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Thread: Adding a crossover to a bass cab

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    Adding a crossover to a bass cab

    I've got a Kustom combo bass amp here with a 15" speaker and a tweeter. Based on the wires coming from the speakers, there was some sort of crossover(I'm assuming here), in the amp. The owner doesn't want the amp, and wants me to turn the combo into a standalone extension cab with a 1/4" and speakon connects. He told me to forget about the tweeter. Easy enough.
    Now he's inquiring about using the tweeter, so I'm trying to figure out what kind of crossover I need to install. What other info do I need to know and what kind of crossover do you all suggest for this?

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    Old Timer olddawg's Avatar
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    Quote Originally Posted by LarBal View Post
    I've got a Kustom combo bass amp here with a 15" speaker and a tweeter. Based on the wires coming from the speakers, there was some sort of crossover(I'm assuming here), in the amp. The owner doesn't want the amp, and wants me to turn the combo into a standalone extension cab with a 1/4" and speakon connects. He told me to forget about the tweeter. Easy enough.
    Now he's inquiring about using the tweeter, so I'm trying to figure out what kind of crossover I need to install. What other info do I need to know and what kind of crossover do you all suggest for this?
    Depending on the tweeter and where you want the rolloff to start. I would just put a 4.7, 3.3, or 2.2uf 50v bi polar cap on the positive terminal of the tweeter, parallel it off of the speaker, and call it a day. You can add an LPad if you want to make it adjustable.

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    Oh wow, ok, thanks! I'm a little unsure of how you mean to make the connections, though. Place the cap between the two "+" connections on the 15" and tweeter?

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    Old Timer J M Fahey's Avatar
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    Yes.
    Join - to - and + to + but cut the +/+ wire and insert the capacitor in between.
    Small detail: what kind of tweeter is it?
    Cone? Horn? does it have a magnet? Post a picture to be certain.
    Because if itīs a piezo, no capacitor is needed but a 47 ohm 1W or larger resistor instead.
    Juan Manuel Fahey

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    Quote Originally Posted by J M Fahey View Post
    Yes.
    Join - to - and + to + but cut the +/+ wire and insert the capacitor in between.
    Small detail: what kind of tweeter is it?
    Cone? Horn? does it have a magnet? Post a picture to be certain.
    Because if itīs a piezo, no capacitor is needed but a 47 ohm 1W or larger resistor instead.
    Ok so it's a Kustom K100. Description says a "round piezo horn". So why a resistor and not a cap?
    https://www.musiciansfriend.com/ampl...ixer-amplifier

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    Old Timer olddawg's Avatar
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    So it's piezo tweeter. The resistor just reduces the power slamming it and balances the load. I always still put in a cap. The element wil last longer because it isn't hammered with the energy of low frequency.

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    Quote Originally Posted by olddawg View Post
    So it's piezo tweeter. The resistor just reduces the power slamming it and balances the load. I always still put in a cap. The element wil last longer because it isn't hammered with the energy of low frequency.
    So resistor and cap in series?

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    Old Timer olddawg's Avatar
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    Quote Originally Posted by LarBal View Post
    So resistor and cap in series?
    Yes.. the resistor before the cap on the positive terminal.

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    Lifetime Member Enzo's Avatar
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    Piezos act kinda like caps anyways, they really don't need crossovers. They just don;t respond to low freq signal.
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    Education is what you're left with after you have forgotten what you have learned.

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    Quote Originally Posted by olddawg View Post
    Yes.. the resistor before the cap on the positive terminal.
    15"+ > resistor > cap >piezo +

    correct?

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    Old Timer olddawg's Avatar
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    Quote Originally Posted by LarBal View Post
    15"+ > resistor > cap >piezo +

    correct?
    Yes Sir... and a straight wire goes from the negative terminal on the 15 to the negative terminal on the
    Tweeter. If the tweeter is too bright you can increase the resistor value.

  12. #12
    Old Timer J M Fahey's Avatar
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    The series resistor is needed because the piezo element **is** a capacitor.
    Actually a ceramic disk with both sides metallized, go figure, just itīs designed to flexand twist following applied voltage .
    FWIW many ceramic caps do the same, and both ways, so they are microphonic.
    I have made a piezo Bass pickup using two 2 x .1uF x 50V cheap ceramics, go figure, sandwiched between bridge and body

    And the piezo element usually measures between .15uF (small round tweeters) to .35uF in the larger rectangular horn ones.

    They are considered "very high impedance" ... at 1 kHz, but like all capacitors, show way lower impedance at high frequencies.

    SS amplifiers do not like that at all, to boot many become unstable and oscillate.

    Now oscillating full blast at, say, 20 or 30 kHz , into a near short impedance is a recipe for disaster, so adding some series resistor makes certain that it does not go below , in this case, 47 ohms.

    As of the series cap, since itīs way larger, the piezo disk still takes all the low frequencies you send it.

    To use simple numbers, if Piezo is .15uF and series cap is 1.5uF, tweeter still gets 90% of any voltage sent to it .

    IF you want to protect it, add a resistor in parallel with disk and use that resistor value to calculate crossover.

    But here keep it simple, just the resistor is enough.

    Add the cap if you wish, for peace of mind
    Last edited by J M Fahey; 04-14-2018 at 08:51 PM.
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    Juan Manuel Fahey

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    Thank you very much for the detailed explanation and help, Juan and olddawg!

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    Old Timer olddawg's Avatar
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    I know the math doesn't lie.. but I can only give my anecdotal experience. Years ago I used to make a lot of floor wedges with a 12" speaker and a Motorola piezo horn. The ones that I didn't use the hi pass cap lasted a couple of months. With the cap I don't remember one failing. It's kind of like not using a rectifier on an LED. Theory and practice. But it's just my humble opinion.

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    ALL Solid State Amps have some DC Offset on their speaker output. Hopefully very small but 100mV or so is not uncommon.
    The additional cap will block that. That MAY take some stress of the Piezo or at least make sure it is getting symmetrical drive either side of 0V rather than a small DC biased drive.
    Can't really see why that might affect reliability but cant discount it either.
    Putting in the cap won't hurt anything and MAY help.
    Cheers,
    Ian

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