At 11:07 PM 7/2/99 +0100, Tony wrote:
What I was really asking is for any kind of
justification of the '240W'
figure. Now, most of these speakers seem to have a 6W DC input rating, so
the output power can't be any more than that.
I wonder if it's something like :
We have a 12V PSU. So we can get 24V p-p across the speakers (use a
full-bridge output stage). Say 8 ohm speakers, 4 of them (2 per channel).
So we get 24^2/8 *4 = 288W. Yes, I know there's absolutely _no_
justification for any of that, but then again it is marketroid-speak....
Boy does that sound like spec. inflation! 24 p-p is just 12 Volt peak in
either polarity with a bridge circuit. That gives 12/8 peak amps and 18
peak watts (like with a +/-12 volt square wave). The RMS voltage and
currents are these voltages, currents divided by sqrt(2), giving a sine
wave power of 9 watts per 8 Ohm speaker, with no voltage drop in the
amplifier. Don't tell me these "240 Watt" speakers are powered by 4
"AA"
batteries ;)... or use one of the many automobile ic amps that give about
4.5 watts with a 12 Volt supply and a 4 Ohm speaker.
-Dave