A D.I. Box is a box that converts a line level signal into a microphone level signal. D.I. Boxes come in two flavours. Passive D.I. Boxers do not need power and are cheap. Active D.I. Boxes need power - either from a battery or from phantom power - and are more expensive to purchase. D.I. Boxes can sometimes take two inputs and have two outputs so they can be used for stereo sound sources such as synthesisers. Active D.I. Boxes might also have a link output which is a line level output to connect a monitor or amplifier.