The S-phone is a 1U, four-channel headphone mixer/amplifier designed for monitor and cue mixing. It provides two master stereo inputs with a stereo link output, plus front-panel master level and per-channel LED metering. Each channel includes three headphone outputs, a stereo auxiliary input, and a two-band equalizer. Per-channel controls include mute and a stereo/2-channel mode switch, enabling flexible, listener-specific headphone mixes.