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. 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. The typical console preamp input impedance of 1,500 ohms would appear to the electronic instrument as a high input impedance of 216,000 ohms. The passive direct box was suitable for most electronic musical instruments but it negatively colored the sound of ones with weaker output signals, such as Fender Rhodes pianos and Fender Precision Basses with single-coil pickups. To accommodate these instruments, active direct boxes were designed containing powered electronic circuitry which increased the input impedance from about 200,000 to above 1,000,000 ohms. In 1975, a 48-volt phantom powered active direct box was designed for Leon Russell's recording studio, its circuitry published in dB, the sound engineering magazine. The sound company Tycobrahe, known for supporting large rock festivals such as California Jam, offered an active direct box for sale in 1977, a model capable of +9 dBm line level output, with a built-in attenuator to compensate for various input levels. The direct box takes a high impedance, unbalanced signal and converts it to a low impedance, balanced signal. This allows the signal to be sent over long cable runs without signal loss, and greater rejection of interference due to the benefit of common mode rejection in a balanced signal. Furthermore, it allows the low impedance signal be sent to the input preamp of a mixing console which is designed to accept input from low impedance microphones. Because all cables used in live sound and recording are 'capacitive', long cables used without a DI box can become a 'low‑pass filter' which reduces the high end frequencies. Using a DI box provides a balanced cable output, which reduces radio frequency noise and electromagnetic problems coming from lighting systems. Most microphone cables are balanced XLR cables; the output from an electric bass or electric guitar is usually through a 1/4' unbalanced cable. Another advantage of DI units is that the DI contains a ground conduction to provide galvanic isolation that can eliminate a ground‑loop hum. A passive DI unit typically consists of an audio transformer used as a balun. The term 'passive' indicates that the unit does not require a battery or other power source to operate. This makes passive DIs less expensive, but it also means that they cannot include preamplifier features which enable the user to boost the gain. The turns ratio on a passive DI is typically chosen to convert a nominal 50 kΩ signal source (such as the magnetic pickup of an electric guitar or electric bass) to the 100–200 Ω expected by the mic input of an audio mixer. Typical turns ratios are in the range of 10:1 to 20:1. Less commonly, a passive DI unit may consist of a resistive load, with or without capacitor coupling. Such units are best suited to outputs designed for headphones or loudspeakers.