Whirlwind Qbox-REV2 Line Tester Inch Jack for Line-In or a 2k Ohm Telephone Earpiece Out A quieter headphone amplifier section for improved listening quality. A handy MONITOR switch that allows the user to listen to a balanced signal or to separate unbalanced signals on pins 2 or 3 (referenced to pin 1), such as a two-channel intercom. A switch that unconnects the two XLRs from each other so tone or internal mic can be sent out of one while you listen to what's coming back on the other. Or connect the XLRs together and "loop-through" or daisy-chain units. An improved, test-tone oscillator circuit that more closely maintains the selected level into a wide range of load impedances..