HIFIMAN EF2C USB-DAC Headphone Amplifier Sublime and Organic Tone from the Vacuum Tube The hybrid design employs two 6J1 tubes for its input stage and incorporates the OPA2604 along with discrete components for its output. Step Potentiometer for Improved Volume Control The EF2C is meticulously designed for discriminating music listeners who demand performance and value. A compact desktop design with an integrated USB-DAC inherits the power and performance from the EF2A.