Objective2 Headphone Amp (with AC adapter and NiMH batteries installed) The o2 amp is highly regarded as a Desktop Headphone amp, or a Portable Headphone Amp, (i.e. AC or DC). (Note: By popular request, the AC power connection for this version is on the back of the unit.). Designed with enough power to drive most full sized headphones, yet quiet enough to be used with IEM's, (In-Ear Monitors). Size is approximately 4 1/4" x 3 1/4" x 1 1/4", (if you include the volume knob, add about 3/4", i.e. depth 4"). Weight is about 8 oz, (11 oz with two NiMH 9v batteries installed). Assembled in the USA. Measured specs have been compared favorably to headphone amps costing multiple times the price of the Objective2. Measurements..... --- Frequency_Response...+/- 0.1 dB --- THD_1_Khz_150Ohms...0.0016% --- THD_1_Khz_15Ohms...0.0023% --- THD_20_hz_15Ohms...0.0023% --- THD_20_Khz_15Ohms...0.010% --- IMD_CCIF_15Ohms...0.001% --- IMD_SMPTE...0.002% --- Noise_(ref 400 mV)...-105dB --- Max_Output_15Ohms...337mW --- Max_Output_33Ohms...613mW --- Max_Output_150Ohms...355mW --- Output_Impedance...0.54Ohms --- Crosstalk_15 Ohms...65dB --- Channel_Balance...0.6dB --- Battery_Life...~6-8.hours