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High Pass Filter for small PA?

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  • High Pass Filter for small PA?

    Hi all,
    I've been using a powered mixer for small gigs and leaving the horsepower (and active crossover, amps and everything else in the 3-way system) at home.
    I'd like to add a single sub to the system to add just a little grunt to the overall sound. My powered mixer has not only powered speaker outs, but outs in the front that aren't powered. I hooked up an amp and the sub (which has a low pass filter in it) and it sounded good, but my small mains I use with this mixer clip when I feed them too much of anything below about 100hz.

    I figured rather than add a crossover and more rack gear (trying to keep this system small) I could add a high pass filter to the small main cabs to keep everything below about 100~150hz out of them so they don't get damaged. If I don't use the sub I can just dump the built in EQ at the bottom, but that makes everything sound pretty thin - that's why I wanted the sub.

    The mixer is a SoundTech PMX10SD powered mixer. The sub is an EV iForce single 18 and my mains are DIY cabs with a 15 (EVM-15B 400w drivers) and a small horn. They are crossed over internally between the 15 and horn and are pretty efficient.

    When I use them in the larger system I have, these go down to the floor as vocal monitors so they wouldn't need sub freq in them there either. My powered mixer does not have a way to go from the preamp section out of the unit to a crossover and back to the internal amps.

    I've gone over my options and considering I do not want to haul more gear than the sub and one amp to add the sub to my system my best option is a sharp cutoff high frequency pass built in the main cabs. Anyone have a high pass filter schematic that I could add to my small main cabs to keep sub freq out of them? I doubt they'd ever see the full 450W, but the closer to that it'll handle the better 'just in case'.

    - JJ
    My Momma always said, Stultus est sicut stultus facit

  • #2
    Do you mean a low-level active filter for before the power amps, or a speaker-level passive filter for after them?

    If the former, I built one years ago, just using a couple of Sallen & Key stages.

    If the latter, I'd just connect a motor run capacitor in series with the speaker, sized to have a reactance of 8 ohms at 100Hz or whatever. It'll probably interact with the speaker's mechanical resonance and start to roll off below the resonance.

    It's not a sharp cutoff, but if the speakers were ported to start with, you get a pretty sharp acoustic rolloff, and I don't think it's worthwhile spending effort and money on designing a high-order passive filter, especially because it'll interact with the speaker and not work like you expect.

    But if you hear something "clip", how do you know it's the speaker and not your amp, in which case no high-level crossover can help?
    "Enzo, I see that you replied parasitic oscillations. Is that a hypothesis? Or is that your amazing metal band I should check out?"


    • #3
      well the first iteration of a HPF is a cap in series with the speaker connection, netting a 6db/oct 1st order filter.

      this can be sufficient in terms of speaker protection, depending on how high you place the break frequency.

      if you want steeper slopes you can add shunt inductors and/or add more series caps. the filter will become more effective but will also likely sound worse due to the degradation of transient response that occurs with higher order filters.