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Thread: About 3-Pole Bump Filters ?

  1. #1
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    About 3-Pole Bump Filters ?

    Hi All,

    (A) Re; Todds' 3-way "bump-circuit for the 2410/2307(HL91) combo .

    - I built up a partial working version of the horn circuit that Toddalin is now using . Through that exercise I became aware that this circuit has it's 3-pole hipass arranged as a "bump-circuit" /before initiating its' mid-frequency roll-off . Giskard had previously displayed this in a voltage drive for the orginating L200B and Zilch confirmed the bump was present in that example of the 3-way crossover that Todd had sent him . ( I knew the "bump" worked in my circuit through a couple of different type of measurements . ) The bump does a good job at shoring up the droopy response of the last lower octave for the 2410/2307 combo .
    - See the pics below for the "predicted" bump versus the realized bump in Todds 3-way crossover .

    (B) Now; A new bump circuit for a 2426H/2307(HL91) combo.

    (i) Overview
    - With a bunch of Lpads in place & with an extra resistor in line with the compression driver ( to simulate a 16 ohm driver ) / the working circuit impedance that the CLC components "see" , calculates out to be @ 6.3 ohms .

    - Here are some component values ( with calculated Fc points )
    C1 = 16.5 uF = 1536. hz
    L1 = 2.1 mH = 476. hz
    C2 = 24 uF = 1056. hz

    - This 3 passive elements form 2 measurable resonances , peaking at approx. 900 hz and 700 hz .



    - If one looks at the "pole spreads" between the CLC elements one will notice that C1/L1 = about 1.6 octaves
    C2/L1 = about 1.1 octave
    C1/C2 = only about .73 octave

    - I don't know the math, necessary to predict where these 3 components are going to form their resonant points. Intuitively, I wouldn't have "guessed" anywhere near those 2 measured points of 700 & 900 hz.

    (ii) A question

    - Does anyone have a math formula ( that they can post ) that would accurately predict which two frequency points the resonant pairs are going to peak at ?

    Thanks <> Earl K
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  2. #2
    RIP 2011 Zilch's Avatar
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    I think that's the "Q" of the filters makin' them humps.

    Think Chebyshev, for example, maybe.

    I'm not seeing them as "bump" filters per se, but that's just perspective, maybe.

    Best I have on the formulas is here:

    http://www.passivecrossovers.com/index.htm

    Click "Drawing the numbers" in the left frame.

    There's more after that at "The filter Q"....

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