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Adding a tweeter to a cabinet

Discussion in 'Amps and Cabs [BG]' started by nil, Nov 11, 2003.

  1. I've got a cheapo 1x12 i've cobbled together from an old ported box and a car sub. Sounds killer (honestly), so much so the 8x10's been sitting idle in the garage for a while now! :D

    I'd like to add a tweeter to enhance it's range. I've got a couple of spare Motorola piezo units that I can use, and I don't need absolute clarity in the crossover.

    How do I correctly wire this in? Is it in parallel with the 12", the +ve side going through a capacitor to cut lows or something? How can I determine what value components to use to get a good crossover point (around 3K)?

    Also, will adding the tweeter to an already 4ohm cabinet cause any issues with impedence - amp or output of the 12"?

    thanks! :)
  2. Ohms law: E=IR

    never mind.

    To calculate C:


    freq is whatever xover frequency you need
    R is the impedance of the tweeter.

    f = 3K, R=8

    C = 0.0000066 which is 6.6 uf. Standard value is 6.8 uf, you could maybe find a non-polar type at radio shack of this value, or try Digikey or Mouser or Madisound to order it.

    Piezos are super efficient though. If your subwoofer is a typical car sub, they are notoriously inefficient and you may have a serious problem of the highs being much louder than the lows. There's a way to calculate what's called an L-pad circuit, I just can't remember it off the top of my head.
  3. Wiring: (sorry I forgot this)

    hook up the woofer full range, and wire the cap in series with the tweeter, from the + input to through the cap, then into the + on the tweeter.

    Hook all three grounds together.

    try it out with out cutting a hole in your box first. then if you like it you can do the mod.

  4. wneff

    wneff Supporting Member

    May 27, 2003
    Woburn, MA
    Piezos don't need crossovers.

    They have a really high impedance (for a loudspeaker) of 1000 Ohm or so, so you just hook it up in parallel to the 12" speaker.

    They start to emit sound at about 2 - 4 kHz, depending on the model, and below that frequency they don't do anything.

    I have a piezo in parallel to my 12" in my GK 200MB (just used the external speaker out).

    If the piezo is too dominant put a resistor in series (I have a 500 Ohm 1/2 W resistor in series).

    Have fun!

  5. Fan-tas-tic! :D :D Thanks so much, i'll give this a go today. I'm sceptical that I even have enough room to mount the tweeter properly, so might end up mounting it internally, firing out the reflex slot! ;)
  6. Stinsok

    Stinsok Supporting Member

    Dec 16, 2002
    Central Alabama
    They are right, just wire it up and go. No crossover needed with the piezo. I added one to my Behringer combo. Can't really tell it when using a bass but there is a night and day difference if I run a CD player through it. Cheap and easy fun!
  7. I can only speak from experience, and I blew up a piezo tweeter running it full range. The Radio Shack one I have now has a cap built in, it must mean something.

    But I guess they're not expensive so whatever.

    They can be pretty harsh sounding too. But if you like hey that's cool.

  8. I'm not a big fan of piezo's, i've just got 4 spare ones! I do find them a bit too harsh and brash...methinks by installing it *inside* the cabinet I can tame it a touch. This is mainly so that my stage sound is a little closer to what i'm sending to the board.

    I've heard that they can blow quite easily, a reason why most piezo tweeter-equipped cabs have a bulb protection.

    As for cheap, the Motorola ones (here at least) aren't really at a throwaway price.

    I'll definitely wire in a cap, maybe even a variable (if I can find one) to allow some fiddling around.
  9. Go to the Parts Express website and look under their technical resources. They have crossover calculators, as well as L-pad calculators, so you can wire your tweeter in properly.

    Or buy one of their L-pad variable attenuators for a few bucks.


    Do it right instead of just sticking it in there.
  10. wneff

    wneff Supporting Member

    May 27, 2003
    Woburn, MA
    Just a little clarification:

    There are two common ways to build a loudspeaker that produces high frequencies:
    1) Build a loudspeaker with a voice coil and a magnet, use a really light and small membrane (titanium) and a tiny voice coil.
    This is a dynamic tweeter and NEEDS a cross over. They blow by putting in too much POWER.
    2) Grow a crystal with the right properties so that when you apply a voltage it expands or shrinks. These crystal are called piezos. They do not neeed a cross over, they can be wired parallel to the bass speaker. They do not blow by too much power, but by TOO MUCH VOLTAGE.

