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What is the meaning of 30W amplifier?

  1. Mar 30, 2017 #1
    When I have gone through spec of amplifier, I saw 30W for output. Is it mean that even if I am connecting a load of less impedance, say 30ohm, will I get 30V output voltage across the load?
  2. jcsd
  3. Mar 30, 2017 #2


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    Probably not. Usually that is the power at a particular impedance -- which should be given as well, somewhere in the specs.
    And 30 W can mean 30 W peak with the (rms) continuous power out considerably less.
    [edit] why don't you post the specs (or a link) so we know what we are talking about
  4. Mar 30, 2017 #3
    You're maximum output voltage is 70.7 volts.
    With a peak sine wave signal, 30ohm load may draw ((70.7)^2)/2/30 = 83W, so your 30W amplifier would clip severely. But if you turn the volume down to 42V peak, you would be all set.
    Last edited: Mar 30, 2017
  5. Mar 30, 2017 #4

    jim hardy

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    Where did that come from?
  6. Mar 30, 2017 #5
    Are you sure that's 30 ohms? Cuz that's a lot

    Maybe a guitar amp?
    Last edited: Mar 30, 2017
  7. Mar 30, 2017 #6
    It's a standard for some sound systems - although perhaps not if it's just 30W.
    It's actually a maximum of 70.7Vrms. It puts the maximum DC at 100V.
    If it's not a 70.7 volt system, it probably doesn't really "clip". Instead, it just gets muddy - or pops a breaker.
  8. Mar 30, 2017 #7
    Oh ok a pa
  9. Mar 30, 2017 #8


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    yeah ... very different to a home stereo which will be either 4 Ohm or 8 Ohm

    We really need the OP @Nikhil N to come back and clarify what he is talking about so that we all don't have to make wild guesses

  10. Apr 4, 2017 #9
    Its just for calculation I took as 30ohm, Actually I have load of 12ohm
  11. Apr 4, 2017 #10
    This is the amplifier I am using. I was using it last few days, but it is not working nowadays. So I am looking for a new amplifier to buy which suits for my experiment. Is there any problem, if we have switched on the amplifier without load? I read some where, for class D we should not turn on without load.
  12. Apr 4, 2017 #11
    Last edited by a moderator: May 8, 2017
  13. Apr 5, 2017 #12


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    Good thing you finally posted the specs.
    Did you miss the 4-8 ##\Omega## ?
    Did you mention 30 ##\Omega## because you have such speakers at hand ?
    Did you also see the working voltage is 12 V, 2 A ? That is 24 W at 100% efficiency ... as the sum of two channels:rolleyes:
    I wonder if someone can guestimate the rms power into 4 ##\Omega## ? and 30 ?

    I think you'd need pretty efficient speakers if you want to use this thing on a motorbike - but then again, I'm not a subject expert.
  14. Apr 5, 2017 #13
    Why not repair and upgrade the old amp it's got to be cheaper than a new one
  15. Apr 5, 2017 #14

    jim hardy

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    Lepai ? They're only about twenty bucks US

    Their true power is a fraction of what's advertised, Google will find test reports that show true power is overstated by about 4X.
    To OP's original question,

    The meaning is it should deliver thirty watts into its load.

    Marketeers of audio stuff know how to lie with statistics
    real useable power is called "RMS" which is not exactly taking the average but very similar.

    For a very brief instant an amplifier can deliver watts nearly equal to (its supply voltage squared ) divided by (resistance of its load) ..
    That is called 'Peak Instantaneous Power" and it's a pure marketing ploy "of dubious integrity", to borrow a phrase from Janis Ian....
    It can deliver that power for only milliseconds not continuously.
    Note the Lepai with supply voltage of nominal 12 volts and a load of four ohms might claim "peak instantaneous power" of perhaps almost 122 / 4 = 36 watts , and they do claim 30.
    I'd expect realistically seven to twelve.

    Manufacturers can include an internal booster power supply to give the amplifier higher voltage, which is how those preposterous car "Boomer " amps work, but don't expect that in your ten dollar Lepai.

    Here's an article to peruse, written by engineers for anyone who wants to learn.

    old jim
    Last edited: Apr 5, 2017
  16. Apr 5, 2017 #15
    Hay thank you for posting that I consider my understanding of audio equipment to still need some corners knocked off and this should help
  17. Apr 6, 2017 #16
    Thanks for this detailed replay.
  18. Apr 6, 2017 #17

    jim hardy

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    It's difficult to find what's inside that Lepai amplifier.

    One source said it's heart is an LA4625 IC

    datasheet at https://www.onsemi.com/pub/Collateral/LA4625-D.PDF includes these graphs

    it's an honest ten watts if you don't mind some distortion. Just a few percent won't be objectionable....

    It's not class D though.

    old jim
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