Alone, Amp1 is a compact inexpensive desktop headphone amplifier that sounds incredible for the money. As a PAIR, Amp1 gives you true balanced sound in a compact package with exceptional sound quality. The key to making this work is connecting a balanced source (cd/sacd/dac/other) to both amps. Using an XLR to 3.5mm cable (or XLR to 1/4“ then 1/4“ to 3.5mm adapter) connect the left output of the source to the input of one Amp1, connect the right output to the input of a second Amp1. So instead of L/R/Gnd on the input the first Amp has Left Positive signal/Left Negative signal/Gnd, the second Amp gets the Right Positive signal/Right Negative signal/Gnd. Since each amp only handles one channel, the outputs have to be tied together to power a set of headphones. Shown below is a dual 3.5mm to 4 pin XLR adapter that allows you to plug in all of your 4 Pin XLR balanced headphones.