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Part 2 - Watt's watt with power amps.

Discussion in 'Amps and Cabs [BG]' started by Petebass, Jan 19, 2003.

  1. Petebass


    Dec 22, 2002
    QLD Australia
    This is a follow up to a thread I started a while back asking why my 400w bass head has can easily overpower PA systems rated as high as 4,000 watts. There were a lot of theories floating around but the one I took on board was that my bass head is only amplifying one instrument, but the PA has to deal with many.

    Suddenly I'm not so sure. On the weekend I was presented with a situation which is unusual here in Oz. The club we were playing at had installed a medium sized PA for the bands to use, but do not supply anyone to operate it. I've done my fair share of PA work so I was "volunteered" to operate it while I play.

    I turned up really early to get in a decent sound check. Since I was the only one there i decided to put the theory to the test and run a back to back "shoot-out". I wanted to see which was louder, my 400w head, or a PA with only one instrument to deal with.

    My amp killed it! It wasn't even close!

    The PA, on each side, had a single EV 18" sub ,(Crossover point was 120hz.) plus 2 EV mid-high cabs with a 15" and a horn. The Amps were Australian Monitor 900w a side for the mid highs, 1,200w a side for the subs (that's a 4,200w PA).

    Sorry to do this to you all again but I'm really back to square one. Maybe my 400w amp is freakishly loud giant killer?
  2. BassWizard55

    BassWizard55 Guest

    Dec 21, 2002
    Rome, Ga
    What's your point?
  3. Petebass


    Dec 22, 2002
    QLD Australia
    Oh sorry, my point in the original thread was regarding the way power amps are rated. Is the test that determines the wattage rating the same for all types of amps? If so, why the discrapancy.
  4. Captain Awesome

    Captain Awesome

    Apr 2, 2001
    Did you get the PA to distort?
  5. Petebass


    Dec 22, 2002
    QLD Australia
    No I didn't distort the PA, i didn't wanna blow it up. The desk had a clip light on each channel and a led display to keep track of input gain. I set the gain so that the bass signal was nice and strong, just below clip. Then I started experimenting with the sliders to get the loudest result possible, keeping an eye on the speaker movement to make sure they were working hard but not so hard they'd blow.
  6. So what exactly did you do to test it? Enquiring minds want to know:D

    Did you run both to the point of power amp clipping?
    Was the EQ curve similar?
    Did you see which was louder or sounded better, say, 30 degrees off axis?
    Does your amp have a power amp clip indicator?
    Did you check the sensitivities of all the cabinets?
    Was the amp close to a wall or in a procenium-type stage?

    There are all kinds of possible reasons for this.
  7. Petebass


    Dec 22, 2002
    QLD Australia
    Mark, I've inserted my answers below in brackets:-

    Did you run both to the point of power amp clipping? ( Yes but I unplugged the speakers first coz I didn't wanna send a clipped signal to them.)

    Was the EQ curve similar? (For the test I set all eq's flat. The PA did however a 31 band eq locked away behind a security grill which we weren't allowed to touch. I played a CD through a flat channel and is sounded fine, so i guess whoever set the 31 band eq up had a pretty good idea of what he/she was doing).

    Did you see which was louder or sounded better, say, 30 degrees off axis? ( I actually walked around the whole room several times. Both systems produced a nice tone with the PA producing more in the sub 50hz than my rig did.)

    Does your amp have a power amp clip indicator? (No but one of my effects pedals does. I have a clipping paranoia.)

    Did you check the sensitivities of all the cabinets? (No I didn't. I'm actuall a bit light on specs for the PA speakers coz they're not mine)

    Was the amp close to a wall or in a procenium-type stage? (My amp was up against a curtain, the wall was a further 3 feet away. At the risk of sounding like a "himbo", can I ask what procenium means:) )

    There are all kinds of possible reasons for this.
    (I'm sure there's logical explaination. I'm open to all suggestions).
  8. Remarks:

    - without actually measuring the watts this test is fairly useless.
    - your amp doesn't have a clip light. So it could have been clipping. The human ear is practically insensitive to 25% of Total Harmonic Distortion. I know my ears are. LF Distortion adds harmonics, which sound louder than the frequencies they are based on. A clipping 400W amp delivers (slightly) more than 400 W.
    - Your amp is probably voiced for bass guitar, so it'll probably have a midbass hump. The PA, with subs and all, probably puts out a lot more serious, clean low end. There's your huge power difference.
    - You can't compare the clipping point of an amp with no load against the clipping point under full load.
    - From 400W to 4000W is only +10 dB, or (perceived) twice as loud, under otherwise equal circumstances.
    - Do I have to continue?

    My 2 cents.


