DI unit  A DI unit (direct input) is an electronic device typically used in recording studios and in sound reinforcement systems to connect a high-output impedance, line level, unbalanced output signal to a low-impedance, microphone level, balanced input, usually via an XLR connector and XLR cable. DIs are frequently used to connect an electric guitar or electric bass to a mixing console's microphone input jack. The DI performs level matching, balancing, and either active buffering or passive impedance matching/impedance bridging to minimize unwanted noise, distortion, and ground loops. DI units are typically metal boxes with input and output jacks and, for more expensive units, “ground lift” and attenuator switches. DI boxes are extensively used with professional and semi-professional PA systems, professional sound reinforcement systems and in sound recording studios. Manufacturers produce a wide range of units, from inexpensive, basic, passive units to expensive, sophisticated, active units. DI boxes may provide numerous features and user-controllable options (e.g., a user-selectable 0dB, 20dB or 40dB pad and/or a "ground lift" switch). They may come in different types of enclosures, usually a metal chassis that helps to protect against electrical interference. Some bass amplifiers have built-in DI units, so that the bass amp's output signal can be connected directly to a mixing board in a sound reinforcement/live show or recording context. DI units are also referred to as a DI box, direct box, or simply DI, with each letter pronounced, as in "Dee Eye." The term is variously claimed to stand for "direct input", "direct injection", "direct induction" or "direct interface". Passive direct boxes first appeared in the United States in the middle 1960s, most notably in Detroit at radio stations and recording studios like "Motown", "United Sound Systems", "Golden World Records", Tera Shirma Studios and the Metro-Audio Capstan Roller remote recording truck. These DIs were custom made by engineers like Ed Wolfrum with his "Wolfbox" and by concert sound companies to solve certain problems associated with amplifying electric musical instruments, especially electric guitars. These boxes typically contained an audio transformer (like the Triad A11J through 1974) with a turns ratio from approximately 8:1 to 12:1, and thus an impedance ratio of around 144:1. With this kind of transformer, the output voltage of the instrument is stepped down to a range compatible with the typical mixing console's microphone preamp.