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Adding a cab loudness question...again

Discussion in 'Amps [BG]' started by Masher88, Jan 11, 2014.

  1. Masher88

    Masher88 Believe in absurdities and you commit atrocities

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    I have a QSC plx3002 stereo power amp. It's rated at 3000 watts RMS into 4 ohms bridged. That's how I run it to my Ampeg 410hlf.

    If I got another 410hlf cab and ran the amp stereo which would be 900 watts to 4 ohms for each side...would there be a significant perceived loudness increase due to more cone area...even though the total wattage rating is less?
  2. Mr. Foxen

    Mr. Foxen

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    Yes, because you cab can probably make about 200w max into sound.
  3. BFunk

    BFunk Gold Supporting Member

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    You would get a noticeable increase in volume. More lows due to constructive coupling. Better dispersion. Lots of good. The difference between 1800 watts and 3000 watts is less than 3dB. But the loss in gain certainly does not tell the whole story. The plusses will definitely make up for the minuses.
  4. Downunderwonder

    Downunderwonder

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    Hlf might be a bit more productive than that I would hope, but the point is the same regardless. No way one cab turns anything like 900w into sound.

    With two cabs you double the cone area and double the power employed, for 6db extra headroom, and speakers at ear level. Now go find a fearful to demo.
  5. Bassmec

    Bassmec

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    Yes this is very true at about 45 Hz so by running the power amp in stereo the OP would expect a very usable 3dB increase in level before fart out with the additional cab.
    With a well adjusted high pass filter, maybe a little bit more before the dreaded flatulence sets in.:bassist:
  6. Bassmec

    Bassmec

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    No you are wrong by about 3dB. + I can make an HLf fart with my Simms Watts AP 200 tube amp running clean on my five string tuned to low B with a fat nordstrand pickup running a flatish bass and amp frequency response.:bassist:
  7. BurningSkies

    BurningSkies

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    You're definitely going to have more volume with two. Mostly because you're not really feeding 3Kw to your speakers right now anyway. That would last for a few seconds, then your cab would crawl away to die and you'd start crying.

    In fact, I'd hazard to say you're not even giving them 900w if you run one per side. Because the thermal rating of that cab would also suggest an excursion limit well below their 450/1000w ad copy.

    In other words, you're probably saying would I get more volume by running 500w into one cab or 500w each into two cabs.
  8. bgavin

    bgavin Supporting Member

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    Dunno about farting out, but the physics of getting louder are straight forward.

    If you add a 2nd identical unit, wired in parallel, you get a +6dB boost in noise.
    You get 3dB from doubling the cone area, and another +3dB from the increased power handling.

    A "unit" is a cab with X speaker count.
    It can be one, or eight. Matters not.
    The cone area doubling comes from a second identical cab, yielding 2x more drivers.

    I own two of this model PLX.
    IMO, it is pointless to bridge this amp into a cab that cannot possibly handle that much input power.
    For maximum noise, my advice is run the PLX in dual-mono mode, where a single input drives both channels.
    This amp produces 60 volts per channel, into a 4-ohm load.
  9. Bassmec

    Bassmec

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    This +6dB equation would only be true if it was a transistor amp that
    gains in power by driving into 1/2 the impedance, but as he has enough power to destroy his cabs anyway, he will only get the coupling gain, like if it was a tube amp with a fixed output in watts whatever the impedance.:bassist:
  10. Bmorefoozler

    Bmorefoozler

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    But, let's be generous here...

    If we can assume that 1 cab can handle 500 watts, well then 2 can handle 1k watts, so power is still doubled. Doesn't matter how much power he has on tap, but how much is actually being passed to the speaks.
    So, if I'm not mistaken, it is in fact a 6db boost.
  11. bgavin

    bgavin Supporting Member

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    What part of this is not clear?
    Or, is this veering off into the "how many angels can dance on the head of pin" thing again?
  12. Bassmec

    Bassmec

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    That still only really works if each cab is not at maximum output.
    Anyway whatever he actually gets it's going to be a very useful increase
    in apparent loudness.:bassist:
    And better still however cautious he has been with one cab, the service life of his drivers is going to improve considerably.:bassist:
  13. Downunderwonder

    Downunderwonder

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    Should be abundantly clear that there is power on tap to even blow both cabs to kingdom come. +6 dB any day plus Sunday.
  14. ThisBass

    ThisBass

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    3dB donation for Headroom (hence the amp shall not clip) => 1500 Watt.
    6dB donation at least for dynamic (we like it "undistorted") => 375 Watt RMS

    A second cab provides nearly +3dB of system efficiency. Theoretically it is +3dB but in in practice a little less.
    But as the QSC tolerates some clipping artefact you will win in any case.
  15. Drunk Heffalump

    Drunk Heffalump Tone that I have. Skill? Oh, that? Well.... Supporting Member

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    Pin dancing angels aside, you're moving more air so yes, you and your chiropractor will notice.....


    :D
  16. Masher88

    Masher88 Believe in absurdities and you commit atrocities

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    Thanks guys. I was just wondering if a 2 identical situation was worth was worth exploring. Seems so.:bassist:
  17. DukeLeJeune

    DukeLeJeune Only immortal for a limited time Gold Supporting Member

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    Assuming fartout is the limiting factor (which is very likely the case), adding the second identical cab will give you 6dB more output.
  18. AlexanderB

    AlexanderB

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    Ampeg's ratings are honest. these cabs just laugh at a five string through a maxed LMII (400W + RMS, in BGM). I run one per channel from an PLX 1804 (900wpc at 4 Ohms) and they are OK up until just below clipping the amp. And I have lots of bottom in my sound...

    So, add +6 dB. There is a very clear increase with two cabs.
  19. AlexanderB

    AlexanderB

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    Bassmec, tube amps have much less damping factor (which determines how well the amp keeps the cones in control) than SS amps. In general, the bigger amp, the more power in the lows can be fed into the cab before cones bottom out. True for PA subs and true for bass amps as well.
    Also, many people think that these cabs bottom out while they actually hear their (small) amp clipping...
  20. Mr. Foxen

    Mr. Foxen

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    Ampegs ratings are thermal, which is nothing to do with farting out, you can't have a rating on that, you have a chart, because it varies with frequency.

    Damping factor ceased to be relevant when cones with damping or their own are used, which will be what you find in ported cabs.

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