Woo Audio WA6 2nd gen Vacuum Tube Class-A Headphones Amplifier (Sliver) Single-ended triode Class A, Output Transformer Coupled, Headphone Amplifier Headphone impedance: 8 - 600 Ohms Rectifier tube x 1, driver/power tube x 2 (brand/model may vary) Point-to-point wiring, Custom made output transformer Selectable Voltage: AC 110/220 V, 50/60 Hz