DI boxes are essential pieces of stage equipment, but to get the most out of them, it helps to know what they do — and how they do it...
The humble DI — or 'direct injection' — box is a fundamental part of every live-sound engineer's armoury of stage equipment, but there is an enormous variety of different models, designs, applications and features, which can be quite confusing, if not daunting, to the novice. The aim of this short article is to examine the aims and requirements of DI boxes, and to identify the appropriate features and specifications.
The fundamental purpose of a DI box is to provide a convenient and safe means of interfacing an instrument (guitar, bass, keyboard and the like) directly with the rest of a stage sound system. That usually means connecting it into the mic preamp of a console, because most live-sound rigs are configured to receive only microphone signals from the stage, via a stage box.
Usually, the instrument to be DI'd will be connected to a local amplifier/speaker combo on stage, so some kind of signal splitting and feed-through provision is generally required to allow the DI box to 'hear' the instrument signal, while still allowing its signal to be passed on to the amp. It's worth bearing in mind that in the case of electric and bass guitars, the amplifier/speaker combination plays a very big part in shaping the instrument's tonality, and that the DI signal derived straight from the guitar will typically sound exaggeratedly bright and dynamic compared to the sound produced acoustically from the speaker. Some specialist DI boxes are available that process the DI signal internally to deliver a more lifelike sound, but it is worth bearing in mind the need to process the DI signal quite heavily, in most cases, to achieve a usable sound!
To understand the technical implications of the DI interface, we first need to know something about the interfacing characteristics of both the instrument source and the mixer preamp destination. Mixing-console mic inputs in a live-sound context typically expect to handle signal levels from maybe as low as -55dBu, up to about +5dBu. Microphones generally have a very low source impedance of about 150Ω, from a balanced source, and expect to see a load impedance of about 1.5kΩ or higher. All modern mics are designed to work in a voltage-matched configuration, where the load impedance (the input impedance of the preamp) is between around five and 10 times larger than the source impedance (the mic's output impedance), to ensure that the source signal voltage is detected as accurately as possible. Some vintage microphones — and we can include the ever-popular Shure SM57 in that — were designed for use with impedance-matched interfaces, where the source and load impedances were the same (typically about 600Ω), but with modern equipment this is rarely the case.
In contrast to microphones, the output of an electric guitar is unbalanced, with a typical level range of between about -30 and 0dBu, perhaps going up to +10dBu for active basses. The source impedance is relatively high, at between 5kΩ and 20kΩ for normal electric guitar pickups, with higher-output pickups tending to have higher source impedances (and higher still for piezo pickups, such as those found on acoustic guitars). Guitar pickups need to 'see' a load impedance of at least 250kΩ if they are to work — and sound — as intended, and often the load impedance is actually much higher than this (anything up to 1MΩ is common). The circuit made between a guitar pickup and the amplifier input is quite complex, and typically has a resonant frequency somewhere around 2kHz to 3kHz. The load impedance determines the strength of this resonant peak: a low impedance damps the resonance more, while a higher impedance allows a stronger peak, and that gives a brighter, more biting sound, which is often desirable to guitarists.
Instruments with electronic outputs, such as keyboards and drum machines, typically produce output levels in the same sort of range: -30dBu to +10dBu. However, the electronic output drivers employed usually have a very low source impedance (100Ω or so) and are generally quite happy with any load impedance over a few kΩ.
Clearly, a typical console microphone input is not ideally matched to the requirements of most instruments: the input impedance is too low, affecting the tone, and the sensitivity is too high, which means that the preamp could easily be overloaded. However, the practical reality is that most stage boxes comprise XLR connectors expecting to receive balanced mic signals, not unbalanced, high-impedance instrument signals. The balanced aspect is important here, because a stage is an electrically hostile environment — there is a strong likelihood of interference and ground loops occurring — and balanced signals are relatively immune to both of these problems, allowing long cable runs without fear of significant signal degradation.
Given all of the above, it makes pragmatic sense to convert an instrument signal to something that looks like a microphone signal, and to do so on the stage as close to the instrument as possible. And that's what a DI box basically does. In short, it:
- Presents a high-impedance load (>250kΩ) to an unbalanced instrument source.
- Provides a signal-splitting function to feed the source signal to a stage amp as well as the front-of-house console.
- Reduces the instrument signal level to a nominal microphone level.
