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power requirements vs rated output for powered pa speakers

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  • power requirements vs rated output for powered pa speakers

    Okay, hopefully someone can clearly explain this. I plan to run powered pa speakers (Yamaha DBR15) off a battery and inverter setup for use in remote locations (ie: no electricity, I don't want a generator running). I would like to calculate battery and inverter size and playing life. The pair of powered speakers I purchased are advertised as 1000W each and do put out some powerful sound. I understand that the RMS value is a fraction of that and that manufactures commonly over rate their products, however when I measure the current draw on the 115 volt supply, it is less than 1 amp. How is it possible this can be called 1000 watt output? Thanks - Doops.

  • #2
    First off, Music is not a constant energy source - it's energy varies from soft segments to loud peaks.
    Crest factor is a big part of this.
    http://www.prosoundweb.com/channels/..._crest_factor/
    "Not a Speaker Designer - Not even on the Internet"
    "If the freedom of speech is taken away, then dumb and silent we may be led, like sheep to the slaughter."

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    • #3
      I would start by purchasing (or renting) a Kill-A-Watt meter and measuring the actual power draw at the outlet for one of your powered speakers under normal operating conditions. With that data you'll be way better off making a battery determination.

      Oops... it appears you've already measured actual current draw. Hopefully that 1A was taken with the sucker cranked up as you would be using them.
      Last edited by wogg; 04-17-2017, 01:05 PM. Reason: Reading comprehension fail
      Electronics engineer, woofer enthusiast, and musician.
      Wogg Music

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      • #4
        A page from the Yamaha Sound Reinforcement Handbook
        Click image for larger version

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        "Not a Speaker Designer - Not even on the Internet"
        "If the freedom of speech is taken away, then dumb and silent we may be led, like sheep to the slaughter."

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        • #5
          Thank you Sydney... I get so tired of reading the term "RMS Power". I know what manufacturers typically mean, and it is widely accepted terminology, but it does not actually exist!


          If you check Yamaha's website, they list the "Dynamic" Power as 1000 W and the "continuous" power as 465 W (probably sinusoidal output). These are output capabilities of the amplifier. Further down the page, there is power consumption listed. This is generally 1/8th power pink noise, and is an industry standard for AC power consumption. This is why a Crown XLS2500 (rated at 2500 watts), has a screen-printed label on the back that states it only consumes 425 Watts. This is the number that will be much more accurate for average AC power consumption, playing music, and would be a good number to use when estimating battery capacity to reach a desired run-time. They state it at 74 W, which would be right on track with your <1 Amp measurement.

          For inverter size, you need to size the inverter so it can deliver the max peak current without tripping some sort of over-current protection. To truly spec that out, you'd need a current probe on an oscilliscope. In reality, you can probably use the 1000 W power number and back-calculate a rough current peak number from the 120 VAC mains. Most likely, you'd be ok getting an inverter rated for 1000W peak output capability.

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          • #6
            Add 15% to your calculated battery load when calculating battery life. There is about 10% to 15% loss in the power inverter on the up convert.

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            • #7
              I had wondered about the efficiency of today's Inverters; the last time ( many years ago ) I used one for a parade sound system it was about 80%.
              There was noise intrusion, so I went back to a 12v based system.
              "Not a Speaker Designer - Not even on the Internet"
              "If the freedom of speech is taken away, then dumb and silent we may be led, like sheep to the slaughter."

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              • #8
                Originally posted by Sydney View Post
                I had wondered about the efficiency of today's Inverters; the last time ( many years ago ) I used one for a parade sound system it was about 80%.
                There was noise intrusion, so I went back to a 12v based system.
                I had a single-unit 2.1 plug-in shelf system I was using as beach boom box (excellent in that application). There was the 110 AC power inverter up-convert and the unit's internal switcher down convert. I estimated 25% losses in the conversion stages.

                I opened it up and found the digital amp ran at 26 V and the DSP at 3.3. So I went to two SLAs in series directly to the amp and a voltage regulator for the DSP. Huge difference in battery life.

                And know that lead acid based batteries behave like people expending energy. You can walk all day long but have to rest after a sprinting a short stint. Same with batteries. You will not get 1 hour of power when using a battery at it's amp rating (e.g., 5 A on a 5 ah battery). So decreasing the current draw adds time from both the reduced ah load and the lower amp draw.

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                • #9
                  Originally posted by Sydney View Post
                  I had wondered about the efficiency of today's Inverters; the last time ( many years ago ) I used one for a parade sound system it was about 80%.
                  There was noise intrusion, so I went back to a 12v based system.
                  Noise is something I did not address... 2 things will affect the noise levels the greatest. The first is the PSRR (power supply rejection ratio) of the amplifier. Better designed amplifiers will have higher PSRR and will equate to less noise on the output. The second thing is the type of inverter. The cheapest inverters will essentially be a modified square wave, or stepped sine. Spend some higher dollars and you can get a pure sinewave inverter that will be much better at keeping things quiet.

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