Originally Posted by James neutron
I think they made the choice for TPA6120 with power output in mind. Given the low voltage at which the soundcard operates (much lower than O2) it's possible that the TPA opamp would output more mW than NJM4556 for the same gain.
Actually, both the O2 and the analog output section of the STX operate at +/-12 V regulated supply voltage. In the case of the O2, the voltage is even slightly less, because of the power management/protection circuit.
However, the TPA6120 can output more power into low impedance loads (although this is mostly useful only for orthodynamic headphones), as it has a higher current limit than even two parallel NJM4556's. Also, sound card manufacturers may prefer the use of a single SMT chip for the headphone amplifier. Still, it is surprising that no sound card uses a similar solution to the O2, given that it can perform so well at a low cost.
Edited by stv014 - 3/21/14 at 3:19pm