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Thread: Test Method For Low Z Input

  1. #1
    Supporting Member TomCarlos's Avatar
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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
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    Lifetime Member Enzo's Avatar
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    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.

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  3. #3
    Supporting Member TomCarlos's Avatar
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    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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    Lifetime Member Enzo's Avatar
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    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.

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  5. #5
    Supporting Member TomCarlos's Avatar
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    I stand corrected.... YES to phantom power on both Pins 2 and 3.

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    Don't forget the joker g1's Avatar
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    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.

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    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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