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Add a cabinet, drop the ohms, nothing really changes, right?

Discussion in 'Amps and Cabs [BG]' started by millard, Nov 11, 2005.


  1. millard

    millard

    Jul 27, 2004
    SoCal
    Yes, I've read the ohms FAQ several times. If the answer is there, it either eluded me or requires interpreting a variety of different replies in hopes of finding the answer...

    My understanding is that if I chain two 8 ohm cabinets to a power amp channel, the effective resistance drops to 4ohms, but does that make the individual cabinet sound any louder since the channel is now needing to drive 2 cabinets? Or do they both effectively get driven at the same level, it's just louder because there are two of them?

    I'm not sure I asked that quite right, let me be a bit more specific. My current amp puts out 200W at 8ohms driving a single 8ohm cabinet. The speaker can get up to 200W of power (theoretically). If I chain another 8ohm cabinet onto the first, the resistance drops to 4ohms and my amp provides 400W at 4ohms. However, there are now 2 cabinets to drive. Isn't each cabinet effectively getting only 200W (just as a single one did before)? Or is there something about the parallel wiring that magically lets each cabinet get 400W?

    I'm guessing it is the first case, so that I can't actually drive the individual cabinets any harder by lowering the resistance, I just manage to not drive the first one less hard by adding one. Of course, I'm now driving twice as many cabinets at the same level, so it is likely to be twice as loud, but I'm not milking any more performance out of my first cabinet (so if it can take 400W, I haven't magically started pushing it to perform harder).

    Thanks...Millard
     
  2. Trevorus

    Trevorus

    Oct 18, 2002
    Urbana, IL
    You are right in the first case. The amount of power a cabinet gets is dependent on the output of the amp, and it's resistance. So, 2 cabs on a 400 watt amp, both making just half the resistance, the power is divided equally to each cabinet. So, 200W each in the scenario.
     
  3. MJ5150

    MJ5150 Terrific Twister

    Apr 12, 2001
    Lacey, WA
    Yep. You'll see alot of people around here suggest adding another cabinet if you have volume issues before running out and buying a bigger amp.

    -Mike
     
  4. millard

    millard

    Jul 27, 2004
    SoCal
    In my case, I'm underpowering my cabinet -- it can take 400W and I'm only feeding it 200W. While I could add another cab, I could also buy a head that can properly drive the one I have. Two cabinets means more to carry.

    FTR, it's a Bergantino HT115 being driven by one channel of a QSC PLX 1202. I'm told the cab performs well when you actually give it the sort of power it is capable of handling. Maybe Santa will bring me a QSC PLX 2402.

    Thanks...Millard
     
  5. Trevorus

    Trevorus

    Oct 18, 2002
    Urbana, IL
    is the PLX1202 capable of being bridged?

    EDIT: It is capable of being bridged. It can run the whole 4 ohm stack at over a kilowatt.
     
  6. millard

    millard

    Jul 27, 2004
    SoCal
    True, but I bi-amp a 2x10 off the other channel. I could bridge and put the HT115 and the 2x10 in parallel, but then I'd have no control over volume levels between the two cabs (true of any parallel setup). For me, anyway, I think the parallel setup only works if they are the exact same cabinet. Otherwise I'm sure I'd be saying "I wish I could get the blah-blah cab a little louder than the yada-yada cab..."

    Millard
     
  7. Passinwind

    Passinwind I Know Nothing Supporting Member

    Not quite. If you read though the FAQs a bit more, you'll see that you'll gain around 6dB, not the 10dB that most people consider to represent "twice as loud". This assumes equally sensitive (ie identical) cabinets as well.
     
  8. Chasarms

    Chasarms Casual Observer

    May 24, 2001
    Bettendorf, IA USA
    If you are actually using a two-channel source w/ a crossover to pass filter and/or EQ the cabs independently, that's one thing.

    But, to sacrifice all that headroom in the name of being able to independently control the volume of the cabinets seems a bit obtuse.
     
  9. billfitzmaurice

    billfitzmaurice Commercial User

    Sep 15, 2004
    New Hampshire
    Owner, Bill Fitzmaurice Loudspeaker Design
    True, when parallel wired.
     
  10. millard

    millard

    Jul 27, 2004
    SoCal
    Yes, I am. The two cabs have different frequency responses, sensitivities, and EQ needs. Just trying to not turn the basic question of the thread into a discussion of my gear.