DI unit

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A professional passive DI box. It is passive because it does not need external power to operate. DI boxes which require a power source (batteries or phantom power) are called active DI boxes. DI Box 8121.jpg
A professional passive DI box. It is passive because it does not need external power to operate. DI boxes which require a power source (batteries or phantom power) are called active DI boxes.

A DI unit (direct input or direct inject) is an electronic device typically used in recording studios and in sound reinforcement systems to connect a high output impedance 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,[ citation needed ] balancing, and either active buffering or passive impedance matching/impedance bridging. DI units are typically metal boxes with input and output jacks and, for more expensive units, “ground lift” and attenuator switches.

Contents

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.

Terminology

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.

History

A vintage Wolfbox custom-made by audio engineer Ed Wolfrum in the 1960s. WolfBox2.jpg
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 help amplify electric musical instruments.

These boxes typically contained an audio transformer with a turns ratio from approximately 8:1 to 12:1 to help with impedance bridging from the high output impedance of an instrument's pickup to the relatively lower input impedance of a typical mixing console's microphone preamp. For example, the Triad A-11J used in the Wolfbox has a turns ratio of 10:1 [1] and thus an impedance ratio of 100:1. The typical console preamp input impedance of 1,500 ohms would appear to the electronic instrument as a high input impedance of 150,000 ohms. [2]

The passive direct box was suitable for most electronic musical instruments but it negatively colored the sound of instruments 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 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 the April 1975 edition of dB, the sound engineering magazine. [3] The sound company Tycobrahe, known for supporting large rock festivals such as California Jam, offered an active direct box for sale in 1977 capable of +9 dBm line level output with a built-in attenuator to compensate for various input levels. [4] [5]

Purpose

There are two related purposes for a direct box: audio and electrical. The audio purpose is to deliver a clean, unaltered instrument signal to a mixing console or recording device, where it may be processed using other devices (equalization, compression, modulation, reverberation, etc.) to meet the needs of the audio production. Without a direct box, an instrument or microphone needs to be connected to an amplifier, which might then be connected to a speaker, another microphone, and then to the mixing console. These electronic stages and cables can color the sound in ways that some audio engineers and musicians find undesirable. Of course the reverse is true—many times musicians and audio engineers seek this coloring because it fits the style of music or recording.

The electrical purpose of a direct box is to bridge the impedance of the input and output. Most electronic instruments and microphones cannot be plugged directly into the pre-amplifier inputs of mixing boards or recording devices because of impedance mismatch. Instruments are typically high impedance, whereas microphone inputs are low impedance.

The direct box takes a high impedance, unbalanced signal and converts it to a low impedance, balanced signal. [6] 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. [7] Furthermore, it allows the low impedance signal to be sent to the input preamp of a mixing console which is designed to accept input from low impedance microphones.

Because all cables are capacitive, long cables used in live sound and recording can become a low‑pass filter which reduces the high end frequencies when used with high-impedance source. [8] Another advantage of DI units is that the DI contains a transformer to provide galvanic isolation that can eliminate a ground‑loop hum. [8]

Passive units

A very simple, inexpensive passive DI Passivedi.jpg
A very simple, inexpensive passive DI
A passive DI with "throughput" in addition to XLR output Radial JDI Mk 3.jpg
A passive DI with "throughput" in addition to XLR output

A passive DI unit consists of an audio transformer used as a balun. The term "passive" indicates that the unit does not require a power source (such as batteries or phantom power) to operate. This makes passive DIs less expensive, but it also means that they cannot amplify signal power. 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. [9] [10] [11] 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.[ citation needed ]

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 power source, 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.

Active units

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

Preamplifiers

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.

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.

Typical applications

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.

Acoustic or electric instruments

DI boxes can be used on instruments with electronic circuitry and pickups 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 mixing board, either directly or through an audio snake. Another example would be an acoustic guitar with pickups, an electric guitar or bass guitar, or a double bass with piezoelectric pickups. These instruments could be plugged into a DI box, and the DI signal would be mixed through a mixing console into a main or monitor mix.

Instrument amplifiers

Some instrument amplifiers, particularly bass 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 bassist who wanted to connect their instrument through a public address system (PA system) or sound reinforcement system at a live show, while keeping the unique sound of the amplifier's preamplifier and equalizer circuitry and the custom settings. In comparison, if an external DI box is used, with the bassist plugging their bass into the DI box and sending the signal to their bass amp, the DI box signal would be the direct signal from the bass' pickups. With the external DI box approach, the sound shaping added to the bass' signal on the amplifier (e.g. boosting the preamplifier, adding overdrive, or adjusting the equalization to change the tone) would not be present in the external DI box signal. Some instrument amplifiers have the ability to turn off the amplifier's equalizer (EQ) through a pre-eq/post-eq switch. This can be used if a "clean" direct output from the amplifier is desired, which does not contain the tone shaping created by the bassist's adjustment of the EQ controls.

