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powering a cab, wattage-impedance question

Discussion in 'Amps and Cabs [BG]' started by [SD], Jun 10, 2004.


  1. [SD]

    [SD]

    Feb 24, 2004
    i read at the QSC forum that Bob Lee said that a good rule of thumb when powering a cab is to have similiar watt power from the power amp to the program watt of the cab and the double of the RMS of the cab.
    Now here is my question.

    I don't know a whole lot about this. But if i get a QSC RMX850 which in bridge mode @ 8 Ohm gives 600W and i connect it to a warwick 411pro (8 Ohm and 600 Watt RMS) would i be getting all the headroom i can get from the cab or i should go for a RMX1450 or a RMX1850HD(bridge mode 1200 watts @ 8 Ohm).

    One last thing, does anyone know a web page where i can get all the theory about how to connect cabs? when to overpower them and when not? etc....

    thanks :hyper:
     
  2. it wouldn't be 'bad' if you got an RMX850; it'd still be quite loud, too. but you should be able to use the RMX1850HD with the 411pro. if i were buying the rig, that's what i'd do.

    i don't know that there's any web sources for sound theory that are particularly reliable technically. i think the best way to learn (it's so 1990s, though) is to talk to the experts -- e.g. guys like Bob -- and learn from experience.

    robb.
     
  3. [SD]

    [SD]

    Feb 24, 2004
    thanks rcz!!! for you answer!

    any more opinions ppl with knowledge out there?

    i thought overpowering a cab would be bad for it
     
  4. Eric Moesle

    Eric Moesle Supporting Member

    Sep 21, 2001
    Columbus OH
    Sure, "overpowering" an amp is not healthy for it, but the question is, what exactly IS "overpowering"????

    Example: say you've got a cab rated at 400 watts, but you have an amp rated at 500 watts with matched impedance. Odds are, when playing at fairly loud levels, your cab is only seeing about 300 watts from the amp at most. Your amp is going to go into terrible distortion before it ever puts out its full 500 watts.

    Just because a 500 watt amp is turned "on" doesn't mean that is the power it is sending to the cab . . .
     
  5. Mcrelly

    Mcrelly

    Jun 16, 2003
    Minnesota, USA
    I had been running 350 (rated a 4ohms) into an 8ohm aggie gs112, then two 112s. now I have a stewart 1.2 and when I run 700w into one gs112 or 1200w into two gs112s. it sure sounds better with twice the power vs. 1/2 power:

    situation one with bass 350 swr head
    two aggie gs112 (300+300w) using 350w into 4ohms

    vs.

    situation 2 with stewart
    two aggies gs112 (300+300w) using 1200w into 4ohms

    situation 1 was loud, but situation 2 is louder and punchier, BUT NOW I'M AFRAID OF BLOWING SPEAKERS WITH TOO MUCH POWER!!! I"m also sharring my rig with other players at church, but everything seems o.k. so far....
     
  6. [SD]

    [SD]

    Feb 24, 2004
    so as Bob Lee said it might be a good thing to get more power a try to double de rms wattage of the cab. But then i shouldn't set the gain of the power amp all the way up cuz that would blow up the speakers right?
     
  7. no, it probably won't blow the speakers.

    speaker ratings are funny because they're much more of a guess than power amp ratings. not only that, but the amount of peak power they can handle is different from the size of amp they can reasonably coupled with. and then the RMS rating is completely different from that.

    RMS ratings on cabs are more of a "heat dissipation" rating than an actual "use this kind of power on this cabinet" rating. a cabinet that is rated for 300W RMS is able to have a 300W sine wave played through it for a long time. but nobody plays sine waves -- they play bass.

    if a cab has a "program" rating, that's a recommendation for amplifier size. and the peak rating is more of a "you'll damage your speaker if you ever exceed this power" type of rating.

    for example, my employer, peavey, rates their PA speakers with all three peak, program, and RMS. our popular SP-5XL loudspeaker is rated for 700W program and 1400W peak. that means we recommend using an amplifier channel up to 700WRMS into 8 Ohms for each loudspeaker. yet the RMS rating is only 350W.

    so you're doubling the power over the RMS rating, but in actuality you're not even getting close to the peak rating (700WRMS = 990Wpeak, which is 410W less than 1400W). so you get a more dynamic sound that is also louder, and you don't have to worry about damaging your speakers.

    however, just as a precaution, try turning everything up, starting with the amp, then the preamp volume, then EQ controls, and then the bass and try to find out when you run out of headroom. it should sound really bad before you're risking any damage. you should do that anyway, not just because you're worried about having too much power.

    robb.
     
  8. Hawkeye

    Hawkeye Canuck Amateur

    Hey RCZ, a truly helpful post. It answered some questions I had about upgrading from my Deltabass head to a Firebass 700. I'm currently using one 4 ohm 115 BX BW cab.

    It helps to know what I would be getting into.

    AND If my understanding is correct (based on your xlnt info), my cab @ 400 Watts RMS continuous & 800 Watts program should be fine with the Firebass head which has 700 Watts at 2 Ohms 475 Watts at 4 Ohms.
     
  9. [SD]

    [SD]

    Feb 24, 2004
    thanks rcz that post really made me see the light! finally! now i'm sure about getting more power!

    i should've found this forum before! it would've answer lots of questions!
     
  10. absolutely. that's a nice little rig, though i myself prefer a 412TVX.

    robb.
     
  11. Mcrelly

    Mcrelly

    Jun 16, 2003
    Minnesota, USA
    talking about gain,amp, volume settings...I usually start with gtr,pre, and amp gains/volumes at zero, power up preamp, then power amp, turn up amp gain full, put my guitar volume at about 3/4 (so I can turn up or down a little without touching preamp later or while playing) then gradually turn up preamp output until its at a usable level.

    before I power down I minimize all levels, kill power amp first then pre, then unplug guitar

    more info than necessary, but thats the way it is.... :D
     
  12. Call me crazy, but Ive been running my SVT5Pro in Mono Bridged mode into a single 410 Gallien Krueger cab rated at 400 rms, and Ive never had a better, cleaner, louder, punchier sounding rig! I was worried sick at first, but apparently its a perfect match, no probs w/ speakers or distortion at all!
     
  13. Mcrelly

    Mcrelly

    Jun 16, 2003
    Minnesota, USA
    today I ran my stewart 1.2 bridged (700w into 8ohm) into ONE gs-112 (300w) and it sounded good. sometimes I think that pushing ONE gs112 is better than NOT pushing TWO gs112s!?!?!