Aphex HeadPod 4 is a professional 4-channel headphone amplifier designed to provide musicians with proper headphone mixes at proper levels. It features three input sources: balanced analog, unbalanced stereo analog, and S/PDIF coaxial digital input. The HeadPod 4 also has individual stereo power amps for each output, ensuring that plugging in multiple headphones with different impedance values will not affect the sound quality or volume.