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cascading active and passive filter sections?

Discussion in 'Electronic Design' started by Eric Griswold, Sep 20, 2004.

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  1. Does anybody know this one?

    I have a sealed speaker whose acoustic rolloff response is a chebychev
    2nd-order highpass filter with a cutoff frequency of 119 Hz and a Q of 1.0.

    I want to add an first-order active highpass filter before the power amp, to
    make a compound, 3rd-order Butterworth response overall.

    I know that a first-order highpass has a Q of .707 so when I multiply that
    by the speaker's Q of 1.0 I will have a final Q of .707 which is
    Butterworth, just how I want it.

    What I don't know, is what frequency to make the active filter, and what the
    final cutoff frequency will be.

    Can I place the active filter at 119 Hz too? Bt then the overall -3dB point
    will move a little higher, won't it? Where will it go?


    eric g.
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