A DI unit is a device typically used in recording studios to connect a high-impedance, line level, unbalanced output signal to a low-impedance, microphone level, balanced input, usually via XLR connector. DIs are frequently used to connect an electric guitar or electric bass to a mixing console's microphone input. The DI performs level matching, balancing, and either active buffering or passive impedance matching/impedance bridging to minimize noise, distortion, and ground loops. DI boxes are extensively used with professional and semi-professional PA systems, professional sound reinforcement systems and in sound recording studios.
DI units are also referred to as a DI box, direct box, or simply DI. 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 electronic 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 with significantly less signal loss (especially in high frequencies) due to the lowering of the impedance, and greater rejection of interference due to the benefit of common mode rejection in a balanced signal. Furthermore, the lower impedance (around 600 ohms is normal) allows an insignificant load to the input of a mixing console or preamp which is also designed to accept input from low impedance microphones.
Passive DI units
A passive DI unit typically consists of an audio transformer used as a balun. The turns ratio is typically chosen to convert a nominal 50 kΩ signal source (such as the magnetic pickup of an electric guitar) to the 100–200 Ω expected by the input of an audio mixer. Typical turns ratios are in the range of 10:1 to 20:1.
The cheaper passive DI units are more susceptible to hum, and passive units tend to be less versatile than active. However, they require no batteries, are simpler to use, and the better units are extremely reliable when used as designed.
Some models have no settings, while others can have a ground lift switch (to avoid ground loop problems), a pad switch (to accommodate different source levels) and a filter switch for coloring the sound.
Active DI units
An active DI unit contains a preamplifier. Active DI units can therefore provide gain, and are inherently more complex and versatile than passive units. Active DI units require a power source, which is normally provided by batteries or a standard AC outlet connection, and may contain the option for phantom power use. Most active DI units provide switches to enhance their versatility. These may include gain or level adjustment, ground lift, power source selection, and mono or stereo mode. Ground lift switches often disconnect phantom power. Some active DI units also have electronic effects units.
A pass-through connector is a second jack, sometimes simply paralleled to the input connector, that delivers the input signal unchanged, to allow the DI unit to be inserted into a signal path without interrupting it. This allows a user, such as an electric bass player, to plug their bass into a DI unit, which routes the bass signal to the mixing board, and at the same time plug the bass into an onstage bass amp for monitoring purposes. Pass-through is also commonly referred to as a bypass. True-bypass occurs when the signal goes straight from the input jack to the output jack with no circuitry involved and no loading of the source impedance. False bypass (or simply 'bypass') occurs when the signal is routed through the device circuitry with buffering and no other intentional change to the signal. However, due to the nature of electrical designs there is almost always some slight change in the signal. The extent of change and how noticeable it may be can vary from unit to unit.
Multiple direct box circuits can be mounted inside one housing. These are used for multiple unbalanced outputs, such as for a bank of electronic keyboards.
Acoustic or electric instruments
DI's can be used on instruments with electronic circuitry and pick ups that do not contain an XLR balanced output. An example of this application would be an electric keyboard that needs to be connected to a mixer board, either directly or through an audio snake. Another example would be an acoustic guitar with pickups, an electric guitar or bass guitar that would be mixed through a mixing console into a main or monitor mix.
Some instrument amplifiers contain built-in DI units, and can be connected to a mixing console directly without needing an external direct box. This would be a typical setup for a person who wanted to run their instrument through a Public Address (PA) system while keeping the unique sound of the amplifier. Some instrument amplifiers have the ability to turn off the amplifier EQ through a pre eq/post eq switch. This can be used if a "clean" direct output from amplifier is desired.
It is common to use both a DI and a microphone on the same source. One method is to connect a guitar amplifier speaker level output to a DI and then run it to one channel of the mixing console, and run a miked guitar speaker signal into another channel of the mixing console. Another method is to connect a DI between the guitar and the amplifier. The DI signal and mic'd guitar speaker can then be selectively blended, with the DI providing a more immediate, present, bright, un-equalized sound, and the microphone providing a more 'live' sound, with instrument amplifier characteristics and some room ambience.
Examples of use
Direct-input tracking is used on almost every electric bass part on Sgt. Pepper's Lonely Hearts Club Band, most prominently on "Lucy In The Sky With Diamonds", "When I'm Sixty-Four", "Lovely Rita", and "A Day in The Life", as well as "Only A Northern Song", "I Me Mine", and the lead-guitar introducing "Revolution". Other clear examples include Dave Matthews Band's "So Much To Say" and Adele's "Daydreamer".
- Whitlock, Bill (2001). "Audio Transformers". In Glen Ballou. Handbook for Sound Engineers (3 ed.). Section 2.2.3: Line to Microphone Input or 'Direct Box'.
- "A Remote Powered Direct Box". dB. 9.
- "Advertisement". DB: The Sound Engineering Magazine. 111. 1977.
- "Equipment Reports". High Fidelity. 27: 117. 1977.
- AES E-Library: Interfacing Electronics and Transformers by Finnern, Thomas
- Electronic Musician, Nov 1, 2001. Scott Wilkinson. Going Direct
- ProSoundWeb Community. Cheepie DI's response by Andy Peters
- ProSoundWeb Community. Behringer Ultra DI-120 response by Andy Peters
- Whirlwind Direct Boxes
- Direct Boxes
- Hodgson, Jay (2010). Understanding Records, p.47. ISBN 978-1-4411-5607-5.
- Schematic for a transformer-based, signal-through DI Box - Jensen Transformer's suggested schematic for a split-signal DI Box
- Direct Boxes - 1998 article by JdB Sound Acoustics giving some technical info on what direct boxes can accomplish