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Power and Cabs.

Discussion in 'Amps and Cabs [BG]' started by WAKOJACO, Jul 1, 2001.


  1. WAKOJACO

    WAKOJACO Guest

    Jul 5, 2000
    Binghamton, NY
    A high power rating for say, a 4x10 cab, is around 700W RMS. I use a 800W amp. to power 2 700W enclosures (1400W total). The power amp. section will definately clip when cranked up, resulting in a loud CRACK on the attack of hard-plucked (mostly low) notes. Does this not mean that I am giving the speakers as much as they can handle? So, even if I had a 2400W amp., with the same speakers, I couldn't get any louder because I'd be clipping the amp at the same point (assuming similar tone settings), right? It seems I need more speaker.

    Most cabs. are 4 or 8 ohms, and the min. ohmage you'll likely want to put on your amp is a 4 ohm load. This means that to maximize sound you'll typically use a max. of two (8 ohm) cabs. with a single amp. So, even if I had more speaker, I couldn't use it because the ohmage would drop too low for a single amp. Why then are people using separate pre/power units with big PA amps with a ton of power?
     
  2. I run high powered amps because my subwoofers are very inefficient. I also use mid-bass drivers because subwoofers don't have any response above 200 Hz. I run a preamp because I like the tone, need something to drive the power amps, and it allows me to bi-amp the subs and mids. Bi-amping lets me avoid the weight, cost, and other issues inherent in high powered passive crossover networks.
     
  3. MikeyD

    MikeyD

    Sep 9, 2000
    Yes or no. Two possibilities occur to me:
    _ (1) You are overdriving the power amp during transients, in which case you need a bigger amp. Your speakers can handle 1400 watts straight, so a bigger amp will work fine. OR...
    _ (2) You are making those small 10" drivers bottom their suspensions on really loud and low "thump" transients - even with that 800-watt amp. You can easily watch this happen if you remove the grille and do it. If it happens more on the low notes with the bass boosted - and you hear a loud "clack" as the cones jump wildly, it could be the coils bottoming. If so, you are inflicting damage on the speakers. In this case, I recommend you seriously consider getting a cabinet better able to handle the lows - such as something loaded with 15" or 18" drivers (or otherwise turning down your low-F bass EQ). Then attenuate the low-F output to the other 4x10.
    No. Amplifier clipping and speaker "clipping" are two different things. See my comments above. You have to be sure which is happening. If you get a more powerful amp, you will get more volume before clipping (the amp). But you may not get more (low F) volume if the speakers are bottoming already.
    - Mike
     
  4. WAKOJACO

    WAKOJACO Guest

    Jul 5, 2000
    Binghamton, NY
    I didn't realize that speakers could clip...just bottom out or fry...and the the power amp. clipping specifically meant that the speakers were taking as much "juice" as possible.

    I always suspected that the loud crack was due to bottoming of the speaker...but the power amp. clip light also flashes on with the crack, possibly creating a momentary ground when the speakers bottom out (?). The 10 inchers are definately dancin' when cranked up, and I was suprised to see how easily they bottom (they're Gol. III's). Maybe it's a single bad driver? Anyhoo, I used to have a 4x10 and a 1x18, and had the same problem. I just crank the limiter and roll off the lows.
     
  5. MikeyD

    MikeyD

    Sep 9, 2000
    Well, with speakers, "clipping" isn't really the proper term, although if one were to plot the displacement vs. time of the loudspeaker cone, it could certainly look like a clipped waveform. When the amplifier itself clips, the waveform's average power goes up. At its clipped extremes, a sine is distorted to a square wave, and the effective power is twice the rated RMS sine output of the amp.
    No - but I could believe that the sudden mechanical impediment to the cone motion might feedback to the amp as a sudden loss of back-emf, meaning the amp is suddenly (and momentarily) driving a motor with "locked rotor". In this case, the current might spike up drastically, which would momentarily add to the amp's burden. The current spike produced by the temporary stoppage of the cone might result in a clipping event at the power amp. This is a guess, since I'm not a power amp expert.
    You should be able to tell if there's a bad driver by listening to each one carefully at moderate volume. If you are bottoming the drivers, you really need to do something or it will cost you lots to repair. If it is simply the amp clipping, again - you need a bigger amp (and/or better line voltage if you are loading it down too much). In this case, you might try borrowing a bigger amp to see if that fixes the problem. However, if you are already bottoming the speakers, all that will do is kill them off faster. So - make sure you are not bottoming them first. If you had the same problem of bottoming using an 18", then you may need to (a) turn down your bass EQ, (b) get more BIG speakers, or (c) get more efficient speakers. Or all of the above.
    - Mike
     
  6. The loud CRACK is your power amp clipping.
    It has run out of head room.
    A bigger-more powerful amp will increase your headroom.

    Oh, BTW... power amp clipping will blow your speaker coils faster then sh*t, and that type of damage is not covered under warranty.

    Take it from me. I just sent my speakers out for repair because I lent my cab out to an idiot who "Spinal Tapped" the amp to "11" blew my speakers and then said it sounded bad during his gig. When he returned it, I plugged in an noticed the speakers now buzzed with the volume set at 10:00. A sure sign of a blown voice coil. New speakers with trade in of old ones... $350. Bonehead pleaded innocent by reason of stupidity.