What does a direct box (or “DI”) actually do? What are the differences between passive and active DI’s? Which one is the best for my source? It’s well understood that direct boxes are an essential part of any stage package. However, as technicians, we probably put less thought into them than any other part of our signal chain. For a lot of us, the DI might just be a “magic box” that we know we’re supposed to use, but aren’t quite sure why. In this article, we’ll explore the basics of direct boxes, and give you some simple guidelines for using them.
What Does a DI Do?
Put simply, a DI is a device that takes a signal from a source, and adjusts it to the parameters your mixer or stagebox’s preamp will be happy with; namely a balanced, mic level, low impedance signal. To illustrate, let’s look at one of the most common sources that we use a DI with: the electric bass guitar.
The signal output from an electric bass is high impedance and unbalanced. The high output impedance means the signal will degrade quickly over a long cable run. Likewise, unbalanced cables are extremely susceptible to RF interference. For this reason, unbalanced cable runs should be kept as short as possible. A direct box will take the incoming signal, and convert it to a low impedance, balanced signal. The result? You can run a long XLR cable to your stagebox or console, without worrying about signal degradation or interference.
For another example, let’s consider a line level source, maybe an electric keyboard, CD player or other playback device. While there are a couple different standards for line level, both operate at an exponentially higher voltage than mic level, which will overload your mic preamp. A direct box steps down the voltage from a line level source, and again, outputs a balanced signal, which your preamp will accept.
Most DI’s have an input pad to attenuate particularly hot incoming signals, a ground lift to deal with any pesky hums, and a throughput (typically labeled “THRU”) to feed an instrument’s unaltered signal to the musician’s amplifier. Your DI might also have a polarity switch; if you’re running an instrument through a DI, and also have a mic on that instrument’s amp, it can be worthwhile to flip that polarity switch and listen to determine which setting sounds better.
Passive VS Active
Broadly speaking, there are two categories of direct box that achieve the described results in two different ways: passive direct boxes, and active direct boxes. Passive DI’s employ a transformer for impedance matching and balancing, while active DI’s use a preamplifier circuit. Active DI’s require a source of power – almost always 48V phantom power from the mixer. So, which should you use for any given source?
As a general rule of thumb, active direct boxes should not be used with devices that have “active” outputs – line level sources, like the keyboard described above. There’s no need to boost the strong line level input, so using an active DI would amount to introducing an unnecessary gain stage, and raising the noise floor in our signal chain.
Conversely, active DI’s can be hugely beneficial when dealing with particularly low signal levels from “passive” sources, like the electric bass we talked about earlier. While it will probably sound fine through any modern passive DI, the active DI’s preamplifier circuit can give the bass a little bit of a lift.
Conclusion
There are countless direct boxes on the market today. Maybe you need to plug in a phone or laptop for playback. Perhaps you’re looking for a DI to color (or even “colour”) your instrument’s sound to evoke vintage gear. Or, maybe you need a streamlined way to plug in multiple keyboards with stereo outputs. Our extensive rental inventory includes direct box options for all of the above, plus a lot more. Give our Rental Team a call at 847-367-9588, or email them at rentals@tcfurlong.com to make sure you have the right selection of DI’s on your next gig.
We’re also authorized dealers for all major professional audio manufacturers. For a purchase quote on a DI, or any other gear you may need, reach out to our Sales Team at sales@tcfurlong.com or call 847-367-9588.
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