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Real vs. Advertised Power Output

Discussion in 'Amps and Cabs [BG]' started by stettoman, Sep 10, 2003.


  1. Just got my rig back from the tech. It checked out as healthy, but for some very dirty pots. The tech. told me that the amp was running well and putting out a healthy 165 watts @ 4 ohms, 85 watts @ 8 ohms. Sounded fine, but this particular amp (Acoustic 370) was advertised at a MUCH higher output rating.

    Don't get me wrong, this isn't a complaint so much as an inquiry. The rig is plenty beefy and handles all my current needs, but I'd really like to know how one is supposed to understand at the music store what he's really getting when buying power equipment. How many rating formulas are there? And which one(s) are closer to the truth?

    Then again, though my tech. told me that the componants on the 370 were operating nominally, maybe power transisters lose muscle over the years...:eek:
     
  2. It's the same with my Mackie M1400 poweramp. In the manual it says 2x500W, but in real life it is 2x400W! I think it's really strange. Is there a common formula to check the power rating?
     
  3. JMX

    JMX Vorsprung durch Technik

    Sep 4, 2000
    Cologne, Germany
    It depends how you measure it. Unless your service tech does it exactly the same way as the manufacturer, you won't get the same result.
    That said, some manufacturers do publish "optimistic" specs.

    But RMS specs are usually reliable enough with most respectable makers.
     
  4. NJL

    NJL

    Apr 12, 2002
    San Antonio
    i wonder what my QSC is putting out
     
  5. Petebass

    Petebass

    Dec 22, 2002
    QLD Australia
    My understanding of the measurement proceedure is to run a sine wave through the amp and to measure the output at the point of the onset of THD.

    The word "onset" may have something to do with it. Brand A may claim 400w but at a much higher distortion rate than brand B. That would explain why some 400w amps are louder than others.

    I remember reading a thread in which the TB heavyweights explained that it would be more accurate to rate an amplifier's output in Volts rather than watts. There was also a discussion of how to calculate the voltage output of your amp. I'm at work now, but I'll have a hunt around for it when I get home.
     
  6. Yup....

    Also, they tend to test amps into static loads (ie dummy resistors). Speakers are reactive loads with complex impedances and are much harder to drive than static loads. An amp that can make 400W into a dummy resistor might well not make anything like that into a speaker load. Some amps just can't drive real-world loads very well, but the currently used specs don't show that.
     
  7. Mcrelly

    Mcrelly

    Jun 16, 2003
    Minnesota, USA
    if you want a laugh or shock search "pyramid dj power amp" on google. these amps are advertised to be huge like: "Pyramid PA1000X 1000 Watt PA Amplifer" "500 W x 2 into 8 Ohms" what they don't always tell you: "(2 x 175W RMS)"(into 8ohms) they are notorious for advertising PEAK! power rather than RMS which is sustaned average power output like most legitimate companies post. THIS AMP SELLS FOR $180?!?!
     
  8. And of course you can push more peak watts by sending a high voltage signal into the input (to the just below explode threshold). Someone already mentioned this one too, but THD or distortion specs can may all of these numbers say anything someone wants them to.
     
  9. Benjamin Strange

    Benjamin Strange Commercial User

    Dec 25, 2002
    New Orleans, LA
    Owner / Tech: Strange Guitarworks
    This is why you should never buy an amp based on power ratings at all, real or advertised. Plug it in, turn it up. Is it loud? Does it sound good? Pay the man.
     
  10. Doesn't the 370 go down to 2ohms?
    look at this:

    http://www.acoustic360.com/images/manuals/370/370d.jpg


    In fact, didn;t one of their 70's amps go down to 1ohm to drive their behemoth 4x15 cabinet?

     
  11. 10% distortion? WOW. Thats pushing the edge of realistic specs a little far. Most people can "hear" distortion above about 5%. Once again refer to the Pyramid specs mentioned before.
     
  12. Sure, it may go down to 2 ohms, but my tech measured @ 8 & 4 and still got much lower readings than Acoustic professed.

    I'm just curious though. The Acoustic boots mahooty and I wouldn't change a thing!
     
  13. Acoustic specced all their amps at VERY high distortion ratings. Yeah, the amp CAN put out the rated power, but it is dirty, which makes it seem all the more loud because it has a greater harmonic content. Your tech probably measured output voltage at the onset of distortion which gave a lower power rating than Acoustic specified.
     
  14. JPJ

    JPJ

    Apr 21, 2001
    Chicago, IL
    Virtually all amp and cab makers distort, or "selectively" report, stats to make their products look slightly better than they really are. Phil Jones (of Phil Jones bass: www.philjonesbass.com), is probably the only guy I've ever met who takes a completely honest and very straigtforward approach to listing performance specs of his products.