About Power
We already discussed a little bit about power - RMS and PMP ratings. Well, RMS is what matters, here, because it is needed in order to use some simple mathematics. (Don't scream. I'm no good at maths so it HAS to be simple!)
Supposing you decide that you can afford a 1000 Watt RMS Amp and four 300 Watt speakers. Never mind the cost, think about the power.
Most amps are about 80 percent efficient at best. That means that 20 percent of the input power is wasted as heat. So, to get 1000 Watts out, you have to put 1250 Watts of energy in. Where does this come from?
NO! Not the stereo, you idiot! I knew you'd say that.
The energy comes from the BATTERY. (The stereo equipment just supplies the signal).
OK, so it's a 12 volt battery in most vehicles (which is usually giving 12 - 13 volts in practice - call it 12.5 volts) and there's a simple mathematical formula.
Watts = Volts x Amps
So, to get 1250 Watts out of a 12.5 volt battery we need 1250/12.5 = 100 Amps !!!
Now, just use your imagination. Your average car starter motor needs about 100 Amps to turn the engine over. Imagine the engine won't start. How long will the battery last while the starter motor turns the engine? Two minutes continuously? Maybe 5 minutes if the battery is fully charged.
Oops!
So, you've gotten your car to the car park, opened the rear lid, there's your beautiful sub, amps and speakers on display and you are broadcasting to the town....
for 5 minutes - then you are asking for a push-start
(
Are you going to look Kool or Fool ?
So, you have a problem. Your car alternator supplies maybe 45 Amps (check your handbook). 45 Amps at 12.5 volts gives you 562.5 Watts continuously as long as your engine runs. Bearing in mind the 80 percent rule, your amplifier will produce just 450 Watts. Pump up the volume and your battery goes flat (at best. At worst, the battery plates buckle and your alternator burns out!)
Fitting an additional or larger battery gives you a few more minutes but that's all. You either make do with a 450 Watt Amp or else you fit a bigger alternator (or two).
But, hey! Why are we worrying. If you can *afford* a 1000 Watt RMS Amp, you can afford to get the electrics uprated, yes?
)
Heat
I want to backtrack to the top of this page. I mentioned that an 80 percent efficient Amp wastes 20 percent of the power going in as heat. So, out of 1250 Watts going in, 250 Watts is coming out of the heatsink fins and the metal casing as HEAT.
Where's it gonna go? Have you ever held a 250 Watt light bulb in your hand while it's on? No, you don't want to try it.
So, the Amp has to be positioned where it can get rid of that heat, otherwise it'll get hotter and hotter and hotter and..
Did you ever wonder why Karl keeps on burning out his Amps? Could it be because he fixes them under the carpet under the front passenger seat? Think about it. Maybe he didn't think about it. Maybe the guy who sells these Amps didn't bother to TELL him. ( Who would you rather buy off. A guy who tells it like it is or a guy who says it's all your fault but he'll sell you another replacement for "only" 500 quid? )
OK, here's a little secret: Hot air rises.
You knew that already! But you always thought "well, if it *can't* rise it just *has* to go sideways".
WRONG! Hot air rises. If it can't rise, it just gets hotter. The only way to move it sideways is to physically move it - with a fan, for example, or a breeze of air from an open door or window.
So, here's what you must do;
Mount the Amp so that its heatsink fins are vertical - this allows maximum airflow to take the heat away.
Mount the Amp near the bottom of a large chamber (eg. a car boot space) with as much air space above as possible. Hot air will be able to rise above the Amp.
Mount the Amp firmly to a metal surface with heatsink compound on the mating surfaces. This allows maximum heat transfer between the Amp and the metal surface.
Oh, yeah, and for goodness sake, mount it where it won't get wet!
