DI unit

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.

A vintage Wolfbox custom-made by audio engineer Ed Wolfrum in the 1960s.

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.[1]

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.[2] 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.[3][4]

The direct box takes a high impedance, unbalanced signal and converts it to a low impedance, balanced signal.[5] 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.[6] 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.[7] Using a DI box provides a balanced cable output, which reduces radio frequency noise and electromagnetic problems coming from lighting systems.[7] Most microphone cables are balanced XLR cables; the output from an electric bass or electric guitar is usually through a 1/4" unbalanced cable.[7] Another advantage of DI units is that the DI contains a ground conduction to provide galvanic isolation that can eliminate a ground‑loop hum.[5][7]

A very simple, inexpensive passive DI

A passive DI with "throughput" in addition to XLR output

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.[8][9][10] 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 less expensive 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. The lack of batteries in passive DIs means that users do not have to worry about batteries losing their power in the middle of a live show or recording session.

Some models have no settings or switches, while others can have a ground lift switch (to avoid ground loop problems or hums), a pad switch (to accommodate different source levels and attenuate too-strong signals) and a passive filter/equalization switch for coloring the sound or tone.

An active stereo DI with pass-through in mono mode only

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.

A number of companies make combination preamplifier-DI units for electric bass, double bass, or for acoustic instruments which use piezoelectric pickups (e.g., a violin, acoustic guitar, mandolin, etc.). These units may be housed in a "stompbox" pedal format, in a small rackmount unit (often less than a full rack space), or in units designed to be clipped to a belt or attached to an instrument. These devices are often marketed as preamplifiers, even though they also contain DI box features.

Preamplifiers for electric bass typically contain gain knobs, sometimes including an overdrive effect unit, equalizer knobs and, for some higher-end units, multiple channels (e.g., a "clean" channel and a "dirty" channel, with the latter containing an overdrive effect).

Preamp-DI units for double bass and other acoustic instruments often omit the overdrive features, but add additional features that help to produce a good sound and tone for acoustic instruments, such as an audio compression effect, a phase inverter switch and a notch filter (the latter two features designed to help reduce unwanted audio feedback "howls"). Preamp-DIs for acoustic instruments often include two channels and a simple mixer, to enable the player to use both a pickup and a condensor microphone. Some preamp-DIs provide phantom power, in case this is required to power a condenser microphone. Preamp-DI units may be battery powered, have an AC mains plug, or both.

Direct boxes are typically used with electric instruments or other electronic musical devices that only contain an unbalanced 1/4" phone output which needs to be connected to an XLR input of a mixing board. 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.

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".[11] Other examples include Dave Matthews Band's "So Much To Say" and Adele's "Daydreamer".[11] The main guitar riff on Led Zeppelin's "Black Dog" was recorded directly through a mic-preamp and fed through a pair of 1176 Peak Limiters.[12] Byrds frontman and guitarist Roger Mcguinn achieved his famous jangly guitar sound in the studio by recording his 12-string Rickenbacker direct-in through a compressor to emphasize the treble.[13]

• Re-amp

• 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