Help me understand amp power

Discussion in 'Amps and Cabs' started by stratman79, May 10, 2015.

  1. stratman79

    stratman79 Member

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    So I was wondering if someone could help me understand about amp power etc.

    So originally I had always (maybe foolishly) assumed that a 100w amp had twice the out put of a 50w.

    After reading about this is would seem to suggest that this is correct in terms of acoustic power, an amp with twice the watch would create 3db more than the other amp with half the wattage therefore doubling the acoustic power.

    Reading on it suggests that because hearing is based on sound pressure you actually need 6db of volume to create double the sound pressure? Therefore if you wanted an amp twice as loud as your 50w you would need a 200w

    Now on reading other bits and bobs it suggests that to create twice the volume you need 10x the wattage... So to get an amp twice as loud as your 50w you would need a 500w amp...

    I normally use a little 5w amp and was thinking about getting a 12w number but if it's only going to have 20% or so power then I'll re think.

    If someone could help me understand I would be very appreciative!
     
  2. LaoTzu

    LaoTzu Member

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    the main point of a larger power section in an amp is to having more head room before your your amp starts to distort or "break up" helps having a better clean sound but it does also mean that you need to turn your amp louder before the break up happens this is why people say 100watt amps are louder. there right but technically there wrong. you just have to turn them up louder to gain the same effect a 50 watt does at lower volumes.
     
  3. GCDEF

    GCDEF Supporting Member

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    Relative loudness is a perception. Most people perceive loudness to have doubled somewhere between 6 and 10dB. 10dB is the most commonly used number when describing what somebody thinks is twice as loud. Doubling the wattage will increase loudness by 3dB, so to get that perceived doubling of loudness you need to ten times the power to get that 10dB increase in output.

    So, a 100 watt amp will have twice the electrical output as a 50 watt amp, but your ears won't perceive it as twice the volume.
     
  4. justnick

    justnick Gold Supporting Member

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    Our hearing is logarithmic by design: 10x increase in the energy in the air is perceived as doubling the loudness. Same reason we use dB's-- a logarithmic scale. We are evolved to be able to cope with a huge range of sound energy,from the minute pressure fluctuations caused mosquito's wings to the massive pressure wave of a thunder clap. The difference in the pressures involved in those two sounds is on the order of a factor million or more. The dB scale and math of amp loudness are just a reflection of the way we hear.
     
  5. teemuk

    teemuk Member

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    ^ Yep, that's how it goes: Ratio of power and perceived loudness is logarithmic so increase or decrease in perceived loudness requires much higher ratios of increase or decrease in output power.

    This is also one good reason why "tube vs. SS watts" concept is inherently flawed: Easily perceptible changes in loudness would require some 4x - 20x differences in produced output power levels. We all know that ain't happening...

    But output power is also a moderately poor indicator of amp's potential loudness. As is, we can compare two identical waveforms of different power levels and determine some average ratios of power output vs. perceived loudness.

    But all this is more or less thrown out the window when waveforms are not identical but have some deviation in overall frequency response and level of compression (e.g. other wave being clipping distorted). At that point we realize that perceived loudness level is also largely affected by frequency, average crest factor of the signal and overall distortion of the signal. In essence, we hear certain frequencies louder than others, distortion louder than "clean", and overall loudness based on signal's average level, not the level of its transients.

    When all these characteristics tend to differ in amps that seemingly produce the same rated output power, and when we throw efficiency differences of drivers into the mix it gets pretty impossible to tell how loud something will be perceived just based on output power rating.
     
  6. mwhy

    mwhy Supporting Member

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    The other part of the OPs question is important: he plays through a 5W amp, and wants more sound -- "what to do?"

    That leads to several more questions: are you gigging, and in what size venues? what cabinet do you have? [1x8, 1x10, 1x12, 2x10, 2x12, etc. all have different "loudness" experiences!] What type of music do you play? Do you "prefer" tube or SS, or does it matter? etc etc ..
     
  7. zenas

    zenas Member

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    Don't forget speaker efficiency. Although the numbers published by the manufacturers and what you really hear seem to be two different things.

    Just thought I'd toss a little petrol on the fire.:)
     
  8. teemuk

    teemuk Member

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    ...or that loudspeaker efficiency actually varies considerably throughout the effective bandwidth of the driver.

    Here's a response of a certain Eminence Legend series speaker:
    [​IMG]

    Note that within 2-5kHz the speaker is about 10 dB more efficient than usually. To acquire same boost with amplification alone would require about 10x more output power to be produced at the aforementioned frequencies. Here the driver alone produces such a gigantic change in SPL.

    Do also note how the response falls heavily after about 5 Khz. This "acoustic low pass filter" cuts the most "fizzy" high frequencies so usually amps with such a limited response can tolerate more distortion/clipping before sounding obnoxious. Tap a DI to speaker output of the same amp and the "non-processed" signal is usually something entirely unusuable.

    In essence, add more low-pass filtering and distortion usually becomes less "fizzy" and more musical. This means amps can be cranked a great deal more, which means they will compress and distort more and therefore also sound a great deal louder than amps that operate ideally clean. It's a big mess of various things interacting together while outcome of this interaction is basically evaluated solely by individual preferences.
     
  9. Figaro

    Figaro Supporting Member

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    Also, different types of guitar amp watts are not equal. 20 tube watts is louder than 20 solid state watts.
     
  10. teemuk

    teemuk Member

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    Watts are watts.

    If one amp is louder than another with equivalent power rating it's merely representing the issues already discussed in this thread: Rated output power alone is a poor indicator how loud something will be perceived since perceived loudness is also effected by several other characteristics and variables than output power.

