The DI Box – What Is It? Why Do I Need One?

What Is a DI Box?

Born out of necessity in studios such as Motown and United Sound Systems to accommodate the electric instruments emerging in the 1960s, direct boxes (also known as DIs, which stands for “Direct Inject”) began as a way to resolve a basic impedance mismatch between electrodynamic guitar pickups and sensitive studio electronics. Although many modern DIs are far more sophisticated than the original models, even now, the primary function of DI boxes is to take an unbalanced, high-impedance signal and convert it to a balanced, low-impedance signal. This allows you to run guitar and bass directly into microphone preamplifiers or to send signal over extended cable runs without losing volume and significant high-frequency information.

Passive DIs

Like the original models, such as Ed Wolfrum’s Wolfbox, modern passive DI boxes typically use a form of balun transformer to convert high-impedance signal to low-impedance signal. This style of transformer features electrically separate windings in the input and output stages, which isolate ground-level voltages and eliminate ground loops. The result is that signal is both impedance matched for a standard mic pre and free from ground hum originating at the input stage. Passive DIs are ideal for instruments with strong outputs, and both their low cost and durability make them the most popular kind of direct box.

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Active DIs

The biggest difference between an active DI and a passive DI is that an active DI includes a preamplifier. This type of DI was originally designed to provide an extra gain to boost the weak output of some passive, single-coil pickups, but the extra gain is great for driving long cable runs, and many modern active DIs include advanced signal routing capabilities and higher headroom than their passive counterparts, making them an excellent choice for keyboards and instruments with active pickups. The circuitry onboard active DIs requires power, which can come via batteries, dedicated power supplies, or 48V phantom power, depending on the model. Also, because they are more technically complex than passive DIs, active DIs usually cost a bit more.

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What to Look for in a DI

Direct boxes have come a long way since the ’60s, and both passive and active models feature a wide range of extra functions and options that can make them extremely versatile. Here are some of the extra features you’ll commonly find on modern DIs.

Multiple Channels

While single-channel DIs are still the most common type on the market, multichannel versions are hardly rare. There are even rackmount DI units for large stage rigs that regularly feature eight or more DI channels. Direct boxes with two channels are ideal for keyboards and other electronic instruments; whereas, special DI boxes for computers and media players can make connecting laptops and mobile devices to your PA totally painless.

Thru/Bypass

A “thru” (short for throughput) or bypass splits the original incoming instrument-level signal to a separate 1/4″ output. This allows the unprocessed signal to be sent to an amplifier onstage as well as to the PA via the balanced XLR output. This is particularly useful for bass, which allows the bass player to use an amplifier only for onstage monitoring, thereby dramatically decreasing stage volume. Bypasses can be fully passive or — in some active DIs — buffered to allow for longer cable runs or effects pedal chains.

Ground Lift

Although direct boxes can do wonders to reduce or eliminate the external noise that plagues unbalanced instrument-level signals, even balanced audio equipment can be susceptible to hum and buzz caused by ground loops. A switchable ground lift lets you disconnect Pin 1 on the XLR jack of the DI box, preventing current from flowing between the DI and the microphone preamplifier along the shielding, thereby breaking the ground loop and eliminating this noise.

Pad

Some DI boxes feature a switchable attenuator called a pad to prevent excessive gain from overloading the circuitry. This circuit decreases incoming signal by a fixed amount (-15dB and -20dB pads are common) to accommodate the high output of active pickups and unbalanced line-level equipment such as keyboards and other electronic instruments.

Polarity Reverse

Sometimes labeled as a “phase” switch, a polarity reverse changes from a standard Pin 2 hot to Pin 3 hot XLR configuration. This function can be useful in several ways. In addition to correcting for wrongly wired XLR cables, a polarity reverse switch can align the absolute polarity of a direct signal with that of a microphone on the same source, a bass and acoustic guitar recording technique often used in studios. Polarity reverse can also help prevent feedback, and it’s a handy feature to have in case the mixer channel isn’t equipped with a polarity reverse.

The Great Direct Box Comparison with Audio SamplesDo different DIs sound different? You may be surprised at how much difference it can make. I know we were. Listen to audio samples in our Great Direct Box Comparison.

Related Tools

As direct boxes have evolved and many similar tools have entered the market, a number of devices similar to direct boxes have become available. Here are a few related tools you may want to check out.

In-line Preamps

There are a wide range of small personal preamps on the market, many of which offer tone-shaping functions specific to an intended instrument or input source. When it comes to instrument-specific in-line preamps, such as bass or acoustic-guitar preamps, you can expect to find an EQ, and compression is common too. There are also in-line microphone booster preamps that are fantastic for live use, such as Cloud Microphones’ Cloudlifter series. One of the major advantages to an in-line preamp is that the signal leaves the stage much stronger than what you’ll get from even an active DI, making it far less susceptible to external noise and high-frequency loss.

Isolation Transformers

An isolation transformer is a lot like a DI box, except that it doesn’t convert from one impedance to another. These devices are great for removing ground-loop hum and isolating sound systems from one another.

Re-amping Boxes

Re-amping boxes serve the opposite function as DI boxes, allowing you to send a line-level signal back to an amplifier with the correct impedance for its input. These are useful for complex live guitar rigs that involve line-level processing. You can send the instrument signal through a DI box to a preamp and a rack of effects and processing gear, then return the output through a re-amping box to the amplifier onstage.
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