need comments on amp design

[metebalci]

I have recently built a millett hybrid headphone amp, and that has caused me to think about how to design an amp.

I have an AKG K501. It is 120ohm with rated max input power 200mW.

To design an amp for it:

1) To drive it, I need something like 5V/40mA

2) I need;
a) preamp stage (maybe simply a volume pot)
b) voltage amp stage (since cd output is 2V, with gain like 2.5 = 5/2)
c) power amp or buffer stage if voltage amp cannot provide 40mA

3) I need impedance matching of output impedance to 120ohm, if it is quite different. We need as close as possible to transfer maximum power, right ?

I am a bit lost with the gain calculations when keeping impedance matching in my mind.

In millett's hybrid, we have an output buffer with 250mA current output but something like 3V voltage output. How can such a thing drive something (120ohm) needing 5V ? Do I read spec of buffer or headphone wrong ?

If possible, you can tell me about circuit theory terms, I am familiar with it.

Thanks in advance.