Take a voltmeter and read the indicated voltages for each transistor type and select pairs that have the closest values. That makes finding pairs a lot easier. This is not required if the output is capacitively coupled to the headphones, but I believe that DC coupling gives the truest sound reproduction. This is done by using unequal emitter degeneration. Based on calculations, the output idling current could be increased to about 15mA by increasing the 20 ohm resistor to about 30 ohms.
Thermal dissipation is really the issue.
I have not measured the output impedance of the amplifier, but since the output stage is a voltage follower with about 0.5A of drive capability, it would be extremely low. It would be somewhat dependent on both voltage swing and frequency. It also makes the driving output look more like a current source than a voltage source, which is not desirable for accuracy in driving loads which have reactive content. Chester Simpson is an engineer in the Power Supply Division at National Semiconductor. Visit the HeadWize Discussion Forums .