DI Box : Do I need one? What does it actually do?

Maybe you're a musician doing live and studio gigs; maybe a live sound engineer or even someone planning on putting together your own home recording studio.  Chances are, since you ended up on this article, you're someone getting started in the world of audio production. If any of the above is true and you're now wondering if you could use a DI box (a.k.a. direct box or direct injection) in your arsenal, then the answer is "you probably could". Shall we?

Why do I need one?

In a nutshell: whenever you're connecting an equipment's output to an input NOT specifically designed for such signal, you should be doing so through a DI box. Let's recap the most correct connections:

• Mic-level signals should be connected to Mic INs
• Line-level signals should go to Line INs
• Instrument-level signals should go to Instrument INs

Pre-amplifiers are usually designed to handle two types of signals: microphone and line level - both are quite different from the signal put out by an electric guitar or bass for example. To connect something which doesn't have a mic or line level output, we can use a DI Box in the middle of the way to interface that connection. Several entry level and even some professional audio interfaces have DI inputs built into them, usually called "Instrument Inputs", designed to handle instrument level signals. Even modern outboard pre-amplifiers will sometimes have DI inputs. In this case, you wouldn't need a DI, since the ingoing signal is matching the expected input level. So, if you're going straight to the mixer or audio interface (instead of plugging your Guitar/Bass into an amplifier), and your equipment has no Instrument Input , go through a DI box. Here are examples of where we can find each signal level:

• Mic-Level: The direct signal from the mic (or the output of a DI).
• Line-Level: Almost any audio equipment outputs (except for power amplifiers).
• Instrument-Level: Output of instruments with coil pickups (guitars, basses, or the famous Fender Rhodes electric piano).

The Signal

Every signal can be described in a few electrical aspects. One of them is Amplitude (voltage), which is directly related to the volume of the signal. Amplitude-wise, the instrument level falls in between the mic and line; but there's something else to be taken in consideration, which is Impedance. Impedance is basically a form of resistance to the signal flow, and not having the correct relationship between output and input impedances will cause signal loss. Impedance-wise, instrument levels have far more impedance (therefore named "Hi-Z", Z being the symbol for impedance) than line or mic level ("Lo-Z") signals. Let's see what happens when you interface different signals and inputs:

Amplitude-wise issues:

• Instrument level signal into a Mic IN: Probably clip and distort the signal.
• Instrument level signal into a Line IN: You might be too low and not have a good signal-to-noise relationship, since you'll have to turn your pre-amp gain way up.

Impedance-wise issues:

• Instrument signal into a Mic IN : since it expects a lower impedance signal, causes signal loss.
• Instrument signal into a Line IN: since it expects a lower impedance signal, causes signal loss.

What does a DI box really do?

Now that we've seen the basic reasons why we should be using a DI-box when plugging an electric instrument, let's take a look into what a DI does and other extra benefits it's use will offer. As per what we've seen so far, there's a couple things you already know a DI will do. Actually, in it's most basic form, there's not much more to it. Here are the three basic features of any DI:

• Level matching - bringing your instrument level down to a mic level signal.
• Impedance matching - matching the impedance of your signal to one that a mic-preamp works well with.
• Balancing the signal - transforming the unbalanced signal from your instrument into a balanced one. A balanced system allows for the rejection of external interference that happens along the cable run (check out the science here - it's pretty clever)

What types of DI are there?

Depending on the gear you have, you might already have a form of DI, built into the Instrument Input. But not all of them do. And it's important to notice that the more you step into the high-end tier of equipments, the more specialized they become end less likely you'll find these types of inputs. Stepping out of the "all-in-one" type of audio interfaces and pre-amps that have DI inputs, you'll have to get a standalone DI Box. There is a vast array of them available from the most basic "just a transformer" designs, to high-end tube DI's. The two main type of designs are:

Passive DI:

• Simpler design, only passive components circuitry - sometimes just a transformer.
• Needs no power source.
• Little noise due to no active components.
• Might add coloration due to the transformer.
• Fixed signal step-down ratio - more limited in terms adaptability to different input levels.

Active DI:

• Needs power (usually Phantom Power, sometimes battery).
• Might add noise due to active circuitry; high-end ones will be design to minimize that.
• Can be more or less colored depending on the design.
• Has a built-in pre-amp. Can handle a wider range of input levels - sometimes with variable gain control.

