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How to build a cab with a tweeter?

Discussion in 'Amps and Cabs [BG]' started by bluesmandan, Aug 9, 2012.


  1. bluesmandan

    bluesmandan

    Aug 17, 2011
    So I'm wanting to build a bass cab with a tweeter, and I don't understand the wiring, and the piece that splits the frequencies, and how that affects ohms and rms ratings. I've tried googling it and am not pulling up anything useful.

    I would think that this has been discussed on the forum already. Can someone point me to the relevant threads? If not, can you explain to me the theory on how this works? I've built guitar cabinets, but nothing with the tweeter and frequency splitter. I don't understand how it affects the wiring, impedance, and wattage.

    FWIW: It's probably going to be a cab with a single speaker and single tweeter. Something small for practice, studio, and small gigs. 4 ohms. Will need to handle at least a 200 watt amp.

    Thanks for any help!
     
  2. Perhaps starting wih this would help.

    Good luck!
     
  3. will33

    will33

    May 22, 2006
    austin,tx
    If you find a tweet that's somewhat close to 8ohms around the pass frequency, can get away with using an off the shelf 3500 or 5k HPF. A lot of bass cabs just do that and let the woofers run wide open. Not technically the best way, but it's done all the time. Can add an adjustable l-pad to control the tweeter level. The overall cab impedance would be the same as whatever your woofers are.

    Search up some build threads/tutorials and use WinISD or other box sim. program to get your woofer/box combination something decent. Different breed o' cat than making guitar cabs.
     
  4. bluesmandan

    bluesmandan

    Aug 17, 2011
    @ will33: why 8 ohms around the pass frequency? My amp puts out 4 ohms.
     
  5. unless you fit a piezo. :bag:
     
  6. will33

    will33

    May 22, 2006
    austin,tx
    It doesn't "put out" 4 ohms. That's just it's lowest allowable impedance. If it's a tube amp with a 4ohm tap, then you need at least a 4ohm woofer. Any speakers impedance risis so high by the treble territory it shouldn't hurt to have an 8ohm tweeter up there. Most tweeters and nearly all off the shelf filters are for 8ohms. If you need somwthing different, you'd have to roll your own, which is a lot more complicated.
     
  7. bluesmandan

    bluesmandan

    Aug 17, 2011
    Oh. I knew that the actual speaker resistance (while in use) fluctuated all over the place, but I did not realize that there was a relationship to frequency as well as power. So an 8 ohm would be fine (or perhaps even better). Interesting...

    This product came up in a search: http://www.parts-express.com/pe/showdetl.cfm?partnumber=266-464

    Its 4 ohm, 300 Hz. So I should perhaps go for an 8 ohm. Is a 300Hz high pass what I should go for? Or something different? What would be "standard" if there is any?
     
  8. will33

    will33

    May 22, 2006
    austin,tx
  9. 1958Bassman

    1958Bassman

    Oct 20, 2007
    Tweeters don't crossover at 300Hz and the crossover needs to be tailored to the driver, not some chosen frequency point. That's looking for a needle in a haystack.

    If you have used a particular cabinet that you liked a lot, it might be easier to copy it. Coming up with something new isn't really for a first-time project because the technical details can't be ignored.
     
  10. bluesmandan

    bluesmandan

    Aug 17, 2011
    Okay, now I'm getting it. 3500 Hz crossover filter, 3500 Hz tweeter, and an attenuator.

    Now about the wattage ratings on all these... for a 200 watt amp, shouldn't they all be rated at least 200 watt rms? Or is the amount of power sent over to the tweeter cut significantly and not needed so high? How much power is actually going to be sent over to the tweeter?
     
  11. Jools4001

    Jools4001 Supporting Member

    Your tone is made up of the fundamental note and various harmonics of that fundamental at a lesser volume layered on top of that.

    Since the highest normally fretted note on a 24 fret 4 string bass (G4) has a fundamental of around 400Hz, anything that is reproduced above that will have far less energy.

    You still require a frequency response that is capable of reproducing all those juicy harmonics because that's where the tone and sparkle live, but the energy at those frequencies will be very much smaller than the fundamental.

    Couple that with the fact that human ears are much more effective at frequencies and tweeters are also much more efficient than woofers and you only need a tweeter to handle a fraction of the electrical power that the woofer does to get the perceived volume in balance.
     
  12. will33

    will33

    May 22, 2006
    austin,tx
    Not much.

    "Power demand" ,don't know if that's the technically proper term, but anyway, it decreases as frequency increases. It's why you can use a 400 watt woofer, 100 watt midrange and 50 watt tweeter all together and not blow stuff.

    You can run into tweeter trouble if you're playing loud, with a lot of treble boost or distortion. That stuff increases the upper haromonics/treble frequencies the tweeter has to play. That's what the bulb fuses are there to protect against.
     
  13. will33

    will33

    May 22, 2006
    austin,tx
    +1 to this.

    There's just less energy behind the high stuff.
     
  14. 1958Bassman

    1958Bassman

    Oct 20, 2007
    That's more like it.

    If you want to be more sure that the speakers will handle what you throw at it, you can use a 200W model. However, you should become familiar enough with your rig that you know if/when it runs out of power. BTW- that point isn't when the volume control is wode open, either. It runs out long before that, especially if you jack up the tone controls and use pedals that provide gain (it's louder when they're engaged than when they're bypassed). Most variable L-Pads don't handle more than 100W but since you're crossing over at 3500 Hz, your amp would have to be putting out a lot more clean power than that to cook the L-Pad or tweeter. Distortion throws any actual numbers out the window because you may not hear it, but that doesn't mean it's not affecting anything.

    As far as how much the tweeter and L-pad will see, you would need to measure the voltage to find out to be sure.
     
  15. 1958Bassman

    1958Bassman

    Oct 20, 2007
    They're not fuses, just bulbs. The filament gets hot when enough voltage/current flows and when that happens, its resistance increases, limiting voltage AND current. Pretty simple concept considering how functional it is.
     
  16. will33

    will33

    May 22, 2006
    austin,tx
    True.

    When the bulb burns out....then it's a fuse.:p
     
  17. 1958Bassman

    1958Bassman

    Oct 20, 2007
    If someone burns out a light bulb by playing an instrument through it, they're clearly over-powering the speaker or there's some other problem. Another good indication of too much power is popping crossover caps that are rated for 35VDC or 50VDC.
     

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