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  1. #1

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    Default How can the power output exceed the power input????

    How can the power output exceed the power input????

    Hello all, i have wanted to know this for a while , some of the avrís on the market have rated power outputs (continuous or rms) that are more than the rated power input in watts , how is this ??

    For example i have a borrowed denon avr 1509 and the rated power input in watts is 360 . and the sums rated power outputs are (75w*5ch) = 375w @ 8ohm . or (110w*5ch)= 550w .
    How can this be ??

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    Two things.

    One, usually "75w x 5ch" is with only one or two channels driven. It's a cop-out, and blatantly dishonest, but almost all of them do it.

    Two, for instantaneous power, you could technically exceed the input power thanks to capacitors and whatnot.
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    Bobman, "If this is true, which 2 channels would get the main power?
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    I think the trick is to understand the ratings used. I could be wrong, but most AVRs aren't rated for full output with ALL channels driven. I.e., the fronts may be driven at that level, but the surrounds are at much lower levels. If all channels drew 'full power' continuously the power supply would obviously not be able to deliver.

    Then there is the difference between rms continuous power (based on simple sine wave at a specific frequency) and music power (which is a more complex signal with transients that are significantly greater than the average power draw.) With a well designed amp, there are reserves in the PS which will allow the amp to deliver clean transients that are perhaps 5 times the level of the rms continuous power rating.

    It get really complicated!

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    I don't understand the question....

    Basically, the rating is saying that if you have all five channels connected but only one or two are being driven, you will get 75 watts on those channels. If you are driving all five you'll get significantly less on all of them. I don't believe there's any kind of "priority".
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    A more legitimate way is that amps use large capacitors. Some expensive power supplies / surge protectors have giant coils that allow for up to 50 amps of current... now this can only be used for a very short period of time as the capacitors will run out and then recharge.

    It is possible for output to exceed input in short bursts.
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    Bobman, Thanks I think I understand now, forgive me for the dumb question, Im still learning.......If im watching a movie in DTS or DD and all 5 channels are being driven with my Pioneer 816 rated at 110 wpc I may only be delivering half that amount to all 5 channels....am I on the right path?:)
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    Quote Originally Posted by Knucklehead View Post
    Bobman, Thanks I think I understand now, forgive me for the dumb question, Im still learning.......If im watching a movie in DTS or DD and all 5 channels are being driven with my Pioneer 816 rated at 110 wpc I may only be delivering half that amount to all 5 channels....am I on the right path?:)
    Yes sir, that's about it. Luckily, chances are you're RARELY driving all five speakers at once.
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    Thanks, I understand the use of seperate amps a little more now, and whadda ya know, I have pre-outs.:D
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    The guy at denon said that “mains is different” and the amp does give the specified power to all channels driven . still none the wiser
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    So the answer is "It can't..." or did I miss something?

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    Didn't one manufacturer have 'power steering'?
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    Quote Originally Posted by RandyBox View Post
    So the answer is "It can't..." or did I miss something?
    Nope, you are correct. Receivers are not rated for 5 or 7 channels driven simultaneously, only 2.

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    Quote Originally Posted by nguyendot View Post
    Didn't one manufacturer have 'power steering'?
    Yes, the Carver AV-705x 5-channel amp has it. There may be others.
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    Quote Originally Posted by speakergeek View Post
    Nope, you are correct. Receivers are not rated for 5 or 7 channels driven simultaneously, only 2.
    well , my normal amp the Cambridge 540r has specs for all channels driven but the L/R power is higher if only L/R are driven. (limitation of continual psu output ?)
    "80W per channel @ 8 Ohms all channels driven/100W per channel @ 8 Ohms 2 channel ".
    and the power consumption is higher than the output. at 850w .
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    Quote Originally Posted by bobman1235 View Post
    Two things.

    One, usually "75w x 5ch" is with only one or two channels driven. It's a cop-out, and blatantly dishonest, but almost all of them do it.

    Two, for instantaneous power, you could technically exceed the input power thanks to capacitors and whatnot.
    Yes. However, there is also confusion about the difference between power and energy. There is also confusion about the difference between the average power input and the instantaneous power output of an amplifier.

    I will explain these differences to answer to your question. For short periods of time, the instantaneous output power can exceed the average input power, however the total output energy is always less than the total input energy.

    Energy is power applied over time.
    Mathematically this is expressed as E = P x t and is measured in Joules.
    1 Joule is 1 Watt for 1 second.

    Likewise power is described as the amount of energy applied per unit of time.
    P = E/t and is measured in Watts.
    1 Watt is 1 Joule per second.

    A more common measurement of power is the kW =1000 Watts.
    Your electrical meter measures kWh. 1 kWh = 3,600,000 Joules.

    So for any device, the energy output will always be less than the power input because there is some energy lost in heat. The ratio of the energy output vs energy input is known as efficiency. The efficiency cannot be more than 100%. Perpetual motion machines don't exist.

    Now energy can be stored as an electrical charge in a power capacitor. With a heavy demand, the capacitors provide an additional current source which allows for the instantaneous output power from the amplifier to exceed the average input power from your power outlet for very short bursts of time.

    Because the time interval is short, since E = P x t, the energy used is actually not that large. In any case, the total output energy is still less than the total input energy and the universe remains stable.
    Last edited by xcapri79; 06-20-2009 at 05:27 PM.

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    Quote Originally Posted by xcapri79 View Post
    The efficiency cannot be more than 100%. Perpetual motion machines don't exist.
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    Quote Originally Posted by ward91 View Post
    well , my normal amp the Cambridge 540r has specs for all channels driven but the L/R power is higher if only L/R are driven. (limitation of continual psu output ?)
    "80W per channel @ 8 Ohms all channels driven/100W per channel @ 8 Ohms 2 channel ".
    and the power consumption is higher than the output. at 850w .
    Should have qualified my post by saying "generally". There are a small hand full of receivers out there actually rated for all channels driven simultaneously, but the common four (Onkyo, Denon, Pioneer, and Yamaha) don't rate all channels simultaneously, only two at a time.

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