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Using a resistor to change cab ohms?

Discussion in 'Amps and Cabs [BG]' started by kringle77, Mar 27, 2005.


  1. kringle77

    kringle77 Supporting Member

    Jul 30, 2004
    Massena NY
    I want to use ampeg 32 ohm speakers to make a 2x10 cab. I will obviously be left with a 16 ohm cab when what I want is an 8 ohm. Could I put a 16 ohm resistor in parallel with the speakers and therefore get an 8 ohm load? The speakers are 100 watts a piece for a total of 200 watts. If this would work, what wattage/voltage resistor would I use. Im an electrician by trade but I have never heard of anyone doing this and don't feel like blowing up my amp by creating a strange load. Another idea would be to put a 32 ohm resistor in parallel with each speaker.....that sounds better to me. Let me know guys, .....a few of you sound very knowledgable about this stuff.
     
  2. IvanMike

    IvanMike Player Characters fear me... Staff Member Supporting Member

    Nov 10, 2002
    Middletown CT, USA
    with a big enough component i belive you could but only 1/2 the power will go to the drivers. the other 1/2 will go to the resistor. you're better off just making a 16 ohm cab, but much better off making an 8 or 4 ohm cab by buying the correct speakers, or buying a premade 8 or 4 ohm cab.
     
  3. Plain Old Me

    Plain Old Me

    Dec 14, 2004
    Or by running 4 speakers...
     
  4. kringle77

    kringle77 Supporting Member

    Jul 30, 2004
    Massena NY
    Thanks for the advice guys.....your right, thats just what I thought. But, I just can't seem to find speakers that sound like the ones from the ampeg 8x10 cab that come in 4 or 16 ohms to give me a total 8 ohm load. Any suggestions on that? :crying:
     
  5. Forget about the resistor. You get exactly the same power going to your speaker, the extra power the amp puts out from the 4 ohm load goes to heating the resistor. Totally useless.

    Randy
     
  6. kringle77

    kringle77 Supporting Member

    Jul 30, 2004
    Massena NY
    I can't believe that nothing else out there seems to sound like the speakers in the ampeg 8x10!!!!! They just seem to have more high end to them. I hate tweeters. By the way, I do intend to use a sealed cab. I want speakers with that tone at 4 or 16 ohms thats it. Where are they? Anyone? :confused:
     
  7. BillyB_from_LZ

    BillyB_from_LZ Supporting Member

    Sep 7, 2000
    Chicago
    Why not just run a 16 ohm cabinet? Unless you use a tube amp that doesn't have a 16 ohm tap, you can use a solid state bass amp with a 16 ohm load. You seem concerned about the relative limited power handling capability of the 32 ohm 10s, running a 16 ohm load on a solid state amp will help limit the amp's power output.

    Or...you could build a 3x10...it's a little bigger than a 2x10 and the input impedance will be 10.7 ohms...a little more than 8...and it would handle 300 watts.
     
  8. kringle77

    kringle77 Supporting Member

    Jul 30, 2004
    Massena NY
    Thats not a bad idea, but I already have some 2x10 cabs. I just want to get the most out of my Aguilar AG500 thats all. I want a 4 ohm load total with two cabs. Thanks.
     
  9. capcom

    capcom

    Mar 23, 2005
    as far as i know: you can not tweak the "impedance" of speakers just by using ordinary resistors... because ordinary resistors (correct me if i am wrong) can not have the real "inductance" of a loudspeaker... what you REALLY need is (again correct me if i am wrong) an apparatus that can act and behave like a "loudspeaker" in order to alter the "ohmage" for your needs...

    best regards...
     
  10. kringle77

    kringle77 Supporting Member

    Jul 30, 2004
    Massena NY
    Ya know, Im not sure now. Usually anything with inductance has a coil of wire, like a motor or magnet........duh, magnet. Yup, speakers are inductive! :rolleyes: I've decided to get as close as I can with speakers that have the tone Im looking for. It's between the eminence 32 ohm direct replacement for ampeg or the eminence Legend BP102 which is twice the wattage. If anyone knows how they compare tonally, let me know and this discussion will be over. Thanks, guys.
     
  11. Joe P

    Joe P

    Jul 15, 2004
    Milwaukee, WI
    I just jumped-over from the neiboring 'Damping Factor' thread, which got me thinking...

    If you put a resistor in series with a speaker, then wouldn't it 'ruin' the D.F.?? (by definition of D.F., I'm pretty sure it would!)

