[Clifford Heath]

Plays Real Basses wrote:
Scott Dorsey wrote:
Unfortunately many don't. Which is why a lot of amplifiers rated 500W
into 8 ohms are only rated 1500W into 2 ohms... where is that other 500W
going? --scott
HEAT...

To explain a bit more, the heat in an amplifier's output devices is
a function of the output current and the *supply rail* voltage, not
the output voltage. To push 500W into 8 ohms, the voltage swing at
the output must be at least 90V peak, or 180V P-P. The output devices
need to be still running in a linear mode before clipping, so the
actual supply rails will normally be 5-7 volts more, say 195V rail
to rail.

When the output is at half the rail, the current (assuming a
resistive load - not the case actually) is over 5 amps, and the
output device on that side is dissipating just under 300 watts.
Now assume an inductive load with a phase angle of 45 degrees.
The maximum current of over 10 amps is reached at the half-rail
voltage on the descending side. The maximum dissipation in the
output devices occurs somewhat after that, when it reaches perhaps
700W (ballpark figures). The ratio of output power to wasted
power is much less with a reactive load.

If you use a lower impedance driver, you reach maximum current at
lower output voltages, so the voltage across the output devices
is *greater*. The effect is that the safe output power will be
less with low 2 ohm speakers than with 8 ohm ones.

I deliberately didn't go into the maths here, but I hope this has
gone some way to answer your question.