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  • Impedance matching?

    OK toob gurus...

    Doing some late night lurking around, and I found yet another subject I'm clueless about (I know add it to the list, right?:snaggletooth:)

    It seems one needs to be careful in pairing tubes and SS equipment. What happens if you have an impedance mismatch? The basic rule of thumb I found says that if using a tube pre and ss amp, the amp should have an input impedance at least 10x the output impedance of the pre.....I think I'm ok there, just barely. But it got me to thinking, do I have a less than ideal equipment pairing?
    Never Argue With An idiot. They'll Lower You To Their Level And Then Beat You With Experience!

  • #2
    With a tube preamp driving a SS PA, the impedance mismatch will affect the low frequency response of the system. The tube preamp will typically have an output coupling capacitor. This forms a high pass filter for the system, that depending on the two impedances, sets the F-3dB corner frequency.

    This also happens with line level tube output stages and SS preamp input stages. This happens anytime an output coupling capacitor is in the previous stage.
    Better living through Audio Nirvana!

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    • #3
      Originally posted by Jason
      OK toob gurus...

      Doing some late night lurking around, and I found yet another subject I'm clueless about (I know add it to the list, right?:snaggletooth:)

      It seems one needs to be careful in pairing tubes and SS equipment. What happens if you have an impedance mismatch? The basic rule of thumb I found says that if using a tube pre and ss amp, the amp should have an input impedance at least 10x the output impedance of the pre.....I think I'm ok there, just barely. But it got me to thinking, do I have a less than ideal equipment pairing?

      What is a standard output Z of a "Tube-Pre"?
      Lance
      _________
      Whatever

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      • #4
        Originally posted by Lanceb1b
        What is a standard output Z of a "Tube-Pre"?
        Is "Z" the symbol for impedance? not quite sure what your asking.

        Never Argue With An idiot. They'll Lower You To Their Level And Then Beat You With Experience!

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        • #5
          Originally posted by Jason
          Is "Z" the symbol for impedance? not quite sure what your asking.
          Yes what is the out put impedance?
          Lance
          _________
          Whatever

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          • #6
            For my pre it is 450 ohms. As you go up in the Rogue line, it gets lower. And then there's something on their top two offerings called unity gain that I'm not sure what it does..

            So if there's an impedance mismatch, my FR will be off??
            Never Argue With An idiot. They'll Lower You To Their Level And Then Beat You With Experience!

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            • #7
              This place explains it all

              http://www.tape.com/resource/impedance.html
              Lance
              _________
              Whatever

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              • #8
                Lance has a very good site with info.

                Here is another way to look at the issue. Here is an simplified circuit for the preamp to PA interface in picture 1 in a circuit modeling program. The frequency response of this circuit is in picture 2. The F-3dB point is ~20Hz. This would limit low frequency response of the system as it is still down 1dB at 40Hz.

                The full answer mostly depends on the output coupling capacitor in the tube preamp and the input impedance of the PA.
                Better living through Audio Nirvana!

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