sluggo wrote:Headphone impedance - if you want the most efficient power transfer, you want your amp's output impedance to match the impedance of the load. But with headphones, efficient power transfer is not really all that important. What's important is that your output is not the limiting factor in your voltage swing. In other words, as long as the amp's output impedance is less than the headphone impedance, you've done about all you need to do. A 600 ohm headphone is a very easy load to drive and your choice of op-amps is less critical. A 32 ohm headphone is more difficult (electrically) and will probably show more differences with different op-amps. A motherboard bragging about having "a high-capacity amplifier which is able to drive 600Ω loads" is not saying much, really.
Yes, a higher impedance load is easier on the amplifier as, at any given gain, it draws less current. BUT a higher impedance load also requires more gain (voltage) to achieve the same volume (db) level as the amount of power being pushed by the amp is less. The problem is that most line outs on sound cards/sources are not designed to have enough gain to provide adequate volume for impedances much above 50 Ohms (but this also depends on driver sensitivity). So, basically a 600 Ohm solid state amplifier chip's main goal is to increase gain (voltage). I imagine these circuits must also have more power/thermal handling capability as they must also be designed to handle a wider range of impedance loads...
So in reality, these 600 Ohm designs DO have to be designed to provide more power than the regular line outs designed for lower impedance loads.