The Behringer MicroAMP HA400 is a very inexpensive, miniature 4-channel headphone amplifier. for stage or project studio use. The unit is equipped with separate operational amplifiers for each channel, and individual channel level controls.
Ultra Compact 4-channel headphone amplifier with individual channel level controls
4 separate low-noise 4580 op-amps(operational amplifiers) for efficient, maximized output levels per channel