Audio Research Reference 10. Tube preamp, balanced circuit with external power supply, frequency range: 0.1 Hz - 200 kHz (+0-3 dB), harmonics: 0.006%, gain: 12 dB (XLR), 6 dB (RCA), electronic volume control: 104 steps, interactive display, Lamp configuration: 8 x 6H30P, 2 x 6550C,2 6H30P in the supply circuit.