The OBH-11 is a miniature audio amplifier, designed specifically to drive a single pair of low to medium impedance (30 - 300 Ohm) headphones from a line level source. The OBH-11 SE is a 'Class A' amplifier with zero feedback and can drive two pairs of headphones simultaneously. It is more efficient to have a small amp to do this rather than waste the power of a full size one when only driving headphones.The OBH-11 and 11SE are compatible with any source at line level. For example: an average output level CD player can be connected directly to the input of the OBH-11/ 11SE, just as it could also be used in conjunction with a stand-alone pre-amplifier or integrated amplifier which doesn't have a headphone output.The OBH-11 is not designed to be connected to the speaker output of an integrated or power amplifier. Output level can be controlled by the high quality volume control on the front panel.