How much amplifier power do you really need?

[petrik]

There is nothing mysterious.

If you calculate power by squaring the power supply rail voltage and dividing by the load impedance, you're calculating peak power.  Amplifier ratings are normally (at any rate, should be) quoted as RMS with a sine wave signal.  The peak power for such a signal is exactly twice the RMS power.

Example: 140 Pro PSU rail is 41 V, so peak power into a 6-ohm load is 41^2/6 = 280 W, vs quoted power 140 W RMS.

Your math agrees with the calculations I posted, in theory.  In practice, however, peak power is not being reached because of hidden code or the protection circuits are kicking in too soon preventing it.  Audio tested the 140 and could only produce 102 W in to 8 ohms and 203 W into 4 ohms before the protection circuits kicked in.  This was also the case with the 220 when HiFi World tested it, "...using 0.2 sec sine wave bursts, power measured 144 Watts (34V) into 8 Ohms and 290 Watts into 4 Ohms.  Into 6 ohms, burst power is 190 Watts, slightly below the 220W figure quoted but not a consequential shortfall."   

Using theses figures in your example the math becomes... 220 Pro PSU rail is 34 V, so peak power into a 6-ohm load is 34^2/6 = 192 W, vs quoted power 220 W RMS.  Which again supports the observation that peak output power is being limited to continuous output power levels.

Do you know were these measurements done properly - like was the SAM turned off? The SAM can limit the power.