Originally Posted by upstateguy
Hummm, makes sense, but I thought resistance = heat?
No, resistance doesn't necessarily mean heat. Heat is produced when there is power being dissipated from a device. The amount of power dissipated is the voltage across that device multiplied by the current through it. When there is little or no current, there won't be heat.
In an amplifier, given a particular output voltage, the amount of current delivered by the amp through the load is dependent on the impedance of the load. To simplify matters, think of impedance = resistance in this case. Via Ohm's Law, I = V / R, where I is the current, V is the voltage across the load, and R is the resistance. Hence, as you can see, when all else is equal, reducing the load resistance (headphone impedance) results in an increase in current. Since the current is directly proportional to the voltage, the louder you set the volume, the higher the current too.
Now, assuming that the amp is class B or AB, then the output transistors are going to dissipate approximately the power supply voltage across them multiplied by the load current as heat. Hence, assuming a certain and continuous output voltage, connecting a lower impedance load will cause the output transistor(s) to run hotter than a high impedance load.
In a pure class A amp, the output transistors are biased such that the quiescent current flowing through them is high, so high that under any reasonable output voltage and load conditions, the quiescent current will be higher than the load current. In this event, the output transistors will run hot constantly, and the temperature will not change with the load impedance. That is, unless the output stage is push-pull and the peak load current exceeds the quiescent current, then the amp drops out of class A and goes into class AB, and the output transistors will heat up further. In a single-ended class A amp, if you try to make the amp deliver more than its quiescent current, it will simply result in clipping.
Now, not all "class A" amps are biased the same. Some are set up to have much higher quiescent current than others, and those will run hotter. The higher quiescent current allows the amp to deliver more current before it drops out of class A (or clip, depending on the output stage design).