- Provides a low-impedance, balanced output suitable for feeding a microphone preamp.
- Isolates the source and destination grounds to avoid ground loops.
- Provides electrical isolation for safety, where necessary.
There are two basic variants of DI box, known as active (powered) and passive (unpowered). The latter are the simplest and comprise little more than a transformer in a box with some connectors. By selecting the winding ratios and impedances carefully, the transformer can be arranged to provide a reasonably high input impedance (up to about 150kΩ) to the unbalanced instrument source, while also stepping down the signal voltage by about 20dB and providing a low-impedance, fully-balanced output suitable for connection with a mic preamp. Job done!
The transformer inherently provides 'galvanic isolation' too, which means that the input circuit is electrically isolated from the output circuit — so there is no direct electrical connection at all, because the audio signal is passed between the transformer's primary (input) and secondary (output) windings as a varying magnetic field. If the transformer is designed carefully, this electrical isolation can withstand a thousand Volts or more, and so provides a useful degree of protection in the event of a serious electrical fault occurring in part of the stage or FOH system. However, good transformers with a wide bandwidth and low distortion are inherently expensive, and it is difficult to achieve particularly high input impedances.
Usually, the signal-splitting function is achieved with a simple parallel connection between the input and 'thru' sockets, but occasionally the 'thru' output can be derived from a separate winding on the transformer. This can be a useful feature for avoiding ground loops if the source instrument is mains powered (a keyboard, for example) and you need to send the output signal to an on-stage, mains-powered amplifier.
Most DI boxes also include some input-signal attenuation (often known as 'pad') options, either via a switch or via separate dedicated input sockets with different sensitivities. The idea is to reduce high-level input signals that would otherwise overload or saturate the transformer. Typically, 20dB and 40dB attenuation options are included to accommodate line and speaker-level sources, respectively — although some care is required when 'tapping' into the speaker output of a guitar amplifier, as valve amps don't like it if they can't 'see' a speaker! Serious damage can occur to the amplifier if you get this wrong, and to the DI box if you put too big an input signal into it!
From a technical viewpoint, if the source signal is quite loud, it makes sense to use the pad facilities on the DI box to reduce the level at source. This approach eliminates the risk of overloading or saturating the DI box circuitry — something which can't be fixed at the FOH console, should it occur — and ensures that the signal reaching the console is never too big to stress the mic preamp unduly.
However, I know some engineers have been caught out when an unthinking musician has spied a pad switched in on the DI box and deselected it to make them louder! The result, of course, is severe overload of DI box, mic preamp, and the entire PA system. I find explaining that I will perform a DIY sex-change operation on anyone who touches the DI boxes once I've set them up usually reduces the risk quite acceptably!
There are a couple of further points to be aware of when choosing or using a passive DI box. Firstly, and as I've already mentioned, good transformers are expensive but are usually worth it from an audio quality point of view. Secondly, the transformer in the passive DI box conveys the audio via a varying magnetic field, but if the DI box is placed near another strong varying magnetic field — such as that radiated by the mains transformer in a stage amp — mains hum is likely to be induced magnetically into the audio signal too. So keep DI boxes well away from strong magnetic fields!
Finally (and, arguably, most importantly), the limitations of transformers are such that the input impedance of a passive DI box isn't always high enough to satisfy the expectations of some guitar pickups. Passive DI boxes work very well with all active sources, such as active basses, electronic keyboards and the like, but the tonality of some electric guitars and acoustic pickups may well be affected in an unacceptable way because of the lower-than-ideal input impedance. Thankfully, though, a solution is at hand: the active DI box!
Active DI boxes employ an active electronic buffer circuit at the instrument input to present a very high input impedance to the source instrument — typically 1MΩ or so. This allows the guitar pickups to work as intended in delivering the expected tonality and sustain, and for that reason — as well as the fact that active DI boxes work well with electronic sources too — they tend to dominate the market.
Some active DI boxes use an output transformer to provide the balanced output and galvanic isolation, but many of the lower-cost models use electronically balanced output circuitry instead. In situations where galvanic isolation might be useful (such as when working with unknown and untrusted PA systems, or in open-air events with local generator mains supplies), an active DI box with a suitably rated isolation transformer output would be a sensible precaution, offering some protection to the player if a serious fault occurred with the FOH system, and vice versa. However, in general, active DI boxes with well-designed electronic outputs will always out-perform transformer-based boxes in terms of headroom margin and distortion. They will also be smaller, lighter, and less expensive because a costly, bulky and heavy transformer is no longer required. Most of my DI boxes have active electronic outputs these days and I have no trouble with them at all — but I always have some transformer-based units on hand just in case I don't like the look of something!
