Want to know how to obtain the best quality audio recording possible? Searching for various gears to add on to your home studio rig? There are many important audio equipment and components that you need to know about, if you are serious about being a decent sound engineer. One tool that is often overlooked, is the DI box. So the question is – What is a DI Box?
In today’s article, we will be discussing about the different aspects of the DI box, and the purpose it serves in the world of professional audio. You will know more about the basic principles behind this component’s design, and also how deeply it affects the outcome of an audio recording, whether be it in the studio or in a live concert situation. Hence, without further ado, let’s start!
Introduction – An overview
A DI unit is essentially a device used in professional recording situations (usually in studios) to connect an unbalanced output signal (line level with high impedance) to a balanced input (microphone level with low impedance), typically via XLR connectors. DI boxes are often used to connect the output of an electric guitar or electric bass to a mixing console’s channel input.
The DI’s electrical component itself is designed to perform level matching, balancing, and either active buffering or passive impedance matching (also known as impedance bridging) to reduce noise induction, distortion, and ground loops. You will find DIs being widely used in smaller-scale PA systems, larger-scale professional sound reinforcement systems and also in sound recording studios.
It would be beneficial to take note that within professional audio industries everywhere, DI units are very commonly referred to as a DI box, direct box, or simply DI. The term “DI” itself is universally understood, at least by audio professionals, to stand for direct input, direct injection, direct induction or direct interface.
Let us look at the breakdown of various topics that we will further discuss:
- Passive DI unit
- Active DI unit
- Typical applications
The first direct boxes (passive) are found in America during the 1960s, notably used in radio stations and recording studios in Detroit (such as “Motown”, “United Sound Systems” and “Golden World Records”). Engineers like Ed Wolfrum and other live sound companies custom made these DIs in order to solve electrical issues, as a result of amplifying electronic musical instruments (electric guitars etc.)
The core electrical component that drives these DI boxes, is an audio transformer (such as the Triad A11J through 1974) with a turns ratio from 8:1 to 12:1 (estimation), thus resulting in an impedance ratio of about 144:1. By using this type of transformer, the output voltage of the electrical instrument will be toned down to a range that is compatible with the typical mixing console’s microphone preamp.
Although the passive direct box worked very well for most electronic musical instruments, it adversely affected the sound of ones with low output signals, most notably the Fender Rhodes pianos and Precision Basses (single-coil pickups). To solve this issue, active direct boxes (with powered electronic circuitry) were engineered, which increased the input impedance from about 200,000 to above 1,000,000 ohms.
A 48-volt “phantom powered” active direct box was first engineered for Leon Russell’s recording studio in 1975, with its design featured in dB, the audio engineering magazine. Soon after, sound company Tycobrahe, famous for running large rock festivals (such as California Jam), started selling active DIs in 1977. Its model is capable of a “+9 dBm” line level output, with a built-in attenuator for input level compensation.
The main purpose of the direct box, is to take a high impedance, unbalanced signal and convert it to a low impedance, balanced signal (suitable for mixing consoles). This will allow electrical audio signals to travel over longer cable runs (often happens in live sound situations), but with the benefit of a major reduction in signal loss (especially in high frequencies).
This is achieved by the lowering of impedance, and greater rejection of electromagnetic interference (due to common mode rejection in a balanced signal). On top of that, the lower impedance rating of around 600 ohms is fairly common, and allows an insignificant load to a mixing console’s channel input (or preamp) which is also engineered to receive microphone level signals (low impedance).
Passive DI units
These units are typically designed to include an audio transformer, that is used as a balun (a device that converts between balanced and unbalanced signals). The turns ratio of this transformer, is optimized to convert a nominal 50 kΩ signal source (such as from the pickup of an electric guitar) to the 100–200 Ω range, which is ideal for the channel inputs of an audio mixer. Common turns ratios are within the range of 10:1 to 20:1.
Take note that passive units are usually less versatile than actives and the cheaper ones are also less resistant to hum. They are however, simpler to use (with no batteries needed) and the quality ones are very durable and reliable. Some models do not have controls, while others have a ground lift switch (to solve ground loop issues), a pad switch (for input signal attenuation) and a filter switch for tone colouration.
Active DI units
These types of DI unit are designed to include an internal pre-amplifier. Hence, active DI units are capable of providing gain, and has a circuitry that is fundamentally more complex, but more versatile than its passive counterparts. All active DI units require a power source, which typically comes in the form of batteries or from a standard AC wall socket.
Some units allow for phantom power supply as well. On most active DI units, there are switches that enhances their versatility. These switches or knobs usually allows gain or signal level adjustment, ground lift, selection of power source, and mono or stereo function. In some models ground lift may disconnect phantom power. There are also some active DIs that have built-in effects units.
This connector is basically a second jack (that may be paralleled to the main input jack) that routes the input signal unaltered, to prevent the DI from interrupting the signal path that it is inserted in. This allows for example, an electric bass player, to plug the instrument into the DI (sending that dry signal to the mixer), and through the bypass output, the bass signal can be routed to the bass amp on stage as well.
True-bypass is the process of the signal (coming directly from the main input) being routed to the output jack with no circuitry involved and no loading of the source impedance. Alternatively, false bypass (or “bypass”) happens when the signal goes through the device’s internal circuitry with buffering, but with no intentional extraneous modification to the original signal.
It is very important however, to take note that due to the nature of electrical components and circuitry designs, there is almost always some minor attenuation in the electrical audio signal. This often changes the tone, however the degree of change and how noticeable it is to the human ear, vary from unit to unit.
Universally, direct boxes are used in conjunction with instruments or other devices that only have an unbalanced 1/4″ phone output, which ultimately needs to be routed to an XLR input (usually on mixing consoles). You can find multiple direct box circuits being mounted inside a single unit’s housing. These are normally useful for multiple unbalanced outputs, for instance, coming from a bank of electronic keyboards.
Acoustic or electric instruments
DI’s are often used with electronic instruments (such as electronic keyboards) and acoustic ones with pick ups that do not have an XLR balanced output. One typical example of this application, is an electronic keyboard that needs to be connected to a mixing console, either directly or through a snake cable (in live sound).
Another common example would be an acoustic guitar with built-in pickups, an electric guitar or bass guitar that has to be routed to a mixing console’s input channels, in order to be mixed down with other channels. Ultimately these signals are then routed to the main mix or monitor mix.
DI units can also be built into instrument amplifiers (usually bass amps), and can then be connected to a mixer directly without the need of an external direct box. This would benefit the user who wants to keep the unique, original sound quality of the amplifier, and have that signal to be sent to the main mix of a PA system.
An on-board equalizer may also stand in the way of retaining the authentic sound of an instrument’s amplifier. Thus, some manufacturers produced amplifiers that have the function to deactivate the EQ section, often through a pre eq or post eq switch.
It is common practice for both DI and microphone to be used on the same sound source. One method is to connect a speaker level output (for instance, from a guitar amp speaker) to a DI unit and then route it to one channel on the mixing console. The signal from the microphone (miking the guitar speaker) is then routed to a separate channel on the same mixing console.
Another typical method is to insert the DI unit on the signal chain, in between the guitar and the amplifier. The DI’s dry signal and the miked up guitar speaker’s signal can then be creatively blended. In this case, the DI is providing a more immediate, present, bright, dry sound, and the microphone giving a more rounded sound, influenced by the instrument’s amplifier characteristics and a little bit of room ambience.
Okay friends, we have reached the end of this article. Did you learn something? Well, I hope that now you have acquired enough knowledge to confidently use this important audio component in your musical endeavours!
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