DI unit
Encyclopedia
A DI unit, DI box, Direct Box, or simply DI (variously claimed to stand for direct input, direct injection or direct interface), 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 minimise noise, distortion, and ground loops
.
DI boxes are extensively used with professional and semi-professional PA systems and in sound recording studios.
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.
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.
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 loudspeaker
s.
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.
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 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.
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.
s 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.
Electrical impedance
Electrical impedance, or simply impedance, is the measure of the opposition that an electrical circuit presents to the passage of a current when a voltage is applied. In quantitative terms, it is the complex ratio of the voltage to the current in an alternating current circuit...
, line level
Line level
Line level is a term used to denote the strength of an audio signal used to transmit analog sound between audio components such as CD and DVD players, TVs, audio amplifiers, and mixing consoles, and sometimes MP3 players....
, unbalanced output signal to a low-impedance microphone level balanced
Balanced audio
Balanced audio is a method of interconnecting audio equipment using impedance-balanced lines. This type of connection is very important in sound recording and production because it allows for the use of long cables while reducing susceptibility to external noise.Balanced connections use...
input, usually via XLR connector
XLR connector
The XLR connector is a style of electrical connector, primarily found on professional audio, video, and stage lighting equipment. The connectors are circular in design and have between 3 and 7 pins...
. DIs are frequently used to connect an electric guitar
Electric guitar
An electric guitar is a guitar that uses the principle of direct electromagnetic induction to convert vibrations of its metal strings into electric audio signals. The signal generated by an electric guitar is too weak to drive a loudspeaker, so it is amplified before sending it to a loudspeaker...
or electric bass
Bass guitar
The bass guitar is a stringed instrument played primarily with the fingers or thumb , or by using a pick....
to a mixing console
Mixing console
In professional audio, a mixing console, or audio mixer, also called a sound board, mixing desk, or mixer is an electronic device for combining , routing, and changing the level, timbre and/or dynamics of audio signals. A mixer can mix analog or digital signals, depending on the type of mixer...
's microphone input. The DI performs level matching, balancing, and either active buffering
Buffer amplifier
A buffer amplifier is one that provides electrical impedance transformation from one circuit to another...
or passive impedance matching
Impedance matching
In electronics, impedance matching is the practice of designing the input impedance of an electrical load to maximize the power transfer and/or minimize reflections from the load....
/impedance bridging
Impedance bridging
In electronics, especially audio and sound recording, a high impedance bridging, voltage bridging, or simply bridging connection is one which maximizes transfer of a voltage signal to the load...
to minimise noise, distortion, and ground loops
Ground loop (electricity)
In an electrical system, a ground loop usually refers to a current, almost always unwanted, in a conductor connecting two points that are supposed to be at the same potential, often ground, but are actually at different potentials. Ground loops created by improperly designed or improperly installed...
.
DI boxes are extensively used with professional and semi-professional PA systems and in sound recording studios.
Why a DI Is Necessary
The Direct Injection 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
Transformer
A transformer is a device that transfers electrical energy from one circuit to another through inductively coupled conductors—the transformer's coils. A varying current in the first or primary winding creates a varying magnetic flux in the transformer's core and thus a varying magnetic field...
used as a balun
Balun
A balun is a type of electrical transformer that can convert electrical signals that are balanced about ground to signals that are unbalanced , and the reverse. They are also often used to connect lines of differing impedance...
. The turns ratio is typically chosen to convert a nominal
Nominal impedance
Nominal impedance in electrical engineering and audio engineering refers to the approximate designed impedance of an electrical circuit or device...
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.
Less commonly, a passive DI unit may consist of a resistive
Resistor
A linear resistor is a linear, passive two-terminal electrical component that implements electrical resistance as a circuit element.The current through a resistor is in direct proportion to the voltage across the resistor's terminals. Thus, the ratio of the voltage applied across a resistor's...
load, with or without capacitor
Capacitor
A capacitor is a passive two-terminal electrical component used to store energy in an electric field. The forms of practical capacitors vary widely, but all contain at least two electrical conductors separated by a dielectric ; for example, one common construction consists of metal foils separated...
coupling. Such units are best suited to outputs designed for headphones or loudspeaker
Loudspeaker
A loudspeaker is an electroacoustic transducer that produces sound in response to an electrical audio signal input. Non-electrical loudspeakers were developed as accessories to telephone systems, but electronic amplification by vacuum tube made loudspeakers more generally useful...
s.
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
Ground lift
In sound recording and reproduction, ground lift or earth lift is a technique used to reduce or eliminate ground-related noise when connecting signal lines between two or more pieces of equipment. It interrupts the ground line at some point. It is particularly effective at eliminating ground loops,...
switch (to avoid ground loop
Ground loop (electricity)
In an electrical system, a ground loop usually refers to a current, almost always unwanted, in a conductor connecting two points that are supposed to be at the same potential, often ground, but are actually at different potentials. Ground loops created by improperly designed or improperly installed...
problems), a pad switch (to accommodate different source levels) and a filter switch for coloring the sound.
Active DI units
Preamplifier
A preamplifier is an electronic amplifier that prepares a small electrical signal for further amplification or processing. A preamplifier is often placed close to the sensor to reduce the effects of noise and interference. It is used to boost the signal strength to drive the cable to the main...
. 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
Phantom power
Phantom power, in the context of professional audio equipment, is a method for transmitting DC electric power through microphone cables to operate microphones that contain active electronic circuitry....
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 buffering
Buffer amplifier
A buffer amplifier is one that provides electrical impedance transformation from one circuit to another...
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.
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 XLRXLR connector
The XLR connector is a style of electrical connector, primarily found on professional audio, video, and stage lighting equipment. The connectors are circular in design and have between 3 and 7 pins...
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
DIs 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 amplifierInstrument amplifier
An instrument amplifier is an electronic amplifier that converts the often barely audible or purely electronic signal from musical instruments such as an electric guitar, an electric bass, or an electric keyboard into an electronic signal capable of driving a loudspeaker that can be heard by the...
s 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.
Vibrational recorder
A high-voltage turntable cartridge can be used with a direct box to directly record the vibrations from the cartridge's needle to a suitable recording device. This is an inexpensive way to create 'field' recordings of vibrations, such as the substrate-borne vibrational communication of insects. This can be accomplished with a cartridge hooked up to a tripod for stability and a DI box for amplification, which can then be hooked up to a recording device or headphones. By setting the needle on leaves or small branches or grass blades, any insect motion or vibrational communication can be heard through the amplified transduced signal (although this may require trying multiple different blades of grass, branches, etc.).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.External links
- 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