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Test Method For Low Z Input

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  • Test Method For Low Z Input

    Techs and friends....

    Perhaps I should have asked this long ago when working an a Peavey XR Series mixer amp - but I am finally getting around to it.

    I am attaching a Snippit of a typical input circuit for a mixer Preamp (would be any type of Preamp Circuit). To the left is the Low Z input; to the right is the High Z input. Testing the High Z has been simple - I used a 1/4" plug and connect a signal generator. But what about a recommended method for testing the High Z input (if you did not have a microphone) and looking for a steady input signal? Would you connect a signal generator to either pins 2 or 3 and the other end to chassis ground? What about running a signal into a Direct Box and then taking that output to feed into the Low Z input?

    Thanks,

    Tom
    Attached Files

  • #2
    You mean test the low Z input?

    I have 1/4 to XLR adaptors in my kit. I have some wired pin 3 hot (2 ground), and some wired pin 2 hot (3 ground). Pick one, and plug it into the XLR on the unit. It will apply a signal to either pin 2 or 3, and ground the other hot pin. That will work well enough to sort out a problem. Ideally check both ways.

    Note, I am still using pins 2 and 3 rather than chassis ground. I am grounding one of those two pins.

    But really... I have my trusty SM58 on a three foot XLR cord sitting next to my bench. If you only want to test functionality of the circuit, most any low Z XLR mic will work. Just bite the bullet and get some sort of mic for your bench.
    Education is what you're left with after you have forgotten what you have learned.

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    • #3
      Ok.... and I have been using alligator clips to do the same thing (directly into pins 2 and 3). I guess the goal is to get a signal into the IC one way or another (phase speaking). And yes, I have a Low Z mic that I use. The other thing I need to get my hands on is a spare condensor mic for testing the phantom power. Otherwise, it is a simple measurement to see if 15vdc appears on pin 3. (Note to some - 15v is an old standard for Phantom Power. I guess we are up to 48vdc these days).

      Thank you Enzo...

      Tom

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      • #4
        Phantom has been 48v for a long time, but some mixers put 15v out there because most mics will work on that, even if not to full capability.

        proper phantom has the voltage on BOTH pins 2 and 3 with respect to pin 1. Pin 1 is usually ground but may not be in some circuits.
        Education is what you're left with after you have forgotten what you have learned.

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        • #5
          I stand corrected.... YES to phantom power on both Pins 2 and 3.

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          • #6
            If you have a DI box, by all means use it.
            I have one of those cheap xlr to 1/4" transformer adapters designed to plug a mic into a guitar amp, I just run it in reverse (with couplers). For test purposes it does the trick.
            Originally posted by Enzo
            I have a sign in my shop that says, "Never think up reasons not to check something."


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            • #7
              Like Enzo said, I made up different adapters for testing.

              Also for running different devices like for background music at a gig.

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