Amplifier output wattage vs. power consumption

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  • espo
    Junior Member
    • Nov 2003
    • 10

    Amplifier output wattage vs. power consumption

    Many amplifier manufacturers rate their amplifiers at continuous multi-channel outputs that exceed the current inputs? ie Sunfire Signature Seven is rated 400 wpc x 7 = 2800watts, continuous all channels driven. The power consumption is rated at 15amp @ 120 volts which is 1800 watts without efficiency loses. Since both power in and out are continuous, the wattage can translate to energy (joules)which is power x time. I thought energy is not lost or created, it just changes state, and some is obviously lost to heat. I can see peak outputs at these rates for short spurts due to energy storage devices, or continuous rates NOT all channels driven. Another example came up with the Definitive Technology reference powered subwoofer rated at 1800 watts to speaker, while the max cosumption is rated at 1500 watts. Although I have a basic recollection of this theory, my physics is rusty and I could be off base.
    If anyone has an explanation to justify these numbers, please do. I hate to think the advertising is false or as missleading as it appears to be.
  • David Meek
    Moderator Emeritus
    • Aug 2000
    • 8938

    #2
    Hi Ralph,

    Welcome to the Guide! Not being an EE, I can't offer a good explanation either - if my memory isn't going out on me, your math works out properly. I'd like to think the "continuous" statement is a simple error, by an under-educated or over-enthusiastic copy editor, maybe? :roll:




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    • brucek
      HTG Expert
      • Aug 2000
      • 303

      #3
      I hate to think the advertising is false or as missleading as it appears to be.
      espo, you have the right idea about power, but you've chosen (in this case) a rather unusual power amp to make your case.

      There's no doubt that power specifications are often manipulated to a manufacturers advantage. This is not new news. I don't think we're bursting anyones bubble with this fact. :roll:

      Often the 8 ohm power spec is stated as a lower figure so the 'double down' to 4 ohms appears more respectable to indicate a higher quality power supply.

      Often in a multi-channel amp you'll see a respectable continuous power output figure that results in amazing overall power when multiplied by all channels, but in reality the spec is only for one channel continuously driven.

      In a conventional (i.e. Class AB) amplifier, when we refer to its continuous power output, this is the continuous maximum power dissipated in the speaker for a given impedance.

      It is determined by the + and - DC power supply rail voltage. For example, in a 300 watt amplifier, it may have a +/- 85volt DC rail voltage, so the maximum voltage when fully driven with an AC input signal that it can drop across an 8 ohm load is the RMS value of the maximum rail minus the amount dropped across the output transistors/resistors stage. Class AB amps are about 70% efficient, so doing a quick calculation you see a +/- 85volt rail can supply about 60v RMS minus about 10 volts lost across the output stage before clipping yields a maximum of about (P=e^^2/R) 315watts for 8 ohms. A normal 120 volt AC 15 amp line can supply about 1800 watts of continuous power . If it draws more than that, the panel breaker trips. Believe me, amps don't draw maximum AC current on a continuous basis - their power comes from their DC power supplies and appropriately sized filter capacitors. The AC supply only keeps the "well" full. This allows for large amounts of current to be supplied on quick demand.

      So, hopefully an amplifier can produce the rated power for sustained periods over all audio frequencies without distortion at fairly low noise levels. That's what's important. Your THD over the entire audio range is what matters.

      Anyway, this is not the case in the carver amp you have chosen in your post. Carver Sunfire amps use a different type of configuration. They have a cute name of Tracking Downconverter Power Supply. Instead of keeping the rail at a constant voltage (as used in a conventional amp), it 'tracks' the input signal. Actually, it tracks at 6 volts above the signal at all times. In a conventional amplifier, the difference between the rail and the voltage at the speaker is dissipated by the output transistors in the form of heat. Large heat sinks are required to dissipate this power.

      In the Carver system, the supply tracks the input signal 6 Volts above the input signal, so the output transistors don't dissipate any more than 6 V x the current flow (voltage x current = watts), in heat. The amp runs very, very cool - I don't think they even use heat sinks. Since music is non-deterministic, you can calculate rather impressive power dissipations across the speaker loads. There are some trade offs in slew rate etc, but they are rather good amplifiers from all accounts. You can decide.

      The fact remains as you have stated though. On a continuous basis, the wall can supply 1800 watts - that's it.

      brucek

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      • Sonnie Parker
        • Jan 2002
        • 2858

        #4
        Seems like a very good explaination brucek... although I only understand about half of what you wrote, I always trust your words. I'm not much on physics myself but have read a little on the Sunfire technology since I own one of Carver's amps.

        espo, I own a Sunfire Cinema Grand and bi-wire current source for the highs and voltage source for the lows. It runs extremely cool regardless of the conditions. It's only 200wpc but it outperforms my previous McIntosh MC7300 @ 300wpc. It has much more punch while smooth and warm but not as warm as the MC7300 which seemed like it just didn't have much power output at all...IMO. Of course I've done little critical listening but have had both amps on a pair of Snell B-Minors with a total Snell 5.1 setup for several previous years. I now have the SCG on PSB Image 6T's with a total PSB Image 5.1 setup and I am a more into home theater. The SCG performs excellent with what few mulit-format DVD's I own. While I don't think I'd ever need the power of the SSS I would still sure love to own one.

        Sorry... I know you weren't asking for a mini review of Sunfire amps but though I'd throw it in to maybe help back up the performance of the technology.






        SONNIE

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        • espo
          Junior Member
          • Nov 2003
          • 10

          #5
          brucek
          I understand what you are saying, and I guess the bottom line is the literature is incorrect in stating 400wpc all channels driven continuously. Energy in = energy out, whether it's dissipated as heat or driving a speaker. Also I believe I read the amp can store 475 joules, which would translate to a total of 400 watts for a little more than a second. Not much reserve for a seven channel amp rated at 400wpc.
          I realize these are only specs, and have little to do with how the amp sounds playing music or soundtracks. It also got some very good reviews. My only interest was whether this innovation was truly remarkable, or smoke and mirrors. :roll:

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