Over the many years I have been an audio engineer I have had the privilege of running sound at thousands of shows of varying types. From large format road concerts, to classical quartets, singer-song writers, D.J.s and even corporate events one fact never fails. There are always inputs on the sound board that need either a little more juice to get the signal to house or need to be padded down in gain and grounded to keep from blowing things up. As I would approach the stage with my direct Injection Box in hand the question of “What’s that thing for?” would inevitably come to bear. In this article we are going to not only explain what a Direct Injection or “D.I.” Box is; what it is used for and why it is an important part of any band, or DJs gear collection.
Let’s start off with a few reasons you may have to use a DI box. For example, the capacitance of cables reacts with source and destination impedances, forming a low‑pass filter. When dealing with high impedances and long cables, this only gets worse, curtailing the high end of the signal. The relatively low impedance created when the signal passes through a DI box enables you to work with long cables without problems.
Secondly, sending a balanced signal to a line level signal or instrument level signal to a microphone level input through a DI box means that RF (Radio Frequency) and EM (Electro Magnetic) interference breaking into the cable can be largely rejected, which is very handy in a hostile and unpredictable environment in which there will be lighting interference and who knows what else. Mic signals are generally balanced, whereas instrument line signals are not.
Thirdly, most PA systems are set up with a mic‑level snake from stage to mixer, and it’s just a lot more convenient, and faster, to rig to work entirely with mic‑level signals rather than a mix of mic and line.
Finally, the balancing transformer in the DI box also provides galvanic isolation between stage equipment and PA equipment, helping to avoid ground‑loop problems and potential electrical safety issues under fault conditions.
Now that we have covered a few reasons to use a DI Box in your signal path let’s get into what kind of sources should be plugged into a DI before going through your sound board.
One of the more common uses of a DI is in connecting more acoustic instruments; these are generally fitted with piezo pickups or contact mics, similar to many acoustic guitars with fitted pickups. The output from the control or interface box of such instruments will usually be ‘instrument level’, much the same as a guitar and again will require a DI box. Some unusual instruments, such as harps, can be fitted with piezo pickups or contact mics, whose outputs are usually at ‘instrument’ level, and will therefore require a DI box to convert them to microphone level to provide for a cleaner and more useable signal. This particular use can also be applied to electric guitars being plugged directly into a sound board when no amplifier is available…it’s not necessarily the preferred means of amplifying an electric guitar, but it definitely has its uses such as providing a clean effects signal to board in addition to a mic’d up amplifier for additional definition of sound. This is a technique frequently used for recording applications.
how to insert a D.I. box into your signal path
Now that we have examined the reasons to use a DI box in line with acoustic instruments, let’s examine why it is that a DI should be used in line with such things as electric keyboards, synthesizers and even electric bass.
In this scenario it is not only that the signal is not of the same impedance, but the fact that the signal coming from the source is incredibly large and dynamic, which when left unchecked can readily overload a sound board input and blow a pre-amp on your channel. In this application the DI box is basically acting as a buffer that will drop the intensity of the instrument signals down to a range that levels out at or at least much closer to what is safe for your inputs on your sound system.
Basic operational schematic of a typical D.I. box
Along with the applications of using DI boxes with high gain instruments comes their use with computer audio outputs and yes, yes…even DJ rigs. I know, most audio engineers dread the presence of a DJ on their stage because their signals tend to be incredibly hot on the output side. Remember, this is exactly what DI boxes are used for. Now I am not trying do down DJs here…many, many of them carry all of the gear they need to plug into any system, anywhere any time. However, with the advent of digital DJing from sources such as laptops, tablets and I-devices the challenges and issues can actually become severe and dangerous to gear on both ends of the equation rapidly…remember, unless your sound system is running off of batteries, that means you are using AC power. AC stands for “Alternating Current” that means that power can leak down the line back to the source of sound. This means that not only can the DJ blow the house gear, but the house gear can blow the DJ’s gear! This is no good guys. Seriously, this is probably one of the biggest misconceptions out there. As a matter of fact that is what causes the hum of a ground loop and the exact reason to use a DI box inline with any DJ setup. Explain to them kindly that it’s not because you don’t trust them…but because you want to guarantee the best quality audio in house and that you don’t trust the stupid machines not to fight and blow each other up.
Example of a D.I. box with optional ground lift
With that being said, there is one final reason to carry a DI with you as part of your gear and that is the beautiful option of “Ground Lift” and “DB+/-“ switches; these give you quite a bit of control over the way the source audio is connecting to the sound board. Let’s take a look at a couple of examples here.
Let’s say you step into a new venue one night to play and the walls are coated with neon lights. This is death to audio, neon lights can cause severe hum in sound systems and being that the venue is small the chance are good that the house sound or stage power will be on the same circuit as at least a few of these dreaded beasties. The solution would be, if you plug your instrument or DJ rig and start to hear nasty hum, plug in your handy DI box from your amp, instrument or DJ rig output to the sound board and flip that nifty “Ground Lift” switch. This will thereby remove your signal ground from the same ground as the house system and *POOF* insta-clean signal…just like magic. In addition to this is the “DB +/-“ switch. If you should happen to plug in your acoustic guitar and it still needs a bit more juice you can simply bump your DB switch to the + side to add an additional preset amount of DB gain, this is usually in the area of 10-20 DB per switch position. Conversely if you plug in your instrument or DJ rig into a DI box and you are still “just too darn loud” then simply reach down and bump your DB switch down by a notch and lower signal to board…it’s literally just that easy.
Now that you have some idea of why you would need a DI box you should decide which make and model suits your specific needs. A decent active DI will set you back about $100, but many people tend to drop their jaws when they see generic ‘active DI’ boxes going for under $30. However the differences in sound quality can be markedly apparent. Remember that just as your microphone should be of good quality construction and frequency response to assure it will put up with stage use and properly represent your vocal range, when choosing a DI box you should make sure that it is made of quality parts, well assembled and most importantly offer the frequency ranges and options you need. Some DI boxes are made specifically for PC audio and have only 1/8” cable for the input, some are made for only instruments having simply 1/4” inputs, while still others are made to handle quite literally every input and output for audio imaginable from 1/’8” to ¼” and RCA. As for recommendations, I’m a fan of the Radial series of products on my stage because they are one of the models with all the bells and whistles. But as with my microphones I have numerous makes and models with different options at my fingertips depending on the instrument or device it is being connected to.
Typical Stage D.I box for use with 1/4″ inputs D.I. used for PCs and digital devices D.I. box with 1/4″, 1/8″ & RCA
With all of these facts in mind try to take some time when shopping, read reviews, research the manufacturers and of course ask fellow musicians their favorite. When you see your audio engineer placing a DI in line with your gear, pick his brain as to why he uses that particular model and what the options it provides are. Believe me, he will respond, and probably with the same fervor as asking a new mother about the picture of the baby in her wallet. Take advantage, we audio engineers are an uber-geeky breed with a love for playing the proud papa when it comes to explaining what it is our gear does. Until next time everybody keep the gain up, volume down and fingers firmly on the frets at all times.