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Optimum Capacitor Value ?

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  • Optimum Capacitor Value ?

    For my purpose , response down to 100 Hz will suffice .
    What would be the value of a single capacitor in series with this :

    http://www.tb-speaker.com/uploads/fi...8623817fad.pdf

  • #2
    Other best value is always 7...

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    • #3
      Trusting TB's specs (should we?); that module's output runs around 86dB up near 10kHz, and drops about -8dB down to about 78dB near 200Hz, below which it drops like a rock. At 100Hz, it's output is around 69dB, that's over -12dB down from its average (of 82dB - around 1.5k) and about -8dB down from the rolloff "knee" near 200. So, in a way, you COULD say it's F3 was at 170Hz, unfiltered ! Also, their FR plot says it's at 1w, which is only 2.0v. At (the "standard") 2.83v, you can add +3dB across the board.

      Not only that, but TB recommends a 2nd order HP @ 60Hz, which COULD be a 300uF (npe) series cap and a 10mH (iron core) shunt coil. About $15.
      This filter pulls the T2 down -6dB @ 60Hz, but (oddly enough) does NOT affect it's response much above that point.
      You can get similar "protection" below 60Hz by just using a single 125uF series cap, but (since the slope is less) you end up being -6dB down up near 125Hz.
      At 100Hz, the 2nd order filter (above) gives you more than +6dB more output, compared to just a simple cap.

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      • #4
        +1. First order filtering offers very little driver protection and excess pass band overlap, while the low Q results in sensitivity loss near the knee frequency. There's really nothing about first order filters that justifies recommending them.

        www.billfitzmaurice.com
        www.billfitzmaurice.info/forum

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        • #5
          I just want to choose an output cap for my amp that will sort of fill
          the protection criteria of T.B.s recomendation for 60 Hz .

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          • #6
            Originally posted by hitsware2 View Post
            I just want to choose an output cap for my amp that will sort of fill
            the protection criteria of T.B.s recomendation for 60 Hz .
            If TB recommends a 2nd order filter, you can't get there with a cap. In fact, because of the driver resonant rise in impedance, you may make matters worse.
            Francis

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            • #7
              If I high-pass the input to the amp with a cap .....
              And then high-pass the output at the same frequency .....
              Won ' t the result be 12 db / oct ?

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              • #8
                Originally posted by hitsware2 View Post
                If I high-pass the input to the amp with a cap .....
                And then high-pass the output at the same frequency .....
                Won ' t the result be 12 db / oct ?
                Yes; if both are (say) 6dB/octave Butterworth filters, the cascade gives a Linkwitz-Riley 12dB/octave filter. However, a cap to the driver may not (probably won't) result in a 6dB/oct Butterworth. The resonant peak of the driver generally screws that up.
                Francis

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                • #9
                  > The resonant peak of the driver generally screws that up.

                  Hence my original question

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