    If you do the math, a driver will be equivally loud when you feed it two identical signals. One produced by solid-state watts the other by tube watts.

    When signals are not identical the comparison is meaningless and moot. When amps are different expect signals to be different too. As is, you will quickly find out that there are similal loudness differences also in equally rated SS vs. SS amps and tube vs. tube amps. Why only make a fuzz about the SS vs. tube scenario...? Beats me, especially when the difference isn't always that tube watts are louder either.

    Tube watts being louder usually falls down to:
    - Comparison made between clean SS amp and distorted tube amp (because tube distortion is claimed to be nice, while SS distortion is claimed not to be)
    - Comparison made using unidentical drivers (wow, what an amateur mistake)
    - Both amps distort, one distortion type is claimed to be more preferable than other (which is of course a plain opinion and nothing objective in scientific terms)

    In none of the cases you see a comparison between identical signals. People listen to two entirely different signals and state that they personally prefer one signal over the other and hence also tolerate listening it louder. This is not a phenomenon that would be based in science or theory of electroinics in any way. It's merely a personal, highly subjective opinion.
     
    Last edited: May 10, 2015
  11. J M Fahey

    J M Fahey Member

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    Not so. (answering post #9)
    The speaker does not know or care where those volts and amperes come from (which is what it receives from that amp) .
    You drive an 8 ohms speaker with, say, 10 volts RMS and it will produce the exact same sound pressure level, no matter where those 10 volts came from, whether tube, transistor, MosFet, analog, digital, whatever.
     
  12. amphog

    amphog Silver Supporting Member

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    Yes, but a complex waveform is different than a 1khz sine wave.
     
  13. teemuk

    teemuk Member

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    Yet amp power ratings are derived with sinusoidal waves. Or should be....

    Because it is the only waveform with ideally only the fundamental frequency in it, thus other frequencies must be distortion.

    So in other words, complex waves are actually combined from sine waves of different frequencies. We call them "harmonics".

    Yes, all this means it's pretty hard to evaluate how amps differ outside that specific rated performance. And how we subjectively perceive it.

    You can have two amps, both claimed to produce 20 watts. However, one of those amps can sustain that average power continuously for seconds, the other starts to sag and distort in a millisecond. OR one amp is rated at %1 THD, the other at 5% THD. ...OR both amps are rated at 5% THD but the THD of one amp is largely low and even order harmonics, while in the other amp it's largely high, odd order harmonics. ...OR these amps have vastly different tone stacks and frequency responses, preamp clipping/compression characteristics and such. A simple output power rating can't cover this all and many effects of those parameters are - in the end - evaluated entirely subjectively. Can you tell which amp is loudest? Which amp sounds nicest? Note: all were rated for 20 watts of output. See where this is going...?

    Need I repeat: rated output power is a poor indicator of perceived loudness. It depends so much on other parameters and individual preferences too.
     
  14. Cosmik de Bris

    Cosmik de Bris Member

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    Power ratings are just that, more marketing than reality. Have a look at how much power is taken from the supply, there is usually a plate on the amp that tells you this. Notice on some amps the rated power output of the amp is 10 times the input power. How can this be?

    Down at the local Hi-Fi store there is an amp and speakers that look like daleks rated at 33000 watts!
     
  15. guitarjazz

    guitarjazz Member

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  16. GT100

    GT100 Member

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    I think that a lot of the tube amps are louder confusion is because tube amps tend to have better more efficient speakers. Solid State amps are usually equipped with cheaper speakers that aren't as efficient.

    Lloyd


    Sent from my iPad using Tapatalk
     
  17. slybird

    slybird Member

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    I often read that a 100 watts is needed to perceive twice the volume of a 10 watt amp.

    I own a stereo 50 watt power amp. Why do I perceive both power section on as twice as loud as only having one side on? This is true for any position on the volume dial. I am not increasing the wattage 10x when I turn on the second power section.
     
  18. J M Fahey

    J M Fahey Member

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    True, and that's a big point.

    The classic comparison is between a 15W SS amp running an unknown cheap 8" speaker , efficiency maybe around 90dB and,say, a 15W tube amp running a couple Celestions (mind you, not cheaper 12L35 but at least G1265 , 12T75, Greenbacks or V30) or at least Jensen 12N or at least one or 2 Eminence Legend or better.

    A very unfair comparison.

    I have tested lots of 100W class Tube amps against lots of 100W class SS heads, think Peavey/Laney/Crate/Marshall/Ampeg , etc. using my shop standard MESA 4x12" cabinet, sporting 2 Black Shadow by Eminence in the bottom closed half and 2 Black Shadow by Celestion in the top open back half and sound has been **bone crushing** with all of them.

    Yes, SS did sound different, a "duller but harsher" sound, not an oxymoron but best description I could find, but SPL of both were very very close.

    I also found that SS amps full blast had the sound I describe above; at low volumes are usually buzzy/icepicky/you name it, but if you fully distort he preamp first (buzzy) and then start rising the Master volume sloooooowly, eventually throwing the (dull) SS power amp into clipping, there is *one* sweet spot where both ends balance and you get a very good and usable sound.
     
  19. DaveKS

    DaveKS Member

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    Imagine because if I'm reading your statement right your also kicking in a 2nd speaker/cab. Most examples here are talking using same speaker for both power configurations.
     
  20. Nota

    Nota Member

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    I wonder why more people don't gig with highly efficient speakers and lower wattage amps. I gig with 30W, which is more than enough through a speaker over ~ 98dB efficiency.
     

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