Church Sound: Passive & Active Direct Boxes, And How They Should Be Used
"DI" is variously claimed to stand for direct input, direct injection or direct interface -- but whatever you choose to call them, these devices are handy problem solvers

July 12, 2011, by Chris Huff

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This article is provided by Behind The Mixer.

 
A DI unit, DI box, Direct Box or simply DI is an electronic device that connects a high impedance line level signal that has an unbalanced output (a.k.a., a piece of equipment) to a low impedance mic level balanced input, usually via XLR connector.

DIs are frequently used to connect an electric guitar, electronic piano, 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 minimize noise, distortion, and ground loops. DIs do not perform impedance matching.

DI (pronounced dee EYE, not “DIE” as in “die feedback, die!”) is variously claimed to stand for direct input, direct injection or direct interface. DI units are used with professional and semi-professional PA systems and sound recording studios.

The basic component of a DI unit is a step-down transformer. Transformers consist of two or more wires that wind many times around a metal core. The ends of each coil of wire protrude from the windings; one pair of ends is the input, and the other pair is the output. The input coil is called the primary, and the output coil is called the secondary.

When an electrical signal through the primary coil, it creates a magnetic field around the coil. The field then induces an analogous signal in the secondary coil, which appears at the output leads.

If the primary has more windings than the secondary, it is called a step-down transformer because the signal level and impedance are lower at the output than they are at the input.

If the secondary has more windings than the primary, it is called a step-up transformer because the signal level and impedance are higher at the output; however, the power does not increase with respect to the input. Step-up transformers are used at the input stage of mic preamps and adapters to connect a microphone to a line-level or guitar-amp input.

Types: active DI units and passive DI units

Passive DI Units
A passive DI unit typically consists of an audio transformer used as a balun. Typical turns ratio is about 500:1, to match a nominal 50 kOhm signal source such as the magnetic pickup of an electric guitar to a 100-Ohm input.

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.

Cheaper passive DI units are more susceptible to hum. Passive units also tend to be less versatile than active. However, batteries are not required, they are simple to use, and the better units are extremely reliable.

Some models have no settings, while others can have a ground lift switch (to avoid ground loop problems) and a pad switch (to accommodate different source levels). Some passive DI units also have a filter switch for coloring the sound.

Active DI Units
An active DI unit contains a preamplifier and can provide gain and are more complex yet versatile than passive units.

Active DI units require a power source, via batteries or a standard AC outlet connection, and may contain the option for phantom power use. 

Cheaper units offering both options may perform far better on fresh batteries than on phantom power, or vice versa, so it is important to test a prospective purchase in the mode in which it will be used.

Most active DI units provide switches to enhance versatility. These include gain or level adjustment, ground lift, power source selection, and mono / stereo mode.

Ground lift switches often (perhaps unintentionally) disconnect phantom power.

A passthrough connector is a second output, sometimes simply connected 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 is essential in many applications.

Passthrough is common on active compared to passive DI units. (By the way, passthrough is 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 widely 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-inch output which needs to be connected to an XLR input.

Headphone outputs
A DI box can be used to receive a signal from any headphone jack, such as those on personal stereo systems 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. The jack cannot normally be used for headphones as well.

Suitable units:
Active
Passive resistive load

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 a snake.

Another example would be an acoustic guitar with pickups or an electric guitar or bass guitar that would be mixed through a mixing console into a main or monitor mix.

Electric keyboards
For best results use the line output(s), unless the keyboard has built-in balanced outputs (some high end units only) which are essentially built-in DI units and should give the best results of all.

If monitor amplifiers are also to be driven directly from the keyboard, the DI unit must have a passthrough connector. Alternatively, take a signal from the amplifier instead, see below.

Suitable units:
Active
Passive balun type

Electric guitar
A DI can be used to take a line in from an electric guitar. When dealing with electric guitars and electric guitar amplifiers, better results will often (not always) be obtained by instead using a microphone in front of the loudspeaker. This is because the tone of the guitar is often shaped by the amplifier and speaker used in the setup.

A DI in the chain before the speaker or amplifier will often result in a loss of fullness or pleasant tone. Using a microphone eliminates hum from ground loops which are often troublesome when using DI units with mains-powered amplifiers. But a microphone will of course pick up background noise which a DI connection will not, and will most often be more susceptible to feedback in live situations.

If an electric guitar is to be connected to a DI and an amplifier is to be connected as well, then the DI unit must have a passthrough connector. Alternatively take a signal from the amplifier, see below. For players using effects (including distortion) built in to their amplifiers, this is the only option, otherwise the contribution of these effects will be lost.

If a passthrough is used, normally the DI unit is between any effects units and the amplifier for the same reason.

Suitable units:
Active
Passive balun type

Electric bass guitar/Acoustic guitar
When dealing with electric bass or acoustic guitar, a DI is most often preferrable to using a microphone on an amplifier. This is because these instruments are often valued in a mix for being clean.

The signal path from the instrument should go into the DI unit and should then pass through to any sort of instrument amplifier. Often any amp used in this setup would be for monitoring purposes only, with the major component of the sound coming from the balanced send of the DI. The DI should be chosen with the specifications of the individual instrument in mind.

Often the best possible tone is achieved by one stage of preamplification. Following this idea, an active instrument, which means that the instrument has a preamplifier inside of it, should utilize a passive DI unit, while a passive instrument, meaning there is no preamplifier inside, should utilize an active DI.

Instrument amplifiers
Some high-end instrument amplifiers contain built-in DI units.

Most of these work as well as or better than any external unit, as they are well matched to the signal, but caution should be used with these as they often are not transformer isolated.

Better results will often (not always) be obtained by instead using a microphone in front of the loudspeaker.

This is especially true of electric guitar. Using a microphone eliminates hum from ground loops which are often troublesome when using DI units with mains-powered amplifiers.

But a microphone will of course pick up background noise which a DI connection will not, and will most often be more susceptible to feedback in live situations.


Suitable units:
Active—
—from line or slave output.
—from loudspeaker output (parallel to loudspeakers).
—from effects loop (may need passthrough).

Passive resistive load type—
—from loudspeaker output (parallel to loudspeakers, and check impedance and power handling capacity of the DI!).

Passive balun type—
—from line or slave output.

Tips and Tricks
One of the most common applications for DI boxes is to connect equipment with high-impedance outputs (such as synths) to a mixer’s low-impedance inputs using long cables. If you were to run a long cable (say, 100 feet) from a guitar to an amp, it would completely load the guitar; you’d lose high-frequency response and add noise.

However, if you connect the guitar to a nearby DI box with a short instrument cable, you can then run a 100-foot mic cable to a mic preamp near the guitar amp. The mic preamp’s output is then connected to the input of the amp.

Brands: ART, Audix, BBE, Behringer, Boss, BSS Audio, DOD, Horizon, Live Wire Solutions, Nady, Pro Co, Radial,* Rolls, Sabine, SMPro Audio, Summit Audio, Tapco, TL Audio, UltraSound, Whirlwind

Ready to learn and laugh? Chris Huff writes about the world of church audio at Behind The Mixer. He covers everything from audio fundamentals to dealing with musicians. He can even tell you the signs the sound guy is having a mental breakdown.



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Church Sound: Passive & Active Direct Boxes, And How They Should Be Used
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