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If you need to send a different mix to the headphones , then yes you'll need either an extra converter as Daniel said, or only have 14 inputs to the 2bus and use the other two for cue mix. If you don't need and individual cue mix you can just send the control room mix to the headphones up the monitor section of the D Command by setting the 'Phones' source in the studio section to 'Mn' using the 'set source' button under the level pot, which will cycle though the Main monitor feed and Cue 1 and 2.