[757labs] 20,000 Watt Car Amp CL Ad, follow-up post

[Scott Dorsey]

There is no such thing was "watts RMS."  You mean "average watts."

If you multiply RMS voltage and RMS current, you get average power.
You can do the math yourself if you don't believe me.

Home stereo gear is normally measured in "average power by FTC method"
since that's what the federal trade commission permits in advertising.
However, professional audio gear, car audio gear, and portable gear
can have power measured any way the advertisers want, and that includes
having someone in the marketing department pull an enormous and random
number out of their rear end.

And you can in fact get any arbitrary amount of power out of a 12V source
as long as you're willing to draw an arbitrary and large amount of current.
This is because we live in the age of the switching supply where it's possible
to readily trade current for voltage and back again, even with DC.

When I was a kid, car stereos all ran on a single 12V supply rail, and
so they were limited to maximum power into a 4 ohm load of 12^2/4= 36
watts/channel peak.  This works out to 25 watts average power (36 * 0.707)
on a sine wave.  

However, we now live in a different world, where car stereo amps have 
switching supplies that pump that 12V input up to as much as +/-50V.
So it can have reasonable rails like a home stereo amplifier would.
Of course, it's still limited by how much current is available from the
car's electrical system, but that's not the manufacturer's concern....
--scott
   (who doesn't even have a radio in his car)