Vintage amps use same amp circuit that would powers the speakers. There is a resistor wired in line to reduce power output, because obviously headphones don't need 50+ watts per side. Some people say high output impedance is bad, but I let my ears be the judge, and what I am hearing is pretty stellar. Especially considering the ridiculously low price I paid for this thing.
Edited by jasonb - 8/7/13 at 7:15pm