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I realise this is a really silly question - but what makes an amp that as a 100 watt output better than one with 25?
There is more to it but an important factor (and one that is often mentioned in alongside watts) is also the aspect of current.
Let's look at it like this -
A car battery is heavy object and is rated at 12v. When the ignition key is turned a large surge of current (a few hundred amps) is delivered to turn the engine on.
A single AA battery (like the ones used is a tv remote for eg.) is rated at 1.5v. Eight of these would add up to 12v.
Replace the car battery with eight AA batteries. Now you have the same 12v as before.
Would eight of these AA batteries start the car engine? No they would not.
Would 50 of these AA batteries (now rated at 75v together) start the engine? No they would not.
The reason is because even 50 AA batteries rated at 75v (as opposed to 12v for the dedicated car battery) would not have enough current to start the engine.
Higher watts does not necessarily mean that an amplifier will control speakers and perform better than a lower powered amp because there can be more to it than just higher numbers/paper watts.
It's better to have an amplifier with a good power supply and one that can deliver appropiate current to each circuit optimally and also (amongst other factors) deliver current rich wattage (rather than current starved wattage) to tackle the loads presented by the connected cables and speakers.
***** Krell's (now discontined) KSA-50 power amplifier is rated at 50w/ch. And yet because of it's (amongst other factors) high current capabilities it can control many speakers more effectively and outperform some other amplifiers of higher power *****
Watts is (not are) a function of Voltage x Current.(Amperes)
so V x A = W
Now your volume control on your amp changes the voltage that is applied to the speaker terminals. For a given voltage, an amount of current will be "drawn" depending on the resistance or impedance of the speaker.
where A = V/R
If the speaker load os 8 Ohms, then at lets say 16V this means that the amp has to deliver 2 amps, which means it needs to be able to produce 16x 2 watts = 32 watts.
Now if at that same voltage our speaker has the same sensitivity, but this time its impedance drops to 4 ohms then the amp has to be able to deliver 4 amps of current, or 16 x 4 = 64 watts. Some "difficult" speakers may even present a 2 ohm load at some frequencies, so suddenly you need 128 watts, all just to maintain the same "loudness" setting. In effect, using watts as a measure of an amps loudness capability is only part of the story, which is why you also need to know its max current capabilty, measured as steady sate and short term peak. Therefore, there is some mileage in the view that you can never have too high a power rating, but then again it is perfectly possible to match a low powered amp to the right speakers and make a lot of noise, where thos speakers are both sensitive, and also are a easy load (i.e dont drop much below the nominal 8 ohm impedance).