Guide to DI Boxes: How to Use Direct Input Boxes

A DI box is a device that allows musicians to connect their instruments to an audio mixing board without adding undesired noise in the process. Most electronic musical instruments send out unbalanced high-impedance audio (i.e., the electric current faces high resistance, measured in Ohms), but audio signals travel better through wires that are balanced with low impedance (i.e., low resistance, as is the case with microphones using XLR cables). To prevent an audio signal from becoming too noisy on its way to the mixing board, musicians and FOH engineers need a way to convert the high-impedance signal from an instrument cable into a low-impedance signal that can run to a mixer's microphone input via XLR. This is where a DI box comes in.

A DI box converts a high-impedance input signal into a low-impedance signal that can be sent to a mixing console. This has four noteworthy effects:

• It converts line-level instrument signals from a 1/4" instrument cable to mic-level audio that can run through a balanced XLR cable.
• It removes the undesirable 60-cycle hum produced by a ground loop.
• It enables long cable runs without adding needless noise to the PA system.
• It allows line-level signals to reach amplifiers thanks to a "thru" output that preserves the unbalanced signal.

An active DI box is a direct box that requires a power source. Some of these boxes use nine-volt batteries, some use 48-V phantom power carried via an XLR cable, and some have a dedicated AC power supply.

A passive DI box is a direct box that does not require a power source. It functions as a transformer, converting line-level inputs into low-impedance outputs via electromagnetic induction.

There are two types of direct boxes on the market: passive direct boxes and active direct boxes. Active DI boxes need a power source, whereas passive DI boxes do not require power. Although they perform the same function—converting unbalanced high-impedance signals to a balanced XLR output—they operate differently and pair with different kinds of instruments.

1. 1. Passive DI boxes increase saturation. A passive DI box is a transformer, which allows it to create a saturated sound under the power of high-output audio. If you're using a direct input with a powerful signal—such as a bass with active pickups or a powerful electronic keyboard—a passive DI box can take that high-input impedance and turn it into a very pleasant, slightly saturated output signal.
2. 2. Passive DI boxes have ground lift. Most well-made passive DI boxes also feature a ground-lift switch, which provides an additional option for removing ground loops, particularly if the instrument you are plugging in is providing its own ground path for an electrical signal. (Electric keyboards often create a ground path, which makes a DI box's ground-lift switch very useful.)
3. 3. Active DI boxes serve as preamplifiers. Because active DI boxes provide a direct injection of electricity into your audio signal, they effectively function like a preamp, boosting high-frequency signals. For this reason, active DI boxes are popular in the studio, and some companies more associated with mixing boards make single-channel and stereo active DI boxes that pair beautifully with a studio console.
4. 4. Active DI boxes pair best with passive instruments. When it comes to live sound, active DI boxes tend to pair best with passive instruments. The lower the instrument's output, the more useful the active DI box will be. Electric basses with passive pickups benefit from active DI boxes, as do acoustic guitars that don't have a battery-powered output. You can also use an active DI box for a guitar with passive pickups, but most guitarists want the sound of their guitar amp in the house mix, so they usually opt for a mic right on the speaker grille.
5. 5. Passive DI boxes pair well with active instruments. As a general rule of thumb, passive direct boxes are a good choice when your input instrument produces a high-intensity signal. Electronic keyboards would qualify, as would acoustic guitars with battery-powered pickups. Many modern bass guitars use active pickups, and these pair well with passive DI boxes. Some electric guitars also have active pickup systems; however, most guitarists derive a lot of their tone from the guitar amp itself, so they typically prefer to put a microphone in front of the guitar amp's speaker, rather than run an unamplified guitar signal into the mixing board.

With many DI boxes on the market, there are two main considerations to take into account when choosing one:

1. 1. Figure out what kind of inputs you will be using. If you're working with a bass or a guitar, you'll only need a single-channel DI. If you're working with a keyboard, you'll benefit from a stereo DI, since most keyboards provide stereo output. If you're running sound off a phone, tablet, or computer, you may want a DI box that has an 1/8" input. You can remove a lot of noise and increase frequency response from these devices when you send them through a DI box and out into a balanced output.
2. 2. Choose a passive or active DI box. This choice comes down to the kind of instruments you'll be amplifying. Passive audio sources like vintage pickups and acoustic instruments go well with active DI boxes, and active audio sources like electric keyboards and powered guitar pickups go well with passive DI boxes. Active direct boxes also sound great as preamps for tracking or reamping in the recording studio.

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