Tech Tip: What Do Direct Boxes Do?

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Learn what direct boxes boxes do and why you might need a DI box onstage or in the studio.

This tech tip appears courtesy of Rudy Trubitt's "Ask Rudy" column at harmonycentral.com

Two recent questions concerned direct boxes.

What is the function of a DI box?
-Steve Ng-Singapore

My acoustic guitar has a built-in pickup and on-board equalizer/preamp. I have always plugged straight into the mixer. Where does a DI box fit in--and is it necessary?
-John Bohannon-Media, PA

You'll often see a direct box (sometimes called a DI) used on electric bass, pickup-equipped acoustic guitar, and occasionally on keyboards and drum machines. They are not commonly used on electric guitar (unless you seek an amp-free tone). But what do they do? A little explanation is necessary, so hang with me a moment.

Radial Engineering ProDI Passive Direct Box
The highly rated Radial Engineering ProDI Passive Direct Box offers an affordable and sonically transparent impedance conversion solution onstage and in the studio.

A bass, acoustic guitar, or synth typically has an "unbalanced" output--either a weak "instrument-level" or somewhat hotter "line-level" signal. Unbalanced signals--especially low-level ones--are susceptible to picking up buzz, hum, or radio frequency interference. The lower the signal level of the source (i.e., your guitar), the more you'll need to boost your instrument at the mixer. Unfortunately, the noise gets boosted along with the source; the longer the cable, the worse the problem.

"Balanced" audio connections and cabling combat this problem. The output of a professional, balanced microphone can run hundreds of feet and stay clean, while an unbalanced mic could be unusable after just a 20- or 30-foot run.

FYI, a balanced cable uses three wires (hot, cold, and ground), and the connections are usually either XLR (like the typical mic cord) or 1/4" or bantam-sized Tip/Ring/Sleeve (TRS) connectors. Unbalanced connections use two wires (hot and ground), like your guitar cord or the typical RCA connector. Balanced cables pick up the same interference, but the noises get cancelled out at the mixer's input.

Behringer Ultra-G DI Box
The active circuitry in the Behringer Ultra-G DI delivers control over input levels and includes a realistic 4x12 cab emulation for direct recording of guitar and bass.

Whew! With that out of the way, we can now answer your questions. A direct box takes an unbalanced signal from a guitar, bass, or other unbalanced source and converts it into a balanced audio signal. In other words, an unbalanced guitar cord comes in, but balanced XLR (mic-style) cable comes out. To get the most from a direct box, your mixing board must have balanced mic and/or line inputs! (Note that some mixers offer both XLR and 1/4" balanced inputs -- check your manual.)

Use a direct box if you are experiencing hum, buzz, or radio interference, or if your instrument's cable is longer than 20 feet. Keep the cord from your instrument to the direct box as short as you can and make up the rest of the run with a balanced XLR cable. Some DIs offer ground-lift switches, which can break ground loops, another source of hum. Finally, many pro-oriented DIs require power, supplied either by phantom power from the mixer or an internal 9V battery.

For a much deeper look into direct injection boxes, read our expert buying guide How to Choose the Right DI Box.

Shop the extensive selection of DI boxes at Musician's Friend.

Tags: Direct Boxes

Comments  

# Dave 2016-12-18 17:13
The DI boxes at our church will not work when we plug
The 1/4 inch into the input. However, they work when we plug the acoustic guitar into the 1/4 inch output
Any explanations for this?
Reply
# alen markvchuk 2016-11-11 20:21
If i buy a Rack mountable Direct Box, does it have to be on stage or is sending the guitar cable 300ft then connecting to the DI box ok? because i know its used to work with reducing the noise. but I'm wondering if the cable length running up to the DI box makes a difference?
Reply
# salientgreen 2016-11-21 10:47
The answer is right in the article, "Keep the cord from your instrument to the direct box as short as you can and make up the rest of the run with a balanced XLR cable."
Reply
# Petar Bakic 2016-10-23 12:56
Hey, just one question:

does the instrument cable going into the DI box have to be balanced too or it can be TR unbalanded?
I understand the one from the DI box to the mixer/amp has to be balanced.
Reply
# Fabione72 2016-10-19 04:47
Often used with Electric Guitar as well, in all cases where the amp is simply too loud to be used, or more generally when it cannot be "mic" (Home recording, complex situation, such as big orchestras, etc.).
Of course the sounds taken straight by a microphone is usually much better, but sometime it is a good compromise.
Reply
# Rohit 2016-10-16 12:13
I have acoustic guitar and I want to connect it from guitar amp. What things I need .
Reply
# Andre Burton 2016-07-22 15:03
I don't know if you answered this or not, but one of the plugins says "Direct Link". What does that do?
Reply
# Bethuel Selvin Raj 2016-07-08 06:35
i cant believe the amount of spam messages left on this site. but anyways, all have been blocked and thats a relief.

by the way, i would like to know, i have two behringer DI Boxes that have no battery in them, but also does not light up when i connect it to the mixer with phantom power turned on. inserting battery does not help either. are the DI Boxes gone for good?
Reply

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