    That means, if you run our piezo in parallel to an 8 Ohm speaker the system handels half the power than at a 4 ohm speaker (because of the high impedance almost no current flows into the piezo, but if the deformation of the crystal, controlled by the voltage, is too high the crystal breaks. I think piezos are 300W or so in parallel to 8 Ohm speakers.

    Since there is almost no current you can't protect it with a bulb - buld protected speakers are always dynamic speakers, which indeed blow easily (small voice ciol).

    And, yes, piezos sound harsh, thats why I have the resistor in front of it so its not dominating my sound.

    The capacitor infront of the Radio shack may be there to push up the frequency the piezo starts to work. The harshness comes I believe mainly from the frequencies around the onset of the crystal vibration. Maybe radio shack's model has a really low resonance and with the cap they can avoid operating it there?

    To put a variable capacitor infront of it is in my opinion not cost efficient (they are expensive). If you want a really good sound and youre willing to spend some money forget about the piezo and go with a "real" tweeter with membrane and cross over. Carvin sells a tweeter for bass for $40, the cross over for $70.
    And then there is Avatat that sells a light 12" with horn for $189.
  11. Cool, thanks for the info!

    I pulled apart my 1x12 and a donor cabinet that housed a couple of the piezos. Found that they were already wired with a resistor (4.7K?), so just used that harness.

    I had to mount the piezo kinda externally sitting in the slot port of the cabinet (not the best)...but holy heck what an improvement in sound - I could hear a helluvalot more and without having to crank my amp as much. Very happy. I can coax some grit outta the piezo, but i'm not sure whether this is the piezo breaking up, or simply fret/string noise.

    I think though i'm gonna mount the piezo inside the cabinet (and ditch the resistor to compensate) probably firing forward out the slot.

    So for power handling...i'm a little confused. I'm running 400w/4ohm into a 300w/4ohm RMS driver. The piezo should be able to handle this for decent periods? Since there's no draw when it's idle, is it's maximum draw minimal?

    Thanks! :D
  12. Petebass


    Dec 22, 2002
    QLD Australia
    Can I just clarify something - are you blocking the port?
  13. Well, yeah, kinda. The horn is bodged in place sitting in the slot, but due to the shape of it, it doesn't really seem to affect much. The horn isn't sealing it, just sitting inside it (so airflow is happening around it OK).

    Not the best to be sure to be sure, that's why I need to mount it internally.

    This has to be the bodgiest cabinet i've ever owned or played through, but for some reason it's one of the best i've ever used! :D
  14. Petebass


    Dec 22, 2002
    QLD Australia
    A couple of things. By partially blocking the port, you could be doing any number of things, none of them good for the speaker. I'm most concerned about when you're pushing the thing hard. The restricted airflow will and best, cause wind noise, and at worst change the characteristics of the speaker cone excursion with possible mechanical damage to the speaker.

    I'm also concerned that the highs will get caught up in the wind and sound like an outdoor gig on a windy day.

    Of course this is all theory. Have you had a chance to run the cab hard yet?
  15. Fair 'nuff theory! :D

    The slot port is "broken up" into 3 by internal braces, so even if I was totally sealing the middle slot (the longest), the two slots either side are breathing fine.

    Haven't noticed any wind noise (apart from the rest of the guys) at all, and I drive the thing hard all the time.

    I guess compared to a "real" cabinet sporting a properly loaded and configured tweeter this cab would probably sound a bit like arse, but it gives me a closer approximation to my recorded/DI sound (which is useful as I usually do our recording also).

    I can post a pic if you like.
  16. Petebass


    Dec 22, 2002
    QLD Australia
    So it's all looking like I'm worried about nothing. Good! At least we've thought it through.

    Pics - yes please :)
  17. alexclaber

    alexclaber Commercial User

    Jun 19, 2001
    Brighton, UK
    Director - Barefaced Ltd
    Yes - I'd expect the air movement from the port past the tweeter to cause a doppler shift in the highs, but it's probably pretty insignificant in context.

  18. rok51

    rok51 Supporting Member

    Sep 2, 2002
    Crawfordville, FL
    Normally, not one to embrace piezos, but the LeSon TLX-1s are the sweetest-sounding units I have ever heard. These are what SWR uses in their cabinets that don't have the Foster compression tweeter. I've added them to my 115 cabinets (switchable) with excellent results. I did use a cap and resistor. Martin sound has them, cheap.

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