Direct Injection (DI) boxes can be real problem-solvers in the studio, but you have to choose the right one for the job.
It's easy to overlook DI boxes, but they play an important role in recording, both in the studio and (perhaps even more so) in live recording. There are several different types of DI box, but all serve two main purposes — they function as impedance-matching devices and they provide ground-lift facilities to avoid ground-loop hum problems caused by multiple ground paths.
The simplest type of DI box comprises little more than an impedance matching transformer with a balanced output. This type of device is known as a passive DI box and is usually intended to work with line-level signals, though some can also handle instruments such as guitars and basses, or even speaker-level signals from power amplifiers. Electrical isolation between the input and output is provided by the transformer and a switch linking the input ground to the output ground can be set to provide a ground-lift facility as shown in Figure 1. Having a well-designed transformer is crucial to the sonic performance of a passive DI box, which is why you'll find quite a price difference between models. Traditionally, the balanced output from a DI box is at microphone level rather than at line level, so that it can be connected to the mic input of a mixing console. Note that the input is also directly wired to a Thru socket. This enables the DI box to be inserted between an instrument and its amplifier without interfering with the normal signal flow, but still permits the engineer to take off a balanced DI feed for recording purposes.
Electric guitars and basses with passive pickup systems need to work into a fairly high input impedance to avoid loading the pickups and compromising the tonality. Typically, an impedance of 1MΩ or above is quoted, though I have used a passive transformer DI box with a 100kΩ impedance instrument input in conjunction with a guitar and achieved perfectly acceptable results. To achieve higher impedances, it is necessary to use an active DI box — one with electronic impedance-matching circuitry.
The active DI box may still use a balancing transformer at the output, as this provides better electrical isolation than electronic balancing solutions. However, because of the relatively high cost of good transformers, the majority of the more affordable active DI boxes are transformerless. Active circuitry requires power, so the usual solution is either to make the DI box run from standard 48V phantom power or to provide a dual solution by including battery power as an option for use in those situations where phantom power may not be available. As a rule, the circuitry is designed so that the battery becomes active when an input is connected to the unit, but is automatically switched off when phantom power is detected at the output XLR. The availability of phantom power is another reason why DI boxes are generally designed to feed into mic inputs.
Active DI boxes give the designer more flexibility in accommodating different types of input signal, so it's not uncommon to find models capable of accepting line, high-impedance instrument and loudspeaker inputs, most often via different sockets. Such a design is shown in Figure 2. In fact both active and passive DI boxes can be designed to accept loudspeaker-level signals, but because of the high voltages generated at the outputs of power amplifiers, resistive networks are needed to attenuate the signal to a safe level. For this reason, speaker level signals should never be fed into a DI box or other piece of equipment not specifically designed to handle them, as serious equipment damage will almost certainly result. Furthermore, power amplifiers are designed to run into a loudspeaker load, so the loudspeaker system or an equivalent dummy load (power soak) must be connected to the speaker Thru socket when working with speaker-level signals. While a solid state amplifier may tolerate running with no load, a valve amplifier is almost certain to sustain damage to its output transformer if run without a speaker or dummy load connected.
In live recording, the obvious use of a DI box is to create a mic-level feed from an instrument, line or speaker source that can be recorded without interfering with the original signal or risking ground-loop hum. In most cases, this simply involves using the ground lift to break the ground connection between the input and output. Note however that when recording electric guitar, and in some cases bass, the DI'd sound taken from the line output sometimes found on the back of the amplifier will require further processing using a speaker simulator before it sounds right. If used as it is, it will contain a lot of unpleasant high-frequency harmonics that would normally be removed by the limited frequency response of a typical guitar speaker. This is particularly important where any form of overdrive effect is being used.
One possible solution is to use a specialist guitar DI box that combines the usual DI box functions with a filter network that simulates the frequency response of a typical guitar loudspeaker cabinet. These are available in both active and passive versions and may either be fed a line input from a suitable preamp output, or be inserted between the amplifier output and the loudspeaker. Usually the latter option gives the most accurate result, as any coloration contributed by the power amplifier stage will also be captured.
The same applies in the studio, and while it is arguably better still to mike up a guitar where possible, sometimes this isn't practical. In the case of bass guitars, using a DI box often produces a cleaner sound, because speaker cabinet rattles and other speaker-related problems are avoided. It's also possible to use the high-impedance input of a DI box for recording clean guitar and bass parts. Note that acoustic guitars fitted with passive piezo electric pickups or bugs need to work into an even higher input impedance than electric guitars, so specialist preamps with very high-impedance FET input stages are recommended, rather than trying to use a regular DI box.
Sometimes the ground-lift facility alone of the DI box can provide a solution; if you have a synth that produces an annoying hum when connected to other equipment, DI'ing it using ground lift may well get you out of trouble.
As I said at the start, DI boxes are all too easily overlooked — they're just little boxes that solve problems (sometimes big problems), so it pays to have a couple lying around. Don't skimp by choosing the cheapest model though, because, like any other recording component, the better-designed ones sound noticeably nicer than the very cheap ones. Try to pick a model that can handle instrument, line and speaker sources, but don't worry about whether or not it has a transformer output unless you're doing a lot of live recording, in which case the improved isolation of a transformer might be worth paying for. And if you do a lot of guitar work, either choose a combined DI and speaker simulator for situations in which the original speaker will still be connected, or choose a combined speaker simulator and power soak for when you want to use a valve guitar amp with no speaker.