Nobsound Hi-End Pure Class A Tube Headphone Amplifier HiFi Stereo Preamplifier This is Hi-End pure class A tube headphone amplifier, also can do as Preamp.Have 4-gear to select output impedance, suitable for a wide range 15-600 headphones, especially for K701 HD650 etc. Also different from the general pure tube headphone amp which drive current is less than 100MA, it's hard to drive the 64 below headphones, this tube amp use the costliest Japan Z11 audio output transformer, achieve optimum impedance matching, even 16 headset is completely no pressure. really makes a huge deal on the quality of sound. Choose different gear according to the impedance of the headphones, For example: 50 gear suit for 16-50 headphones; 150 gear suit for 50-150 headphones.