There's no right or wrong choice between a passive or an active DI Box. More important than that is to choose the correct box for your application. Some boxes are more in the "jack-of-all-trades" category and will sound alright in the vast majority of sources; some others are specifically designed for a certain sound and use. There are direct boxes for the electric bass for example (the Aguilar Tone Hammer comes to mind), others specific for notebooks, some others might sound better on a guitar or keyboard. It's a matter of researching the correct box for your specific need.

Features

As we have seen, a DI box main purpose is to match level and impedance and to balance a signal. But most boxes will come with some extra features that might come in handy. Here's what you might expect:

• PAD: usually a -15db or -20db (number might vary) switch to accommodate hotter (higher-amplitude) signals.
• Link (sometimes "Thru" Jack): an output with the unchanged input signal. Usually sent to an amplifier.
• Polarity Switch (φ): inverts the polarity of the signal. Can be useful when recording the DI and an amp at the same time to adjust phase relationship between the two signals.
• Ground-Lift: can help preventing unwanted noise caused by ground-loops.
• Filter: Usually low-pass filters set to a very high cut-off to prevent high-frequency interference.

Some more specialized features:

• MERGE: combines the "Input" and the "Trough" into a stereo input that sums into mono on the output.
• Variable gain control: allows for the input of a great array of input sources (see the Retrospec Audio Juice Box for example)

How do I use a DI box ?

The Behringer's DI100 is probably the most famous DI box in the world. Let's analyze what each input or feature does.

• Attenuation (-20dB): This button lowers the signal for a total of 20dBs.
• The other (-20dB): Another button that does the same. If both are pressed, they lower the signal for a total of 40dBs. If you only wish to press one, there's no preferred order.
• Input (P10): Here's where you should connect your instrument-level signal. It supports Balanced inputs (via TRS balanced cables), but usually that's where you'd connect your unbalanced instrument. You bought a DI for that as well, right?
• Input (XLR): This works just like the other input, but for XLR connectors. Supports balanced inputs.
• Link: The unchanged input signal. Consider that as an output. Usually, that's where you'll plug a cable to send signal to the amplifier.

The Front

• IN/OUT: That should be called On/Off instead. The 'OUT' position disconnects the battery to preserve battery life. The 'IN' position uses the battery to power the unit. Phantom Power will power the unit and turn it on both on 'On' and 'Off' positions. So if you're only connecting it through Phantom Power, you can leave it on "Off "(Out) to assure it won't  use the battery and it will work anyway. While "On" (In), it will automatically switch between battery and phantom power once you turn on the phantom.
• GND Lift: If turned on, it interrupts the connection between the input and output electrical ground. This generally reduces noise, which leads to be an attractive function to many. However, the best practice is to try to keep it unpressed - on studio usages it may do no harm, but on live, thousand-watt-setup usages, lifting the ground may lead to serious electric shocks if a certain set of conditions is satisfied. There are even some reports of deaths caused by a lifted DI ground.
• Output: That's where you're connecting the XLR that will go the either the audio interface's input or the mixer's. Your previous Unbalanced signal is now Balanced.
• Battery Port: Where you place the 9v Battery that will be used on non phantom powered usages.

Example: How to record a guitar's amplified signal and its clean copy, at the same time, with an audio interface

Here's step by step on how to properly use a DI. It works just the same if you're connecting a mixer instead of an interface.

1. Connect your guitar to the DI Input (with a P10 TS cable)
2. Connect the Link/Thru to the amplifier (with a P10 TS cable)
3. Place a microphone in front of the amplifier's cabinet and connect the Mic's XLR output directly to the Interface Mic Input.
4. Connect the DI's Output on a second Mic Input on the interface. (with a XLR cable)
5. Turn on the interface's Phantom Power for the DI channel.
6. Voilà! You're ready to record

Important: If your interface has only one Phantom Power master switch for all channels, no problem. Dynamic microphones should be able to receive the power without any trouble. You should not do this if you're using a ribbon microphone on the amp, as they might get fried with phantom power. Don't do that unless you're sure your ribbon mic supports phantom power! Condensers need phantom to work, so If your mic is a condenser and your interface has individual phantom power switches per channel, you should enable them for both the the Mic and the DI.

In Conclusion

If you're into audio, be it live or recorded, you might want to consider adding one or some DI Boxes to your toolbox. They'll provide:

• longer cable runs
• better noise (interference) rejection
• better interfacing with a wider array of audio equipment
• extra features that might save the day (ground lift and pad for example)

These boxes are absolutely present in 100% of professional studios and stages around the world for a good reason. And they will definitely help on home studios as well. So do your research, find the one that better suits your needs and have a good session!