My Adcom 555II preamp has a +-10V swing and (as per manual) 100+ ohm drive capability on the headphone output. The AKG 702 headphone is advertised with a 62 ohm impedance @ 1 kHz.
I can monitor preamp headphone output (while it's connected to the headphones) with a scope/spectrum analyzer and see that that a sine wave at frequencies from 100 Hz to 5 kHz is undistorted at peak levels up to ~9V, and then it starts to soft-clip, pushing up the third harmonic to -30 dB at 9v peak. At these levels the headphone output is unbearably loud. At levels below 8 V, THD is < 0.05%. Without headphones, the output is undistorted (THD < 0.05%) right up to 10V peak, and then it hard-clips.
The Adcom 555II manual further says that 'best performance is achievable with headphones with approximately 600 ohm input impedance'.
So the question is whether I'm compromising headphone performance by connecting them straight to the preamp. I notice at my 'normal' listening levels, voltage levels are ~0.1VRMS or less.
Could I expect a noticeable difference by using a separate headphone preamp?
