The amplifier in this case is based upon the CMoy, a common headphone amp design. [ErikaFluff] added a MOSFET on the output to drive his relatively low impedance (32 ohm) Grado headphones with ...

All is not lost however, because it’s possible to make a simple headphone amp with this circuit if you ... so an alternative might be to use a MOSFET source follower as a buffer.