A Direct Box, often abbreviated as DI, is an essential device for connecting high-impedance instruments, such as guitars, basses, and keyboards, directly to the microphone inputs of a mixing console or audio interface. The main function of this equipment is to convert the unbalanced, high-impedance signal from the instrument into a balanced, low-impedance, microphone-level signal. This conversion is crucial because it allows the signal to travel long distances through XLR cables without suffering frequency degradation or picking up external noise.

Most Direct Boxes have a feature called Ground Lift, a switch that interrupts the ground connection between the input and output to eliminate hum caused by ground loops. There are two main categories: passive models, which use internal transformers and do not require power, being very robust and ideal for instruments with strong or active output; and active models, which use electronic circuits and require power via battery or Phantom Power.

Active pickups typically have a much higher input impedance, which helps preserve the brightness and detail of instruments with passive pickups and weak signals.