Signal = voltage in voltage based amps
Signal = current in current mode amps.
When voltage amps drive HP, current is generated by V=IZ where Z= impedenace.
Current is function of voltage with respect to phase.
And magnetic flux is in phase with current, not with voltage.
Because magnetic flux = H is only function of current. This means that only current generates magnetic flux, and voltage can be generated without any current for example open circuit having maximum voltage but zero current that generates no magnetic flux.
Of course voltage generates magnetic flux in relation to impedance=Z, but a phase amount delay.
Sorry for my adoption of math.
Especially impedance Z is function of frequency, when it approaches to resonance the phase will delay or advance in 90 degrees.
Because all these relations, voltage amplifier drives by voltage signal and HP responds only to current, so that there is a phase difference between signal generator ( driver) and signal producer (HP) due to impedance that depends on frequency. The higher the frequency, the larger the phase difference between driver (AMP) and driven (HP).
Edited by Joong - 12/4/13 at 4:05pm