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Amp question...just tell me if I'm right or wrong.

Discussion in 'Amps and Cabs [BG]' started by BubbaZed, Apr 28, 2015.


  1. BubbaZed

    BubbaZed Supporting Member

    Apr 25, 2014
    Tallahassee, FL
    So I have an amp rated at 500 W at 4 ohms and 300 W at 8 ohms.

    I connect an 8 ohm cabinet rated 400 W. Thus, the cab is now getting 300 W delivered to it.

    I then daisy chain a similar cabinet (8 ohm, 400W) to the existing cabinet.

    So now the amp is delivering 250 W to each cabinet.

    Correct?
     
  2. iualum

    iualum

    Apr 9, 2004
    60453
    You're right ;) . The amp's pushing 500 RMS @ 4-ohms, 250 to each box.
     
    B-string likes this.
  3. BubbaZed

    BubbaZed Supporting Member

    Apr 25, 2014
    Tallahassee, FL
    Thanks. I just needed some confirmation. I called 2 major chains (who shall go nameless) and I knew they weren't giving me the right answer. Only confused my pea brain.
     
    Last edited: Apr 28, 2015
  4. Concour, you are correct.
     
  5. Correct, but only if the required signal level and volume/gain control settings are such as to produce that much power. For the same example, connecting a 300w cab will not draw 300w unless the signal and volume/gain settings allow the amp to be driven to 300w. Cabinets do not draw power. You probably know that, but, based on various posts here, it seems as though a fair number do not.
     
    SteveCS likes this.
  6. I only saw "amp delivering" and no mention of cabs (incorrectly) "drawing" power until your post?
     
  7. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    You said similar cabinets. It matters if they are the same or not. You could have two cabinets that are both the same impedance and power handling capacity that perform differently. If they are different designs, one cab might be more efficient, for example, than the other and will be louder.
     
    Last edited: Apr 28, 2015
  8. The OP literally connects a cab and the amp 'delivers' its rated power into that load. I'm saying that connecting a cab to an amp does not cause the amp to deliver its rated power into that load, and by extension, that connecting a cab with a power handling that is lower than the rated power of an amp into that load will not result in the cab being 'overpowered'. I suspect that the OP knows this, but many seem not to, so I thought it was worth clarifying.
     
  9. BubbaZed

    BubbaZed Supporting Member

    Apr 25, 2014
    Tallahassee, FL
    As the OP, I understand what you guys are saying and I understand that my cabs will never see the full power delivered from the amp based on gain and volume settings. Yeah, I know that hooking up a 500W amp to a 500W cab won't instantly gleefully give me 500 W. :DI'm trying to verify that the amp divides the power equally into both cabs. Essentially two 8 ohm cabinets in parallel are 4 ohms hence the wattage is equally divided.

    I'm asking this because I plan on adding an extension to my existing system and want to know who's getting what.
     
    Last edited: Apr 28, 2015
  10. For two identical cabs, the answer is yes, the power from the amp (whatever it happens to be, not necessarily the full rated power) is equally divided between the two cabs.
     
    Last edited: Apr 28, 2015
    BubbaZed likes this.
  11. edencab

    edencab

    Aug 14, 2013
    Toronto, On
    ok......if one cab is rated 200 watts and a 2nd at 400Ws...they both get 250 watts delivered and one cab maybe happy at loud volume/gain settings and the other may protest...correct
     
    Last edited: Apr 29, 2015
  12. Hopkins

    Hopkins Supporting Member Commercial User

    Nov 17, 2010
    Houston Tx
    Owner/Builder @Hopkins Guitars
    It depends on if one cab is pushed beyond its physical limit based on EQ and volume. Power handling numbers really mean very little.
     
  13. It's always best to keep things simple and use two identical cabs. It's safest to overspec your cabs by 50%, so if they need to handle 200w, use 300w cabs. And always keep in mind that typically the thermal power rating (which is generally what manufacturers use) will be about double the excursion limited (fartout) power handling. The fundamental (before bass boost) only constitutes about 20% of the output, so things seem to be pretty safe. However, a little bass boost can easily increase that percentage to well over 50%. Complicating things further is the fact that power handling specs are not standardized, and are often inflated (for increased sales).
     
  14. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    I like to go beyond that just to be safe.

    My rule of thumb is the cab should have at least a capacity that is twice the continuous RMS rating of the amp. If the amp is rated at 300W, for example, I would want a cab that is rated for at least 600W.
     
    B-string likes this.
  15. Lo-E

    Lo-E

    Dec 19, 2009
    Brooklyn, NY
    Yes, you're correct; two 8 Ohm cabs will share the output power from the amp equally. Two identical cabs will also be able to handle and utilize that power equally. Unmatched cabs may have differences in efficiency, drivers and/or power handling so they may not be equal in volume or in their ability to handle the power from the amp, but if they are both 8 Ohm cabs, they will still share the output power equally - it's then just a question of what each cab does/is able to do with that power.
    As Astrosonic helpfully points out, matched cabs makes everything a lot easier. That doesn't mean you have to use matching cabs, of course - and many people have used unmatched cabs with great results and no mishaps, but you leave more up to chance when you start mixing cabs. Caveat emptor, and all that fun stuff....
     
  16. edencab

    edencab

    Aug 14, 2013
    Toronto, On
    thanks Lo-e ...this seems to be the key answer "......but if they are both 8 Ohm cabs, they will still share the output power equally - it's then just a question of what each cab does/is able to do with that power."
     
  17. The impedance rating of cabs is just that - a rating. The impedance varies with frequency. A graph of the impedance of a ported cab without a tweeter will have a double peak in the bass (peaks flanking the tuning frequency), and a rising impedance above that. A (dynamic) tweeter adds a large impedance peak near the crossover frequency. The chances of non-identical cabs sharing the same impedance curve are next to nil. The rated impedance is just a nominal (sort of average) value. The reason for identical cabs is that they will have the same impedance curve, and so present the same load to the amplifier for any given signal. Unmatched cabs, even if they share the same format (210, 212, 410, 215, etc.) will differ in impedance, and so the power will not be distributed equally. In some cases the difference will not lead to serious power distribution problems, but in others...
     
  18. edencab

    edencab

    Aug 14, 2013
    Toronto, On
    thanks Astro....so I am on the hunt for another Eden tweeter-less D115....they are heavy beasts though....
     
    AstroSonic likes this.
  19. Those are fine cabs! Good luck with the search.
     
  20. BurningSkies

    BurningSkies CRAZY BALDHEAD Supporting Member

    Feb 20, 2005
    Syracuse NY
    Endorsing artist: Dingwall Guitars
    Right.
     

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