power amp ratings

Discussion in 'Amps and Cabs [BG]' started by Daniel Elliott, Feb 24, 2010.


  1. A question for you techie types

    If a manufacturer claims their bass head is 1000 watts at 5% THD, what would you guess would be the power rating at 1% THD?
     
  2. bongomania

    bongomania Commercial User

    Oct 17, 2005
    PDX, OR
    owner, OVNIFX and OVNILabs
    200 W. No, really, that's my honest guess. I'd also say that any power amp brand that uses a 5% THD spec as the basis for claiming it's a 1,000 W amp is not to be trusted at all.
     
  3. sleepytime

    sleepytime

    Nov 10, 2007
    Chicago, IL
    5% THD has been a fairly common rating for instrument amplifiers. (Power amps more typically rated at .01% - .1%) I have seen amps that give specs at 1% and 5%, the difference is not all that significant. As a guess, I would say you should see 800 - 900W @ 1% THD, a difference you are unlikely to hear.

    Considering how many amps give no spec for THD, I don't think using a 5% reference is anything to get excited about.
     
  4. billfitzmaurice

    billfitzmaurice Commercial User

    Sep 15, 2004
    New Hampshire
    Owner, Bill Fitzmaurice Loudspeaker Design
    Of far more importance than THD is S/N, the signal to noise ratio. A high S/N ratio is the source of foggy mids, and the dreaded hiss. Good quality power amps run over 100dB S/N, as do high quality PA consoles/mixers/effects etc. The minimum acceptable for high quality sound is 90dB; even the cheapest of hi-fi receivers are capable of that. Bass amps OTOH commonly come in at 80dB and less. The difference between 80dB S/N and 100dB S/N is literally 100 times the noise.
     
  5. B-string

    B-string Supporting Member

    Pass, no way of knowing if the amp will do 1% or less. Can't imagine any amp made today that would not, but still no way of knowing without testing.
     
  6. Hi.

    I'm with bongomania on this, 200W 100Hz RMS give or take a few watts.

    5% THD a standard?

    On tube amps, sure, but a SS rating that high would make me to look for another option.

    Regards
    Sam
     
  7. Yes it will do less than 5% THD. In this case, THD is the amount that the output signal is being clipped (or most of it anyway). In other words, if you turn the volume down, the distortion is reduced. It is a way of exaggerating the output rating of an amp.

    For example, the Yamaha P7000S power amp publishes a power output rating of 750 watts per channel driving a 1kHz signal at 1% THD. And at 0.1% THD driving a signal that is 20hZ to 20kHz (pink noise) it produces 700 watts. The second rating is much more realistic. Many bass amp manufacturers rate their amps by driving a 1kHz signal to 1% THD and measure the output. The argument being that generally THD is not audible to 1%. Whereas some amp companies will drive the output to 5% THD and measure the output at that point. PA amp ratings generally rate at 0.1% and in some cases 0.05%.

    I just wish everyone would do it the same way.
     
  8. Hi.

    The way I was taught the subject, distortion (whether it's total harmonic, just one harmonic, or the base waveform deformation) is the difference between the original and the amplified waveform.
    Clipping was another subject alltogether.

    Whether the distortion reduces or not when reducing amplifying factor, depends greatly of the circuit topology. Some designs are more prone to waveform deformation than others, class B usually more than class A for example.

    Most of the unavoidable distortion characteristics came from unmatched discretes (or tubes ;)) being pushed over the linear portion of their operating curve (but NOT over the curve itself = clipping)

    Perhaps I was taught wrong?

    The fact that a simpler, more unstable design may produce more power with higher THD, doesn't necessarily mean it'll do even the half with less distortion (hence the 200W guess).

    Regards
    Sam
     
  9. bongomania

    bongomania Commercial User

    Oct 17, 2005
    PDX, OR
    owner, OVNIFX and OVNILabs
    FWIW I was taught the same way. All clipping is distortion, but not all distortion is clipping.
     
  10. B-string

    B-string Supporting Member

    Same here.
     
  11. all circuitry will add a certain amount of distortion to the signal, but when you overdrive a power amp, distortion increases dramatically so when you clip an amp, the THD rating goes up.
     
  12. JohnMCA72

    JohnMCA72

    Feb 4, 2009
    Don't you mean low?
     
  13. bongomania

    bongomania Commercial User

    Oct 17, 2005
    PDX, OR
    owner, OVNIFX and OVNILabs
    I think you're missing a piece of the puzzle. While it's true that "all circuitry will add a certain amount of distortion", there is a wide range of imperceptible or barely-perceptible distortion, and then there is another range of significant distortion that is not clipping.
     
  14. That may be, but in this case what is happening is an amp manufacturer is driving a 1kHz signal to the point of 5% THD and measuring the power output. It's a pretty safe bet that it will produce a signal that is less than 1% THD at a lower volume. It is a solid state amp with a tube preamp btw.
     
  15. sleepytime

    sleepytime

    Nov 10, 2007
    Chicago, IL
    As a point of reference, I was just looking at the manual for a Peavey Mk VI.

    290W @ 1% THD
    320W @ 5% THD

    Can't say if the amp in the OP would exhibit similar characteristics, but this is in line with what I have observed in the past.
     
  16. That's about a 10% difference so if that applies to the first amp, it would be 900 watts.
     
  17. Hi.

    In that case (which You conveniently left out), the 1% THD output will probably be about 50W or less through the signal chain unless you find an exeptionally good pre amp tube(s).
    Or measure only the power amp section.

    You now have at least two amplifying stages, and getting 1 % THD figures from the front end is next to impossible if a tube sound is what you're after.

    BTW, measuring the output of an amp isn't rocket sience. Just find out the max power to a known load before clipping and forget the THD.

    Regards
    Sam
     
  18. sleepytime

    sleepytime

    Nov 10, 2007
    Chicago, IL
    So if the amp makes 50W @ 1% THD, what would the output power be at 5% THD? How about 3%? Draw us a curve. Better yet, post amp specs that show a 2000% increase in power from 1% to 5% THD.
     
  19. sleepytime

    sleepytime

    Nov 10, 2007
    Chicago, IL
    Looking at ratings in general, I have a bunch of .pdf manuals, so I did a quick check:

    Fender......................................................Rated power @ .05% THD, 100 Hz
    Ampeg, Phil Jones.......................................Rated power @ 5% THD
    Acoustic, GK, Hartke, Markbass, Mesa............No distortion spec provided
     
  20. greenboy

    greenboy

    Dec 18, 2000
    remote mountain cabin Montana
    greenboy designs: fEARful, bassic, dually, crazy88 etc
    As with MI enclosures, best to treat MI amp specs with very little attention. But when you look at reputable pro power amps the specs are actually referenced and accurate - and deep enough that you actually need to know a little to fully interpret them.
     
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