DI Boxes and Their Use in Live Sound
The DI (Direct Input or Direct Injection) Box is one of the less visible but nevertheless valuable parts of a PA system.
Usually, it is a small box with one or more inputs (usually 1/4″ jack sockets, although some also have phono and/or XLR input sockets) and one or more outputs (always one balanced XLR socket, sometimes one or more jack sockets as well).
A DI box converts an unbalanced, high impedance signal (the kind of signal generated by most pickups and contact mics) into a balanced, low impedance signal (the kind of signal required by most desks). It also isolates the output signal from the input signal (and most also incorporate an earth lift facility), so it can be used - as a temporary measure in an emergency - to cure ground loop problems in other parts of the signal chain (e.g. between desk and power amplifier).
DI boxes come in two main flavours: Active and Passive.
Active DI Boxes use electronic circuits to convert and isolate the output signal from the input signal. For this reason, they always require power. Most active DI boxes can use batteries - historically 9V PP3, but 1.5V AA are now common too - or phantom power (and our own can use either source). However, some DI boxes cannot run on phantom power (i.e. they need batteries), and some cannot use batteries (i.e. they won't work unless the desk can supply phantom power). A few varieties will run from separate power‑supply units.
Passive DI Boxes use transformers to convert and isolate the output signal from the input signal. They are usually cheaper than active DI boxes, and do not require any power. However, the reactance of a transformer increases as frequency rises, so most passive DI boxes will exhibit some high-frequency signal loss. In all but the cheapest passive DI boxes this will not greatly affect the signal within its useful range, and the degree of difference is comparable with the difference between dynamic and condenser microphones.
If all else fails, read the manual!
You plug a lead carrying the source signal into the input socket, and connect a balanced mic lead to the output socket. You connect the other end of the mic lead to the desk or multicore, switch the phantom power on, and forget about it, unless:
You will benefit from using a DI box if:
It is good practice to install new batteries in all battery-driven devices before the start of any show (so you can be confident they won't die during the encore). This involves the bother and expense of changing batteries frequently, as well as remembering to change them. A passive DI box does not need batteries. If phantom power is not usually available, a high-quality passive DI box will probably be your best option.
Otherwise, active DI boxes are generally a better choice. However, you may want to look for some or all of the following facilities, whichever type you go for: