i think that it didn't burn cause power suply from computer on 12V rail has maximum 18A, but that current is maximum which can power suply produce, so when i attached diodes i think that from 12V to the LEDs goes amount of current just needed by LEDs, nothing more. I saw on my headphones that their maximum output is 100mW and rated impedance is 32ohm, so can we maybe calculate the current here?
---------- Post added at 21:55 ---------- Previous post was at 21:42 ----------
i found some characteristics for tip31c transistor, if it can help you, here you go..
NF-L , >3MHz , 140V , 3A , 40W