the output side
                www.rock-bot.com
                Live-Band-Karaoke

                bassist and sound reinforcement

                Comment


                • #9






                  Quote Originally Posted by Unalaska
                  View Post

                  AFAIK there is only 3 wires on the 1/4 side of the transformer (middle is shield?) and only 2 on the output side




                  maybe
                  -------------------------------------------------------------------------------------------------------------------------------------------------------------------
                  Former product development engineer: Genz Benz, a KMC Music/Fender Musical Instruments Company, continuing factory level product support and service for Genz Benz

                  Currently product development engineer: Mesa Boogie

                  Comment


                  • #10






                    Quote Originally Posted by Unalaska
                    View Post

                    I've got a signalflex DI from the 90s, it has no ground lift on it so I want to install one. I put the switch on the balanced output side between pin1 and the chassis.




                    If the DI box has a plastic XLR jack make sure there is not a ground going to a contact for the mating shell. If the XLR is metal, you will need to check your cables. Many have pin 1 tied to the shell and that will defeat the ground lift switch.
                    http://LBPinc.com/di

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