What's the Deal w/ Preamp Input Z?

Discussion in 'Preamps / Channel Strips' started by mark4man, Sep 19, 2005.

  1. mark4man

    mark4man Active Member

    Apr 18, 2003
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    [Sorry to post another inquiry (related to my prior post) so soon, but]...

    Most of the preamps I'm looking at have Input Impedance specs around 1.4k to 1.6k (some are higher...TrakMaster Pro @ 2.5k; & fmr audio's RNP @ 5k.) My best mic has an Output Impedance of 100 Ohms. I had always heard that the pre's Input Z should be exactly 10 times that of the mic's Output Z.

    Is this true? If it is...how important is it? Should I be worried about sending a 100 Ohm signal thru anything greater than 1k? Is the 10X rule a "minumum" #? Where can I get info on mic load & pre Z?


  2. jonyoung

    jonyoung Well-Known Member

    Dec 31, 2003
    Input impedance should be AT LEAST 10x, doesn't have to match at 10x. It assures proper power transfer, S/N ratio.
  3. dpd

    dpd Active Member

    Sep 29, 2004
    ^^^^ Proper POWER transfer comes from matching the impedances. However, you don't want to match impedances.

    The optimum S/N ratio will depend on the complex output impedance of the mic and the complex input impedance of the preamp, the actual signal level the mic outputs and the self noise of the preamp. It can get messy if you try to calculate it.

    Too high an input impedance and you get too much noise from the preamp front end. Too low and you lose signal from the mic, need more gain, and preamp noise again becomes a problem.

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