Passive and Active Direct Boxes : How They Should Be Used

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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.ka. 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 of DI’s; 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 kΩ signal source such as the magnetic pickup of an electric guitar to a 100 Ω 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. Active DI units 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. 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" 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

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Comments

  1. Stephen Iddison says

    Dear Chris, I am running a Vestax VCI-300mk2 DJ Controller and wish to use Active speakers. On my unit are two unbalanced 1/4 jack outputs whilst on my speaker I have the option of XLR/1/4 JACK input. I need to run 6 meter cable and as a result may suffer some humming. Would a good DI box solve the problem here and if so what would you recommend that is not to expensive please.

  2. Luis Mierles says

    Very usefull with technical language but easy to understand really a great help to understand the Di issue…

  3. nimmtheory says

    I have tried simply turning the volume down on my active bass guitar and the input signal is not distorted by the designed for passive preamp. The problem is that I can still play a passive system bass wide open with higher amplifier output volumes. The active bass at lowered input volume will sound cleaner, but the output volume at the speaker is lower. I set the active bass at several settings and a passive instrument will still out perform it’s modern active brother through this tube amplifier, even when using lower gain preamp tubes. I must assume the the design of the amp prevents effective usage of active instruments. I do like the idea of the line signal reducer which will help keep vintage gear from being damaged accidentally.

  4. says

    Can a passive input of an amplifier be converted to an active input?
    I was wondering if there is some sort of box that I could run my active guitar/bass through so it will work affectively in a amplifier that only supports passive system instruments.There are alot of amplifiers out there that should not be used with a active instrument since it overpowers the preamp.

  5. Cajundaddy says

    It’s all about matching gain. Some amplifiers are more sensitive than others but most guitar amps are set up to accept input from distortion pedals, DSPs and boost pedals which can be pretty hot active signals as well. Compare your active guitar gain levels with a common boost pedal and if they are close you should be good to go.

  6. nimmtheory says

    “If your signal is too hot for the amp input, turn down at the guitar until you are no longer over driving your amp. Easy breezy.” COMMENT: CAJUNDADDY
    You must have fallen asleep, The amplifier is best used with a passive instrument. Active systems are just not that good. My other amp has a switch to dummy down the preamp, WOW.

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