Boostaroo R234 Revolution Headphone Amplifier and Sound Enhancer (Discontinued by Manufacturer) 17.5 mW per channel Impedance: 5.5 Ohms 22-23dB boost in headphones up to 600 Ohms Signal to noise ratio -110dBA from clipping 20 to 20,000Hz +/- 2 dB (EIAJ CP-307 Method).