‘DI’ (Direct Injection) boxes are an essential and all too often underestimated component of any live sound rig.
Direct Injection simply means plugging an instrument’s output directly into a mixing or recording system, via a suitable matching device. Any electronic instrument, such as a keyboard, drum machine, electronic drums and electric guitar/bass is a candidate for Direct Injection. The reason we need DI boxes rather than simply connecting the output of the instrument directly to the stage-box and multicore is that the output level, connection format and source impedance of the instrument may not always be compatible with driving long cable runs to a mixing console. The output also may not be tolerant of globally switched phantom power (although some acoustic instrument pickup systems and electric guitar preamps include active electronics capable of providing a balanced output without the need for a separate DI box, and some operate from phantom power).
Put simply, a DI box’s purpose in life is to provide a convenient means of transferring the signal from an unbalanced, highimpedance instrument pickup, or from an electronically generated sound source such as a synthesiser, to a mixer’s mic inputs as cleanly as possible in a balanced format and with a low enough impedance to be happy driving long multicore cables. The source signal level can vary enormously depending on the type of pickup or electronic source connected, so many models have switchable input sensitivity settings or pad switches. Since the instrument (or its amplifier) may well be grounded via a mains power supply, the possibility exists for a ground loop to form via the multicore and PA mixer’s power supply, so most DI boxes also include a ‘ground-lift’ switch, to eliminate ground-loop hum in such situations. Most also include a ‘Thru’ socket that connects across the input socket, allowing the DI box to be used as a signal splitter, between the instrument and its amplifier, providing an isolated DI feed to the PA mixer at the same time as feeding the on-stage amplifier.
The piezo crystal-based pickups found on many acoustic instruments require a particularly high-input impedance (ideally two million ohms, or more) to work correctly, but most guitars fitted with this type of pickup also have matching preamps on board. Electric guitars and passive basses need to work at an impedance of between 250,000 and 1 million ohms—usually the higher the better—while synthesisers and other electronic sources are happy with anything above about 10,000 ohms.
As the stage multicore usually connects to the mixer’s microphone inputs, it is usual for a DI box to produce a microphone-level output so that it can be plugged into the stage box alongside the normal microphone signals. DI’d signals don’t suffer from spill in the same way that miked sources do, and there’s less risk of acoustic feedback, although acoustic guitars, double basses, violins and so-on that have contact pickups may exhibit a degree of ‘microphony’ (microphone-like pickup due to sympathetic resonance) and so a measure of spill may still exist. Likewise, the resonant body of an acoustic stringed instrument such as an acoustic guitar can still lead to acoustic feedback problems as their vibrations couple back to the pickup system via the strings, although the feedback threshold will typically be much better than it would when using a microphone.
Although a DI is sometimes used with electric guitars, it is rarely taken directly from the instrument itself, as the amplifier is such a key element in creating the final sound. Bass guitars will often be DI’d directly, but many bass amplifiers are now fitted with a balanced DI output which allows them to be plugged directly into the stage box. However, some have only an unbalanced line output in which case a separate DI box is still needed. Guitar players who use a specialised preamp rather than a conventional combo-amplifier—such as one of the Line 6 POD family—may feed its output via a DI box to convert its line level outputs to balanced mic-level feeds that can be handled by the mixer’s inputs.
Most guitar players still prefer to use an amplifier on stage, and the usual strategy for reinforcement is to use a directional mic close to the speaker grille, as there’s little spill penalty and the mic captures all the tonal nuances of the amplifier and speaker cabinet. There are, however, specialist DI devices that can accept the very high signal voltage from an amplifier’s extension speaker outlet and turn that into a balanced, mic level DI feed, usually with some electronic filtering to emulate the frequency response of a typical guitar speaker cabinet. This filtering is known as ‘speaker emulation’. Some guitar amplifier designers are now obliging enough to include a line-level output (often unbalanced) with speaker emulation built in, and you can connect this output to your mixer via a standard DI box.
In a similar way, keyboards and other electronic instruments may be DI’d directly, or a DI feed may be taken from the keyboard amplifier’s line-out or DI socket. If taken from an unbalanced line-level output, the signal would normally go via a DI box to convert it to balanced mic level. As keyboard amplifiers are designed to reproduce a clean, uncoloured sound, there’s usually no tonal penalty in DI’ing their line outputs directly into the PA.
Excerpt from The SOS Guide to Live Sound: Optimising Your Band’s Live-Performance Audio by Paul White © 2014 Taylor & Francis Group. All Rights Reserved.