Likely cause of low bass output in PA speaker?

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by doc, Sep 5, 2019.

  1. doc

    doc Member

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    I picked up a pair of Carvin passive PA speakers (12" woofers, single horns) at the pawn shop really cheap, knowing they'd need some work. I suspected in limited testing at the shop that one of the tweeter horns was likely out, but the woofers seemed okay. I hoped I'd get lucky and the compression drivers were okay and it was a connection or crossover issue, but suspected not. Sure enough, one of the compression drivers was dead. I also noticed that the one without the dead tweeter seemed to have a bit less bass. I had hoped to source an exact replacement compression driver, but found two problems with that - one is that the drivers appear to be epoxied to the horns, and second is the drivers appear not to be available. So I ended up getting some replacement horns that fit the holes (and seem to be very exact replacements) and two new Goldwood drivers from Parts Express. I just installed those tonight, and now the top end seems good on them. The one with the weak bass though seems more strident, and the bass is noticeably less than the other speaker. I think the woofer is okay, it doesn't make weird noises but just seems weaker than the other, so I'm convinced the crossover in that one has a problem. I don't have a crossover schematic, and the exact one doesn't seem to be available. My first thought is that maybe the capacitor in the low pass filter to the woofer in the weak one is blown, but I'm not sure how that would make it noticeably less bass output. I didn't have time tonight to mess with pulling the crossover, but hopeful one of you gurus has a good idea of where the problem is based on this much info.
     
  2. doc

    doc Member

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    Pulled the crossover to look it over. Someone has been in there before, and it sort of looks like the inductor for the low pass has maybe been replaced, and it looks like the correct part. The soldering isn't great, but nothing obviously wrong. Other than that it looks ok. After messing with it a little, it seems like it not only has a bit less bass than the other speaker, but the treble is hotter. The difference in the amount of bass isn't huge. Maybe there is also a problem with the high pass as well, since the treble seems hotter than the other one. I guess the next step is to desolder one end of each of the caps and check the values.
     
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  3. Glitch Magnet

    Glitch Magnet Supporting Member

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    You could also swap the crossovers and see if the problem moves.
     
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  4. doc

    doc Member

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    Hey, that's a good idea. Thanks.
     
  5. PushedGlass

    PushedGlass Member

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    You could also hook the 12s up to a power amp without the crossovers and see if they're working the same. I'd even connect them both to the same output channel to be sure. I had a Hartke 4x10" Transporter XL cab and discovered one day that one of the 10s was weak. Not dead, just weak. Don't know if maybe the magnet was out of spec or the voice coil was miswound, but something about it was off.
     

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