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Design Goals
A small power amplifier is what is needed to power dynamic headphones. When designing a power amplifier, design backwards. Start with load, move onto the output stage, then to the input stage.
Although the Grado headphones are not the lowest impedance headphone available (some Sony headphones are a brutal 8 ohms), they are close at 32 ohms. Fortunately, the Grado does not require more than a few millivolts to hurt your ears. On the other hand, the Beyer and Sennheiser headphones present a relatively easy 300 ohm impedance load to the amplifier. However, with the 300 ohm headphones, a higher voltage swing is needed, something on the order of 1-3 volts. This tenfold difference in load impedances makes the design a little more difficult than if only one ohmage were presented to the amplifier, as a no gain amplifier with large current swing potential would be ideal for the Grado and some gain and less current would serve the Sennheiser better. But as we never know when some new killer headphone might come out or what its impedance might be, an amplifier that can cover the spectrum would make more sense.
So here are our amplifier's design objectives: enough gain to bring the 1 volt line level signal up to 3 volts and enough current delivering ability to drive a 32 ohm load to at least 0.7 volts. The implicit design objective when dealing with headphones is LOW NOISE. With the diaphragm at your ears, any excessive noise will mar the listening experience.
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