With an active box, the electronic buffer circuit (and output driver, if present) needs power, of course, and that is usually provided either by an internal battery, or via phantom power from the mixer coming down the microphone cable to the DI box's output socket. In many cases, an automatic internal power-switching system will use phantom power if present, but revert to internal batteries if not. Usually, the battery supply is switched on only when an instrument is plugged into the input, but some boxes have a separate on-off switch. Be aware that if the battery is running low, the headroom margin through the electronics will be reduced and the peak distortion correspondingly increased quite dramatically!
Active DI boxes are often equipped with useful extra facilities, such as output polarity reversal, high-pass filtering, dual inputs with signal mixing (handy for stereo sources feeding a single stage line), footswitch output muting (handy when changing guitars), and separately active or transformer-buffered outputs to feed a second amp, a separate recording system, or a dedicated tuning display.
The output polarity feature can be useful if the facility is not provided on a console, especially if the DI signal is being blended with the miked output from a stage amplifier. Polarity inversion in the amplifier electronics and/or microphone could lead to unpleasant phase cancellations when mixed with the DI signal, and being able to flip the polarity of the DI box signal can often save the day. Similarly, being able to roll off the low end at source minimises any 'mud' and helps to clean up the mix if a suitable high-pass filter isn't available on the FOH console.
Active DI boxes are available from as low as £15, and go all the way up to about £130, but while there are some good low-cost active DI boxes around, my general policy in this area is to invest in one of the high-end products. My reasoning is that a good DI box will provide 40 years or more of reliable service, always delivering a high-quality sound worthy of the highest-quality source instrument, and will never be the weak link in the chain.
The differences between DI boxes basically come down to build quality and ruggedness, the design of the electronic circuitry (especially the power supply arrangements), the features and facilities included, the headroom margin, and the quality of the transformer. The cheapest models tend to be less robust and more easily damaged, and they don't usually sound as good, because of higher noise, restricted headroom and higher distortion. As a result, they are more likely to be replaced as they break, wear out, or their sonic limitations are realised — so over time, it makes more sense to get a quality DI box to begin with.
Everyone has their own preferences when it comes to DI boxes, but the passive units I use are either the Radial ProD2 for stereo active sources or the EMO E520 for mono sources. It's interesting to note that the Radial JDI Duplex passive box, which is a better-specified functional equivalent of the ProD2 using better transformers, achieves 3dB more headroom, 7dB more dynamic range, a more extended high end and less phase shift. These are subtle technical improvements, but audible in demanding situations nonetheless. Clearly, the quality of a passive DI box is set almost entirely by the quality of the transformer it employs, and while £100 or so might seem expensive, the cost is a small proportion of the value of typical source instruments.
When it comes to active DI boxes, my favourites span a wider price range. My oldest units are the Canford Audio active DI box and the BSS AR116. The latter was the forerunner of the current AR133, while the former was originally manufactured by Technical Projects, before becoming a Canford product. The Canford box has the unique ability to switch the 'thru' output from a simple parallel connection to a transformer-isolated (and balanced, if required) output — something that has proved invaluable for curing on-stage ground-loop problems in the past. Both of these units have transformer outputs, are built into extremely robust cases, and have stood the test of time perfectly (being around 30 years old now and still going strong).
I also use the Radial J48 a lot for its very clean sound and handy dual-input merging feature. The J48 doesn't use an audio transformer for the output — it is actively balanced instead, and its input impedance is quite moderate for an active DI box (around 250kΩ), which makes it a useful alternative to more common 1MΩ boxes sometimes. It also has a clever switched-mode internal power-supply system, which provides a higher-than-usual headroom margin.
At the other end of the price scale — but without compromising audio performance in the slightest — I use the Orchid Electronics Micro DI and Muting DI boxes. The former is the smallest active DI box I know of (it runs on phantom power only, so no battery compartment), and its diminutive size has often proven very handy, allowing it to be attached discreetly to instruments or amps. The Muting DI box allows convenient tuning via a second non-muting output, as well as bang-free changing of guitars and basses. Both these units are transformerless, but sound great and are extremely cost-effective.