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Power amp guru's please help...

Discussion in 'Amps and Cabs [BG]' started by Son of Bovril, Nov 28, 2006.

  1. This Yamaha P2500S amp I bought has the following power specifications:

    look at the Yamaha P2500S spec sheet

    1 Khz THD + N = 1%

    - 8ohms stereo - 275W * 2
    - 4ohms stereo - 390W * 2
    - 8ohms bridged - 780W * 1

    20 Hz - 20 Khz THD + N = 0.1%

    - 8ohms stereo - 250W * 2
    - 4ohms stereo - 310W * 2
    - 8ohms bridged - 620W * 1

    1 Khz 20 ms non-clip

    - 2ohms stereo - 650W * 2
    - 4ohms bridged - 1300W * 1


    Regarding the power output level, what does (1 khz, 20 ms non-clip) mean? and does this mean I can run it safely at 4ohms bridged?

    Because in the same manual it says it's minimum impedance for bridged mode is 8ohms, but all the advertising for the amp has it rated at 1300watts, which as far as I can see, is only possible at 4ohms bridged...:confused:
  2. where's BOB when I need him!?
  3. Bob Lee (QSC)

    Bob Lee (QSC) In case you missed it, I work for QSC Audio! Commercial User

    Jul 3, 2001
    Costa Mesa, Calif.
    Technical Communications Developer, QSC Audio
    The "20 ms" thing probably means that instead of using a continuous, steady sine wave signal to measure maximum output power at the threshold of clipping, they use a 20-milllisecond burst of a 1 kHz tone. Their method is probably similar or identical to the old IHF power measurement method, which used a 1 kHz sine wave with repeating cycle of 20 ms at full power and then 480 ms at 1% of full power (-20 dB, referenced at full power). The amp would only have to put out full power for 20 milliseconds, and then would have 480 milliseconds to charge its reservoir caps back up before the next 20 ms burst.

    The power measurements that result will describe the amp's very short-term power capability but not its sustained power capability, which may be much lower.

    Most power amp companies today use FTC (very rigorous) and/or EIA (slightly rigorous, but not as forgiving as IHF) methods of rating power.

    I'd say your amp can be used for bridged 4-ohm usage, but it might not take being pushed very hard without overheating or current limiting. I don't think it would harm anything to try it out that way and see how it performs.
  4. Out of the given power ratings, which is it best to follow? Which would be comparable to the rating given to amplifer heads?
  5. tadawson


    Aug 24, 2005
    Lewisville, TX
    The one listed as "20 Hz - 20 Khz THD + N = 0.1%" is by far the most standard and honest . . . . the 1% number is usable for a bass/MI rig, but pretty much nobody in the PA and audio world in general rates amps at that high a distortion point. The first number is at the onset of clipping - IE, pretty much totally clean . . .

    - Tim
  6. ok, so I should be able to run it at 4ohms then if I'm not pushing it too hard, but I probably won't be getting the full 1300watts out if it...?
  7. I may be wrong in my understanding, as im just getting to grips with these ratings right now.

    But what i think it means, is you'll be able to get transients that push the 1300 watts, but the majority of it wont be, on saying that with SS amps nobody actually runs them at the rated outputs, thats just the maximum output it can achieve before the onset of hardclipping. I think.
  8. Thanx mohawk, I think that is what I was trying to say, but you managed to say it so much clearer :)

    My friend who runs a PA company and really knows his stuff said that the yamaha amps have pretty good protection systems, so I should just try it at 4ohms bridged and if it shuts down then I know it can't handle it, and hopefully nothing gets damaged...

    I don't think I'll be pushing it that hard anyways, plus I will be running a dbx compressor (with a hard limiter) in front of it which should help...

    I just need to wire a cable correctly now...
  9. Ah right, cool :)

    They do look like the buisness ! And if your running a compressor id dare say you'll be able to squeeze that bit extra out of it if your cutting out some of the peaks! :)
  10. A9X


    Dec 27, 2003
    True. And the difference at 8R is 0.4dB and 4R is 1dB.
    Most power amps put out a low measured distortion up until the threshold of clipping which in real terms is the 1% point. Drop back 1dB (barely audible) and you'll have the approx specified power rating of most amps, depending on the method used to perform the test.

    Yamaha have fallen prey to the marketing people in their advertising by using the biggest numbers possible to try to pull the punters in who go for the numbers without neccessarily realisng what they actually mean in practice. Unfortunate as it doesn't accurately represent what in essence are fine and reliable amps. I think I made this comment a while back in another thread on poweramps in reference to my Yamaha P7000S.

    Otherwise, I agree with Bob that 4R load should work in practice, providing the amp doesn't get too hot. Power will likely be a bit more than the 8R rating, maybe 1.5dB.
  11. just to close this post for now, Yamaha finally got back to me regarding the 4ohm issue...

    They say it is perfectly fine to run all of the P-series amps at 4ohms, so at least I don't need to worry about that anymore :)

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