You can use this ultra-compact stereo headphone amplifier with any of 4 headphones at the same time. Four listeners in the studio can listen to the main mix while recording, and each listener can adjust their own volume using a dedicated output level control. HA400 contains 4 high-power stereo amplifiers that can maintain the highest sound quality at maximum volume. The ultra-low-noise 4580 op amps provide outstanding audio performance. They are the same op amps found on full-size audio equipment and are supplied with a DC 12-volt adapter.