    On the other hand, I wonder if one could 'tune' a rig to sound more 'loose' or 'tubby' or whatever - more tub-amp-ish - by INTENTIONALLY adding a little bit of DF-reducing series resistance?!

    It'd be easy enough to experiment with...

    There are also T-pads, which can reflect a different, calculated, controlled impedance each way (one to the source, and one to the load). I wonder if there's anything worth looking into with that for some fine-tweakage of 'amp coloration'?

    Joe
     
  12. billfitzmaurice

    billfitzmaurice Commercial User

    Sep 15, 2004
    New Hampshire
    Owner, Bill Fitzmaurice Loudspeaker Design
    The problem with tweaking via resistors is that the wattage values would be far higher than anything commonly available. Higher than 20 watt resistors are quite rare. As altering for damping factor, audibly doing so with low Qts high Bl pro-sound drivers would be almost impossible.
     
  13. Joe P

    Joe P

    Jul 15, 2004
    Milwaukee, WI
    Hmm...

    ..So if you wanted to take your 500W amp with a DF of 100 for 4-Oms, and you wanted that 4-Ohm speaker to experience a DF of 10 instead...

    You'd have to make a .04-Ohm source look like 0.4-Ohms; that's .36 more Ohms that you need... Check.

    So now with this new DF of 10, .36/4.4, or .082 of the voltage is being dropped across the series resistor. ..so since power is E-squared-over-R, and we're dropping only .082 of the voltage, we'd be dissapating only - ahh... about 3.69V across .37-Ohm... 37.7W is dumped in the series resistor.

    It'd cost you 38W out of your available 500 (7.6%) to reduce your damping factor by an order of magnitude - down to 10.

    I suppose you'd need a 100W resistor, just so the thing isn't running super-hot.

    I doubt that .36 is a standard value, though.

    Hm.

    Joe
     
  14. Bob Lee (QSC)

    Bob Lee (QSC) In case you missed it, I work for QSC Audio! Gold Supporting Member Commercial User

    Jul 3, 2001
    Costa Mesa, Calif.
    Technical Communications Developer, QSC Audio
    Using a resistor to change the ohms of a cabinet is like dragging a boulder to change the miles per hour of a car. ;)

    Resistors have a place in loudspeaker cabinets, such as in a crossover where they're often used to help match up a tweeter's response to a woofer. But to just waste power shifting overall impedance … no, that's not a good idea.
     
  15. bassman314

    bassman314 I seem to be a verb, an evolutionary process... Gold Supporting Member

    Mar 13, 2005
    Bay Area, CA
    This would be called an attenuator. Theya re available, but they ain't cheap. I usually see them around $300. THD makes a good one, and most of the usual suspects carry them.
     
  16. Sure you can. That's why resistors and impedence are both measured in ohms. The difference is resistors' impedance is constant for DC and any frequency of AC signal. Impedence is the "resistance" of a circuit at a given AC frequency. Your 16 ohm speakers aren't 16 ohms all the time over the entire audio freq spectrum. Cone inertia, backpressure, porting, crossovers all conspire to bounce the impedence the amp sees around a fair amount depending on the frequency.

    Its still a horribly bad idea, but you will change the impedance the way it was described.

    Use what we electrical engineers call "wires" to connect your speakers. Period. Big thick juicy "wires".

    Set your cruise control at 55mph. Your 250HP hemi engine is only putting out 50HP. Now hit the brakes and the gas until the gas is floored, but you're still going 55mph. Your engine is now putting out the full 250HP. But you're still going 55. You have nothing to show for the extra power you've applied. Except you're converting the extra 200HP into heat with your brakes.

    That's exactly what happens when you monkey around with resistors to get the impedence matched. All the extra power you pull out of the amp goes into making heat in the resistor. You get the exact same power going to the speaker. The same volume. You just add stress to the amp.

    The goal is to use additional speakers to match the impedence so all the additional power goes into speakers to producing more sound.

    All the points about finding the right resistor values and wattage handling capacity are quite valid, but are still equivalent to discussing the best most efficient way to drive a nail through your hand without damaging the hammer or the nail. Who cares how to do it correctly, the point is you dont' want to do it at all.

    Randy
     
  17. billfitzmaurice

    billfitzmaurice Commercial User

    Sep 15, 2004
    New Hampshire
    Owner, Bill Fitzmaurice Loudspeaker Design
    The point is that even if you did so the likelyhood that you could hear any difference when using a good quality speaker is exceedingly slight.
     
  18. Steve

    Steve

    Aug 10, 2001
    I would think a competant, creative reconer would be able to rebuild those speakers with different voice coils to get you where you want to be....