LD Systems HPA 4, 4-Channel headphone amplifier, 4 Adjustable headphone outputs, 6.3 mm Stereo jack connectors, RCA L / R and 6.3 mm jack input, Switchable mono stereo, Operates with external DC power supply, Max. output power: 4x 100 mW, Input impedance: 1 kOhm, Incl. 24 V DC power supply