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 and live engineering. 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. 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. Always think of the DI box as a microphone – the better it is, the more expensive it will be. 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 microphone 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.
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 microphone inputs.