It is common to use both a DI signal and a microphone in front of the speaker cabinet or combo amp, in both live sound and recording settings. One method is to connect a bass guitar amplifier's speaker level output (via a pad, to attenuate the signal) to a DI and then run it to one channel of the mixing console, and run a miked guitar speaker cabinet 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 and speaker enclosure characteristics and some room ambience (natural reverb).

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". [12] Other examples include Dave Matthews Band's "So Much To Say" and Adele's "Daydreamer". [12] 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. [13] 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. [14]

See also

Related Research Articles

<span class="mw-page-title-main">Effects unit</span> Electronic device that alters audio

An effects unit or effects pedal is an electronic device that alters the sound of a musical instrument or other audio source through audio signal processing.

<span class="mw-page-title-main">Audio power amplifier</span> Audio amplifier with power output sufficient to drive a loudspeaker

An audio power amplifier is an electronic amplifier that amplifies low-power electronic audio signals, such as the signal from a radio receiver or an electric guitar pickup, to a level that is high enough for driving loudspeakers or headphones. Audio power amplifiers are found in all manner of sound systems including sound reinforcement, public address, home audio systems and musical instrument amplifiers like guitar amplifiers. It is the final electronic stage in a typical audio playback chain before the signal is sent to the loudspeakers.

<span class="mw-page-title-main">Mixing console</span> Device used for audio mixing

A mixing console or mixing desk is an electronic device for mixing audio signals, used in sound recording and reproduction and sound reinforcement systems. Inputs to the console include microphones, signals from electric or electronic instruments, or recorded sounds. Mixers may control analog or digital signals. The modified signals are summed to produce the combined output signals, which can then be broadcast, amplified through a sound reinforcement system or recorded.

Balanced audio is a method of interconnecting audio equipment using balanced interfaces. This type of connection is very important in sound recording and production because it allows the use of long cables while reducing susceptibility to external noise caused by electromagnetic interference. The balanced interface guarantees that induced noise appears as common-mode voltages at the receiver which can be rejected by a differential device.

<span class="mw-page-title-main">Instrument amplifier</span> Amplifier with loudspeaker for use with musical instruments

An instrument amplifier is an electronic device that converts the often barely audible or purely electronic signal of a musical instrument into a larger electronic signal to feed to a loudspeaker. An instrument amplifier is used with musical instruments such as an electric guitar, an electric bass, electric organ, electric piano, synthesizers and drum machine to convert the signal from the pickup or other sound source into an electronic signal that has enough power, due to being routed through a power amplifier, capable of driving one or more loudspeaker that can be heard by the performers and audience.

<span class="mw-page-title-main">Guitar amplifier</span> Electronic amplifier for musical instruments

A guitar amplifier is an electronic device or system that strengthens the electrical signal from a pickup on an electric guitar, bass guitar, or acoustic guitar so that it can produce sound through one or more loudspeakers, which are typically housed in a wooden cabinet. A guitar amplifier may be a standalone wood or metal cabinet that contains only the power amplifier circuits, requiring the use of a separate speaker cabinet–or it may be a "combo" amplifier, which contains both the amplifier and one or more speakers in a wooden cabinet. There is a wide range of sizes and power ratings for guitar amplifiers, from small, lightweight "practice amplifiers" with a single 6-inch speaker and a 10-watt amp to heavy combo amps with four 10-inch or four 12-inch speakers and a 100-watt amplifier, which are loud enough to use in a nightclub or bar performance.

<span class="mw-page-title-main">Preamplifier</span> Electronic amplifier that converts weak signal into strong signal

A preamplifier, also known as a preamp, is an electronic amplifier that converts a weak electrical signal into an output signal strong enough to be noise-tolerant and strong enough for further processing, or for sending to a power amplifier and a loudspeaker. Without this, the final signal would be noisy or distorted. They are typically used to amplify signals from analog sensors such as microphones and pickups. Because of this, the preamplifier is often placed close to the sensor to reduce the effects of noise and interference.

<span class="mw-page-title-main">Sound reinforcement system</span> Amplified sound system for public events

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Line level is the specified strength of an audio signal used to transmit analog audio between components such as CD and DVD players, television sets, audio amplifiers, and mixing consoles.

