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High Pass Filter for Bookshelves

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  • High Pass Filter for Bookshelves

    I'm building an amplifier for a pair of bookshelf speakers (currently an Overnight Sensations MT kit) to pair with a subwoofer.

    What's the simplest way to high-pass the speakers to ~70-80Hz? Can I put a large capacitor (~250uF) in series with the woofer portion of the OS crossover without affecting the performance? Or any bookshelf crossover, really.

    Or is there a simple line-level circuit I can build to do the same thing? The amplifier I am using is a Sure 2x30 TPA3118 board (AA-AB32472, not sold on PE). I was going to pass the audio signal (Chromecast Audio) through a 50k audio pot and then split the signal into this amplifier and the low-pass subwoofer crossover board #320-670 (which would feed a separate subwoofer amp).

    Thanks!

  • #2
    Definitely go active (before the amp). Adding a cap before the speaker helps, but the amp is still trying to hit those low frequency voltage swings and will clip earlier.

    If you can find an input impedance on that amp module, a simple cap in line will get you 6dB / octave with the formula C = 1/(2 * pi * R * f).

    Also, you can buy little in line bass blockers that are essentially the same thing, I forget what they're called. Here they are: FMOD
    Last edited by wogg; 05-01-2017, 07:53 PM. Reason: Found 'em.
    Electronics engineer, woofer enthusiast, and musician.
    Wogg Music

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    • #3
      Thanks for the info!

      I saw the FMOD jacks and I didn't feel they were a good fit because a) they were RCA, which won't exist between main volume and the amp, and b) they are pricey at $25.

      Assuming 30k input impedance and 80hz high pass, your formula yields 0.066 uF or 66nF. Does that sound correct? What voltage rating should the cap have?

      Related dumb question, is there such a thing as a variable capacitor that would allow adjustment of the HP frequency?

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      • #4
        Yup, 66nF is correct for 80Hz and 30k ohm. The voltage rating isn't critical, the most voltage you'll see on the cap will be whatever the peak input voltage would be, so only a few volts.

        There are variable caps, but they're more expensive and harder to find. They used to use them in old school analog AM/FM tuners commonly, but most of that is digitally controlled now. If you can find one, it's likely to be in the pF range and not really useful for what you're trying to do. I'd check Digikey.
        Electronics engineer, woofer enthusiast, and musician.
        Wogg Music

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