Behringer Microamp HA400 Ultra Compact 4 Channel Stereo Headphone Amplifier Mini to the max! You can use this super-compact stereo headphone amplifier with practically all headphones—four of them at the same time, to be exact! For example, you can let up to 4 different people in your studio listen to the main mix while the recording is being made, and each listener can determine his own volume level using the dedicated output level controls. The HA400 contains 4 high-power stereo amplifiers that maintain the highest sonic quality even at maximum volume levels. Ultra low-noise 4580 operational amplifiers are included for outstanding audio performance—these are the same op amps found in "full-size" audio equipment—plus, a DC 12-Volt adapter is included.