LINE5 Multi-channel Headphone Audio Signal Switcher Stereo Headphone Amplifier Black As the 4 input 4 output headphone audio signal switcher Connect the headphones headset / signal decoder players and other audio equipment to the IN1 to IN4 interface products back A985 audio signal switcher, mainly used for switching various multi-channel headphone audio signal, which can be used for the same earphone or earplugs to compare different player / headphones, or a headset is connected with the signal, connecting a plurality of earphone / earplugs, earplugs headphones / quality comparison Product dimensions: 9.5*6.5*2.8cm(L*W*H) Note: Most of time, our product works great. But if you found is has a hum, the solution is bought a 3.5mm Ground Loop Isolator ,it will help you solve it,