What You Need to Know About Direct Boxes

July 22, 2014
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I vaguely recall when I realized that a DI was an essential bit of bass technology. It was sometime around my first gig at a club. The sound guy, who had spent the last 45 minutes carefully miking drums, hurriedly came up to me and asked if my amp had a DI or if I needed one. I had no clue what he meant, so I just blinked and anxiously hoped he would answer his own question for me. Perhaps recognizing my adolescent ignorance, he peeked at the back of my rig, saw the xlr jack, and saved me further shame. Then I started playing a bit. I was louder than I’d ever been, each note blanketing the room in a deeply satisfying layer of bass-y goodness. I was in the house for the first time, and I was hooked, all thanks to this mysterious DI thing.

Since then, I’ve learned a thing or two, and given how indispensible DIs are to bass playing, so should you. In their most basic form, DIs perform a few vital functions. They: (1) Lower the instrument’s output impedance, (2) convert an instrument-or line-level signal to mic level, and (3) convert an unbalanced instrument output to balanced.

Let’s dive into what all this means, starting with lowering output impedance. To understand impedance, it’s necessary to understand electrical resistance. In a simple circuit consisting of a battery and a resistor, the resistor will resist the current flowing from the battery—the higher the resistor’s value, the greater its resistance to current. As a result, there is a voltage drop between one side of the resistor and the other. Since the power source in our simple circuit is a battery, its output is direct current (DC), meaning it has a steady amplitude and goes in only one direction. The audio from a bass, however, is alternating current (AC). As opposed to DC, AC varies in amplitude over time (this variation is what defines the frequency) and alternates between positive and negative cycles. Essentially, the signal alternates direction through a circuit. In an AC circuit, several other factors beyond resistance are influential, importantly inductance and capacitance. Unlike resistance, which uniformly resists the flow of current, inductive and capacitive parts of a circuit resist the flow of current, but the amount of this resistance varies according to the frequency of the AC signal’s fluctuations. To reflect the complex influence of resistors, capacitors, and inductors on a signal, we use the term “impedance.” Output impedance measures the way another electronic device “sees” the impedance at the instrument’s output. Similarly, input impedance measures the impedance to a signal as represented at a component’s input. Suffice it to say that a low-impedance signal is better suited to longer cable runs, because a high-impedance signal will lose more high frequencies to the cable. (In both cases, this loss is proportional to the cable’s length.) Also, the inputs of most mixers are designed to accept the low-impedance output of a microphone. Thus, a DI makes the high-impedance output of your bass better suited to running a long cable and makes your signal appropriate for input into a mixer. All important stuff when you play live or in a studio.

To accommodate the extraordinary diversity of instrument, microphone, and mixer types, the audio industry has long employed standards for the levels of different audio signals. In this case, the “level” of a signal refers to its AC voltage, although the unit of measure in this case is the decibel (dB), which describes the output of a signal relative to a fixed reference voltage. Microphones have the lowest level, keyboards and the outputs of preamps operate at a relatively high level referred to as “line” level, and instruments like the bass have wide-ranging output levels that fall somewhere between mic and line levels. Professional audio mixers, such as those used in a club or studio, are designed to accept mic-level inputs. Thus, another important function of a DI box is to convert the instrument-level output of a bass (or the line-level output of a keyboard) into a mic-level output appropriate for a mixing console.

As if all of the above weren’t impressive enough, DI boxes have another important trick up their sleeve. By converting the “unbalanced” output of a bass to a “balanced” output, a DI makes your signal less vulnerable to noise. Here’s why: The typical instrument cable consists of two conducting elements, a central “hot” conductor surrounded by a braided “shield.” The output of your pickups travels along the hot conductor, and the shield serves as the ground, i.e., the return path that completes the audio circuit. While the shield does offer some protection from radio frequency (RF) interference and low-frequency interference (the hum that an AC power line may induce in a signal cable), it’s not very effective. The unbalanced line is especially vulnerable when it’s longer than about 15 feet. To remedy this, a balanced cable uses two conductors and a shield that’s connected to ground, but not essential as a signal conductor. A DI box takes the unbalanced line from a bass and splits it into two out-of-phase signals, sending each down one of the two conductors. As the signal travels down the cable, it picks up the same interference that an ordinary unbalanced line would. The magic comes at the input of the mixer, where the signals are once again phase-reversed (bringing your bass back into phase) and then combined. Since any interference picked up along the cable run is identical in both conductors, this noise—once it’s phase-reversed and combined at the mixer—gets knocked out. It’s a concept similar to humbucking pickups, where noise enters the system in phase and then gets eliminated when combined out-of-phase. The standard jack type for balanced signals is XLR, although sometimes ¼" TRS cables are used.

Considering all the geekiness above, it’s hard to imagine that there’s even more going on in the average direct box. There are both passive and active types, wide-ranging approaches to design, electrical-isolation benefits, and the whole “ground-lift” thing. Space dictates we address this stuff in an upcoming installment. Nevertheless, I hope the next time you stick a xlr into the back of your amp, or gratefully accept that little black box from the sound guy, you take a moment to appreciate the marvelous little bit of engineering involved.

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