Originally Posted by Remeniz
The rule is this: You can never go wrong with an amp more powerful than the speakers.
If you turn up a small amp thats running high power speakers, the amp will start to distort and its the distortion that kills speakers.
Really? I have to say that I never knew this rule...
perhaps that's because it's complete bollocks.
the rule is actually the exact opposite way around.
are you saying that I can connect my 100W guitar head amp to a 3W speaker and not expect it to break the speaker? no, it would break the speaker.
because the voice coil in the actual speaker driver couldn't handle that amount of power.
take a look at that calculator,
3w into an 8ohm load uses ~4v at slightly over half an amp.
100W into an 8Ohm load is driven by 28Volts from the amp with a current of 3.5A
in short, try pushing 100W into a speaker that can only handle 3W and you're going to melt the voice coil.
your rule that the amp can never be to big for the speakers is just plain wrong!
Anyway, now we'll look at fundamentally the reason that this rule has come into audio folk law, which is partly described by what you wrote next...
If your amp is too small, (not too small for your speakers, but too small for your purpose), then you're likely to turn the amp up a lot, and this will lead to it distorting, we call this distortion clipping,
the reason that this can be bad for speakers is because in extreme cases of clipping there is almost no sinusoidal type waveform left as you would expect to see in music, you're left with a waveform that bounces between the upper and lower voltage rail limits of the amp, so that amp is effectively always driving full power into the load (speaker).
there is a very simple reason that this is bad, speakers are not designed to be always on, they are designed for a voltage that oscillates and varies, so that the speaker transitions between high and low voltages, with times in the middle where they are not driven at full. (or near to full) load.
if the speaker is always seeing what appears to be a constant voltage then there is little to no cool down period that can happen within the speaker, and once again without cooling down you can see the voice coil breaking...
so lets put these together...
firstly, what I said at the start, you can't put a 3w, (or 50W or 75W) speaker on a 100W amp and expect it to last long -At full power.
However, you could attach a 50W speaker to a 100W amp assuming that you were running it at around two thirds of the amps capacity.
20v instead of 28v into 8ohms is 50W.
conversely, you could over load a 3w amp till it was clipping and put that into a 100W speaker with no trouble.
but attach a 75W amp constantly at full drive with a distorted clipped signal into a 100W speaker and you'll soon find out that RMS is based on the power handling of a wave form that isn't constantly on full load, (i.e not a square wave).
you'll also find that PMPO (Peak Mean Power) and instantaneous power handling ratings are just that, peak and instantaneous, not constant...
the speaker can't dissipate the heat generated fast enough, and you still melt the voice coil!
the trick is this...
Choose your speakers and your amp wisely.
they don't have to be the same brand for speaker maker and amp maker.
they should be roughly rated to be around the same. but should also be as loud as you want them to be.
there is no point in buying a 10W amp when you really want a 50W amp for it to be loud enough.
when you look at amps, you should think about the headroom that you want.
headroom is the remaining ability to crank it up further once it's loud enough.
as said before, if you want the amount of noise that a ten watt amp can produce at full power, then what you're hearing from that 10W amp is likely to be distorted, it's better to get a 30W amp and only turn it up 3/4 of the way so that if you wanted to, it could still go louder, but you don't turn it up louder cause then it'd sound bad (clipped and distorted). getting an amp with more capacity than you need will ensure that you don't need to turn it up to the point where it distorts.
with regards to loudness, sound is measured on a logarithmic scale, so don't fall into the trap of thinking that a 1000W amp is ten times as loud as a 100W amp, it's actually only twice as loud.