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Active vs Passive Direct Boxes?

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by: MLS_Admin
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Date: Sat, 5 Jan 2013 Time: 12:16 AM

by Quinton Jones

Why do you need a direct box if you can plug a guitar or keyboard directly into a mixer with no problems? That's a great question! It's easy to ignore DI's, but they play a vital role in the recording process, both in the studio and especially in live recording. There are several different types of direct boxes, but they all serve two main purposes: they function as impedance-matching devices, which change line level signals into a microphone level signals and they provide ground-lift capabilities to avoid that nasty ground-loop hum caused by faulty wiring.

More importantly, a direct box allows you to get a fatter cleaner sound than plugging directly into a mixing board. This is because a balanced signal can be run for long distances without picking up EMI (electro-magnet interference) or RFI (radio frequency interference). If you've ever heard a radio station suddenly coming out of your speakers, then you know what I'm talking about.

A DI will generally have a ¼" input, an XLR output and a ¼" thru jack that can be used to route the input signal to another sound source. This configuration allows a balanced microphone signal to be sent to the mixing console via the XLR output and, at the same time, allows you to plug your instrument into an on stage amplifier via the thru jack. DI's are also equipped with a ground lift switch, which will allow you to safely lift the ground without the fear of shocking yourself. This method lifts the audio signal's ground, but not the electrical ground which we do not recommend.

DI's come in two types - passive and active. The most obvious difference between them is, one needs power and the other does not. A passive DI is ready to use straight out of the box and needs no external power. They are very easy to set up - just plug in and play. However a DI with active electronics needs some sort of external power to run the internal circuitry. They can be run from standard 48 volt phantom power directly from the mixing board or, if phantom power is not available, they can be operated via a 9 volt internal battery. Active DI's can accept many different types of input signals, which make them much more versatile than their passive counterparts, but they are also more expensive.

Applications:

  • Keyboard - direct boxes provide the cleanest sound possible
  • Electric Guitar - direct box signals need to be run through an amp simulator, like a Line 6 POD, for some extra processing to fatten them up
  • Acoustic Guitar - a direct box provides a clean, but sterile sound. The DI signal can be blended with a microphone signal to add warmth/depth
  • Bass Guitar - a direct box provides the cleanest sound. The DI signal is usually blended with a microphone signal to add a beefier bottom end

I Recommend:

Radial and Countryman direct boxes for their ruggedness and quality.

About the Author

Quinton Jones is a Mixing/Mastering Engineer with over thirty years experience in the music industry. He is currently the Director of Operations at MusicPro Magazine. If you found this article helpful, then take a look at more articles and tutorials at: MusicPro Magazine


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