  9. There are a load of factors invloved in this stuff, as you can see. What you're experiencing is probably the cumulative effects of a bunch of physical factors plus some psychoacoustic factors. I think it would be difficult to characterize this without serious measurement equipment. You'd need a spectrum analyzer (JBL Smaart or something or even a 1/3 octave RTA would work) a tone/pink noise generator, an SPL meter, an oscilloscope to check output waveforms (to see whether amps are actually clipping or not) and a wattmeter. It's probably a combination of frequency response, power amp behavior, cabinet efficiency and cabinet directivity.

    And, you don't need to sweat a bit of clipping :D Thermal damage doesn't occur all at once it's more of a cumulative thing. The odd peak even clipped severly isn't anything to worry about.
  10. Agreed on all the other points.
    I'd think that the clip lights would come on at very close to the same input voltage with no load attached. Not exactly probably because the load would be purely resistive at *infinity* ohms. But pretty close. That's been my experience in the amplifier tests I've done at any rate....
  11. I think the power rails will effectively drop at least a few volts due to the "dead" time of the rectifier bridge and the filter caps.

    Let's say an amp has 80 volt power rails and 20,000 microfarad per rail as buffer caps. The ripple under no-load condition will be close to zero, while with a 4 ohm load (about 650 watt RMS at 12.8 Ampere) will be roughly V=I/(2fC)=12.8/(2x 50Hz x .02F)=6.4 volt, worst case. That leaves 73.6 volt left for clean output, which means only about 550 clean watts.

    The truth is somewhere in between, because the lower output of 550 watt won't lower the supply voltage as much as 650 watt, but that aside.
  12. Petebass


    Dec 22, 2002
    QLD Australia
    I'll answer Mark's questions first. The stage was a thrust variety. Those procenium stages are indeed a beetch (I've been calling em "room-in-a-room" stages but I'll sound smarted from now on). If anyone has any tips on how to pull a nice sound on these stages, please fill us in. The school gyms have the added problem of the enormous reverb, even when the room is full.

    My cabs on the night were an old 15" JBL E-145 (400w, 8 ohms) and a Nemisis 4x10 (450w at 8 ohms).

    I've digested all these points and will go away and think about it. But it certainly changes my perceptions a bit. It makes it harder to compare amps knowing that a pre-power amp arrangement with 4000w may or may not be louder than a 400w head. Kinda makes it hard to plan my next rig. I might just keep the one I've got........
  13. :eek:
  14. Here's an experiment: try sending the preamp out of your head to a big power amp, and run your cabs off the power amp. Compare that with the head powering the cabs. The only problem is that you'd need to find out at what level your head's power amp actually begins clipping to make it a valid excercise.

    I'd also say that if your 400W head is loud enough for your purposes and you like its tone, then there's not really any need to go with a pre/power amp setup. But that's up to you, of course :p
  15. That makes sense. But the difference, that would make when projected back on the input signal would be pretty small, no? A few millivolts maybe?
    Or am I out to lunch? :p
  16. jondog


    Mar 14, 2002
    NYC metro area
    Why would you even consider changing? Yours is hella loud! Stop worrying about specs and crank up your bass!!!
  17. Hey Petebass -

    Sounds like your bass rig really belts out serious SPL.

    Just curious...In doing the comparison, running thru the PA, were the left and right front-of-house in phase?

    Just for comparison - our PA, on each side, has a double 18" JBL SR sub ,(Crossover point about 120hz.) plus 1 Yamaha mid-high cab with 2x 15" and a horn (total 4x15's and 4x18's). The amps are QSC PLX3002 @ 900w a side for the mid highs, Crest CA12 @ 1,200w a side for the subs (that's a 4,200w PA also).

    My 600 watt 4x10 bass rig is plenty loud, & tends to compliment the bass guitar a bit better than the PA... But in terms of 'apparent SPL' (haven't done actual measurements).......

    Our PA absolutely decimates my stage amp.

  18. Petebass


    Dec 22, 2002
    QLD Australia
    Blimp, your PA has very similar specs to the one we hire most of the time. I'm often asked to turn my amp down coz I'm not in the mix. That's how this whole thing started.

    I run 2 cabs to your 1 blimp, that may be part of it. This is the scary part. When I do gigs with my original band (as opposed the the regular cover band gigs), my rig can not compete with the guitarist's marshall. I actually have to run 2 seperate amps with 4 cabs if I wanna be heard.

    I've organised to borrow a DB meter for a while. Take a few readings more for curiosity than for the benefit of science. I once read that Angus from AC/DC runs 16 marshall quad boxes and the volume at his Mic stand with just guitar playing is 135db..... makes you wonder.......