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Ka-Boombox With Unconventional XO Design Question

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  • Ka-Boombox With Unconventional XO Design Question

    It only took 15 years to make it happen but I built a boombox using the dual 25 watt full-range amp from ApexJr, four of the Vifa/JBL 4" mid-woofs (PE # 299-495) and a pair of mini 3/4" domes with Neo magnets - super-small, from about 2002 at PE but can't find the part number. Attached is the crossover design, let me explain the method to my madness: The woofs are 4 ohms, that plate amp was't going to be happy at 2 ohms with the woofs paralleled, so I put them in series. But, I only wanted the second woofer on each channel for a bit of bass boost. If this had been a parallel situation I would have done something like a 2 or 3 mH coil on the boost woofer. But being in series all I could think of was to "bypass" that woofer at higher frequencies, netting a bit of bass boost. My first attempt using a 250uF cap shunting the boost woofer proved to be much too big, I could tell mainly by the decided lack of cone movement compared to the full range unit (which for this ghetto-box setup actually sounded decent as these things go). So, for now I've pulled the cap all together and let it fly, and it still sounds fairly "smooth" - decent bass, clean highs, mids aren't ratty. Looking for A) a technical critique of the idea of a shunting cap to net a "boost" in this series network. And B) hopefully the answer to A will explain why it's A) an ineffective idea, or B) suggest an more appropriate value. I'm not looking for legit baffle-step comp, I'm looking for more driver surface area below ~ 250 Hz. Using a .64 ft3 box tuned to about 65Hz.

  • #2
    Right-off I finally figured out that my box is twice the size it needs to be, I kept getting this woofer mixed up with the 299--432 Vifa/JBL driver which takes a box twice that required or the 495's (all of which were on closeout at PE at the same time in like 2002). So I can probably improve bass output by filling some of the internal volume with solid material.

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    • #3
      The tweeter is a closeout Onkyo, PE# 269-776. I have one on each side suspended from the metal grille coaxially in front of a woofer.

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      • #4
        Funny, but I THINK your oversized box is giving you about +3dB of boost near 70Hz. I'd leave that alone right now.
        Your 250uF "bypass" cap is actually cutting the bass below about 700Hz. I'd skip it.
        For fun, try halving the cap, to about 1.5uF.

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        • #5
          That's interesting because I sort of fibbed on my schematic. I originally had a 4.75 cap on the tweet, thought it was just a tad too low and while I was in there (it's like Pandora's box to get in there) I pulled the 250 uF cap, couldn't find a 3.3uF cap in the moment and just threw a 1.5 uF in there. Again I'm surprised how warm it sounds, using the "Classical" EQ setting on an iDevice pumps it right up - it's being used in a print production environment so that should ensure there's _some_ bass to get above the machine noise.

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          • #6
            Photos

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            • #7
              I know I've seen those grilles before - many years ago. What were they from?
              Co-conspirator in the development of the "CR Gnarly Fidelity Reduction Unit" - Registered Trademark, Patent Pending.

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              • #8
                I hate to burst your bubble, but, these were actually purchased new from Amazon last week (shhhhhhh, I know, they aren't from PE - they didn't have this exact size). But I agree, they do look familiar. What's funnier is that these have an indentation for a small rectangular badge, so they could have been used by multiple brands.

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