Originally Posted by Uncle Erik
The amount of power reaching the headphones depends on the impedance match between the amp's output impedance and the impedance of the headphones. The closer the two impedances match, the more power that reaches the headphones. You use the amount of power reaching the headphones to determine how loud the headphones will get.
i dont think thats right.
the output impedance of the amp has NO absolute effect on the ability (or inability) to deliver power to a load. output impedance will have an effect on amp/driver damping factors, but an amp with a high output impedance can theoretically put LOTS of power into a low impedance load. the low-impedance amp driving high impedance loads is also well known.
the power through a driver is the same as any other load. measure 2: current through the load, voltage across the load, or load impedance and bust out the calculator. depending on your mood and honesty you can use RMS values, or P-P values for audio.