How To Use A Di Box

Direct Injection (DI) boxes can be existent trouble-solvers in the studio, simply you lot have to choose the right i for the job.

USING DI BOXES: artwork header image of BSS DI box. Photo: Original Richard Ecclestone It's easy to overlook DI boxes, only they play an important function in recording, both in the studio and (perhaps even more so) in live recording. In that location are several different types of DI box, merely all serve two primary purposes — they part as impedance-matching devices and they provide ground-elevator facilities to avert footing-loop hum problems caused by multiple basis paths.

Simple Passive Designs

The simplest type of DI box comprises little more than an impedance matching transformer with a counterbalanced output. This type of device is known as a passive DI box and is ordinarily intended to piece of work with line-level signals, though some tin can also handle instruments such as guitars and basses, or even speaker-level signals from power amplifiers. Electrical isolation betwixt 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 functioning of a passive DI box, which is why y'all'll notice quite a price difference between models. Traditionally, the counterbalanced output from a DI box is at microphone level rather than at line level, so that information technology can exist connected to the mic input of a mixing console. Note that the input is besides directly wired to a Thru socket. This enables the DI box to exist 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.

Electrical guitars and basses with passive pickup systems need to piece of work into a fairly loftier 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 musical instrument input in conjunction with a guitar and accomplished perfectly acceptable results. To attain higher impedances, it is necessary to apply an active DI box — one with electronic impedance-matching circuitry.

Active DI Boxes

The agile DI box may still use a balancing transformer at the output, as this provides better electrical isolation than electronic balancing solutions. Still, because of the relatively high cost of skilful 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 bachelor. Every bit a rule, the circuitry is designed so that the battery becomes agile when an input is continued to the unit, but is automatically switched off when phantom ability is detected at the output XLR. The availability of phantom ability is another reason why DI boxes are mostly designed to feed into mic inputs.

Active DI boxes give the designer more flexibility in accommodating different types of input indicate, then information technology'due south not uncommon to observe models capable of accepting line, high-impedance musical instrument and loudspeaker inputs, most often via dissimilar sockets. Such a design is shown in Effigy ii. In fact both active and passive DI boxes tin exist designed to accept loudspeaker-level signals, simply considering 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, every bit serious equipment damage will near certainly event. Furthermore, power amplifiers are designed to encounter a loudspeaker load, so the loudspeaker arrangement 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 continued.

DI Boxes For Guitars

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 merely involves using the ground elevator to break the ground connexion between the input and output. Notation all the same that when recording electrical guitar, and in some cases bass, the DI'd audio taken from the line output sometimes institute on the dorsum of the amplifier will require farther processing using a speaker simulator before it sounds right. If used every bit it is, it will contain a lot of unpleasant loftier-frequency harmonics that would normally exist removed by the express frequency response of a typical guitar speaker. This is particularly important where any course of overdrive consequence is existence used.

I possible solution is to apply 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 exist inserted between the amplifier output and the loudspeaker. Normally the latter option gives the virtually accurate result, as any coloration contributed past the ability amplifier stage will also be captured.

A DI box with a built-in speaker simulator, such as the Hughes & Kettner Red Box Pro, can be used to condition the signal from the line output invariably found on the back panel of a guitar amplifier these days. A DI box with a congenital-in speaker simulator, such equally the Hughes & Kettner Blood-red Box Pro, can be used to condition the signal from the line output invariably institute on the dorsum panel of a guitar amplifier these days. The same applies in the studio, and while it is arguably better still to mike upwardly a guitar where possible, sometimes this isn't applied. In the case of bass guitars, using a DI box frequently produces a cleaner sound, because speaker cabinet rattles and other speaker-related issues are avoided. It's likewise possible to use the high-impedance input of a DI box for recording clean guitar and bass parts. Note that audio-visual guitars fitted with passive piezo electric pickups or bugs need to work into an even college input impedance than electric guitars, and then specialist preamps with very high-impedance FET input stages are recommended, rather than trying to apply a regular DI box.

Sometimes the ground-elevator facility solitary 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 elevator may well become you out of problem.

Every bit I said at the kickoff, DI boxes are all too easily overlooked — they're just piffling boxes that solve problems (sometimes large problems), so it pays to have a couple lying around. Don't skimp past 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 musical instrument, line and speaker sources, but don't worry nigh whether or not it has a transformer output unless yous're doing a lot of live recording, in which instance the improved isolation of a transformer might be worth paying for. And if you practise a lot of guitar piece of work, either choose a combined DI and speaker simulator for situations in which the original speaker will withal be connected, or choose a combined speaker simulator and power soak for when you lot want to use a valve guitar amp with no speaker.