SMSL SP400 Balanced THX Headphone AmplifierSMSL SP400 Balanced THX Headphone Amplifier Innovated designed with all discrete output stage circuit, which has ultra low distortion, as low as 0. 001%, and the output current for amplification. 256 steps Relay volume control, for quite background, and Pre Amplifier Function with XLR output A fully balanced circuit design from inputs to outputs. Use electronic volume control to ensure the volume level are the same for left and right channel. Audio