A D.I. or direct inject unit is a device that connects a high-impedance, line level signal to a mic-level device. Usually a box, or less commonly a multichannel racked or unracked unit, the box contains buffering circuitry that allows you to connect line-level equipment, or commonly, acoustic guitars pickup outputs, to consoles and studio gear operating at low-impedance mic level. Common uses are simply to interface pieces of gear - DI's also impart tonal quality to the sound and so choices are important. Detailed explanation with reference to content from Wikipedia, the free encyclopedia Detailed explanation A DI unit, DI box, Direct Box, Direct Input, Direct Injection or simply DI is an electronic device that connects a high impedance, line level, unbalanced output signal to a low impedance mic 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 bridging to minimise noise, distortion, and ground loops. DIs do not perform impedance matching. DI (pronounced dee EYE) is variously claimed to stand for direct input, direct injection or direct interface. DI's are extensively used with professional and semi-professional PA systems and in sound recording studios. Passive DI units A passive DI unit typically consists of an audio transformer used as a balun. The turns ratio is typically chosen to match a nominal 50 kΩ signal source (such as the magnetic pickup of an electric guitar) to a 100-200 Ω 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. 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 simple 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. 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. 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 no 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. Typical applications Direct boxes are typically used in instances of instruments or other devices that only contain an unbalanced 1/4" output which needs to be connected to an XLR input. 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. Instrument amplifiers 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 though 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 miked 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. Headphone outputs A DI box can be used to receive a signal from a headphone jack, such as those on personal stereo systems, laptops, MP3 players, samplers or keyboards. If the signal is to be connected to a single input then a mixing facility is required in the DI unit. If stereo is required, then either two DI units or a single stereo unit can be used. Used in this way, a DI box can also be used to lengthen cable runs from say a guitar amplifier or a keyboard to a mixing desk, by placing the DI box near the source, and running low impedance balanced cable(s) to the desk. Valve (tube) Amplifiers Valve (tube) amplifiers need to 'see' a loudspeaker connected to their outputs to prevent damage, and should not be used with DI boxes unless a pass-through link to a loudspeaker is in place, or the DI box is designed for use with these amplifiers. Some Power Soak devices feature a DI or line out (such as the THD Hot Plate); obviously this is showing the post-amplifier signal and so will feature distortion (for distorted sounds) that has not gone through any speaker cabinet filtration - although the device may offer its own filtration circuit. This may well be useful later to go through speakers for reamping, and some engineers even swear that this 'tizzy' distorted signal is useful in the mix itself. Generally you would take your DI'd signal from the effects chain before distortion, and add distortion and amplifier chain later. Re-amping To re-amp or resend a recorded line level signal back through studio preamplifiers for tonal adjustment, the signal needs to be attenuated by 40-50dB commonly. A DI box can be used to do this although its not necessary for compressors, EQs or other studio equipment that accepts line levels.