<span class="mw-page-title-main">Pickup (music technology)</span>

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<span class="mw-page-title-main">Bass amplifier</span> Electronic amplifier for musical instruments

A bass amplifier is a musical instrument electronic device that uses electrical power to make lower-pitched instruments such as the bass guitar or double bass loud enough to be heard by the performers and audience. Bass amps typically consist of a preamplifier, tone controls, a power amplifier and one or more loudspeakers ("drivers") in a cabinet.

<span class="mw-page-title-main">Distortion (music)</span> Type of electronic audio manipulation

Distortion and overdrive are forms of audio signal processing used to alter the sound of amplified electric musical instruments, usually by increasing their gain, producing a "fuzzy", "growling", or "gritty" tone. Distortion is most commonly used with the electric guitar, but may also be used with other electric instruments such as electric bass, electric piano, synthesizer and Hammond organ. Guitarists playing electric blues originally obtained an overdriven sound by turning up their vacuum tube-powered guitar amplifiers to high volumes, which caused the signal to distort. While overdriven tube amps are still used to obtain overdrive, especially in genres like blues and rockabilly, a number of other ways to produce distortion have been developed since the 1960s, such as distortion effect pedals. The growling tone of a distorted electric guitar is a key part of many genres, including blues and many rock music genres, notably hard rock, punk rock, hardcore punk, acid rock, and heavy metal music, while the use of distorted bass has been essential in a genre of hip hop music and alternative hip hop known as "SoundCloud rap".

<span class="mw-page-title-main">Guitar speaker</span>

A guitar speaker is a loudspeaker – specifically the driver (transducer) part – designed for use in a combination guitar amplifier of an electric guitar, or for use in a guitar speaker cabinet. Typically these drivers produce only the frequency range relevant to electric guitars, which is similar to a regular woofer type driver, which is approximately 75 Hz — 5 kHz, or for electric bass speakers, down to 41 Hz  for regular four-string basses or down to about 30 Hz for five-string instruments.

Re-amping is a process often used in multitrack recording in which a recorded signal is routed back out of the editing environment and run through external processing using effects units and then into a guitar amplifier and a guitar speaker cabinet or a reverb chamber. Originally, the technique was used mostly for electric guitars: it facilitates a separation of guitar playing from guitar amplifier processing—a previously recorded audio program is played back and re-recorded at a later time for the purpose of adding effects, ambiance such as reverb or echo, and the tone shaping imbued by certain amps and cabinets. The technique has since evolved over the 2000s to include many other applications. Re-amping can also be applied to other instruments and program, such as recorded drums, synthesizers, and virtual instruments.

<span class="mw-page-title-main">Bass effects</span>

Bass effects are electronic effects units that are designed for use with an electric bass and a bass amplifier, or for an upright bass and a bass amp or PA system. Bass effects are commonly available in stompbox-style pedals, which are metal or plastic boxes with a foot-operated pedal switch or button which turns the effect on and off. Most pedals also have knobs to control the tone, volume and effect level. Some bass effects are available in 19" rackmount units, which can be mounted in a road case. As well, some bass amplifiers have built-in effects, such as compression, overdrive or chorus.

<span class="mw-page-title-main">Live sound mixing</span> Blending of multiple sound sources for a live event

Live sound mixing is the blending of multiple sound sources by an audio engineer using a mixing console or software. Sounds that are mixed include those from instruments and voices which are picked up by microphones and pre-recorded material, such as songs on CD or a digital audio player. Individual sources are typically equalised to adjust the bass and treble response and routed to effect processors to ultimately be amplified and reproduced via a loudspeaker system. The live sound engineer listens and balances the various audio sources in a way that best suits the needs of the event.

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<span class="mw-page-title-main">Professional audio store</span> Retail business

A professional audio store is a retail business that sells, and in many cases rents, sound reinforcement system equipment and PA system components used in music concerts, live shows, dance parties and speaking events. This equipment typically includes microphones, power amplifiers, electronic effects units, speaker enclosures, monitor speakers, subwoofers and audio consoles (mixers). Some professional audio stores also sell sound recording equipment, DJ equipment, lighting equipment used in nightclubs and concerts and video equipment used in events, such as video projectors and screens. Some professional audio stores rent "backline" equipment used in rock and pop shows, such as stage pianos and bass amplifiers. While professional audio stores typically focus on selling new merchandise, some stores also sell used equipment, which is often the equipment that the company has previously rented out for shows and events.

<span class="mw-page-title-main">Triad Transformers</span>

Triad Transformers are currently produced by Triad Magnetics, a Southern California-based electronics company. Headquartered in Perris, California, the company manufactures and distributes power magnetics and transformers.

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