Often abbreviated DI (for direct insertion), a device commonly used to convert high impedance unbalanced signals from a stage instrument (like a guitar, bass or keyboard) into low impedance balanced signals. This puts the signal at the proper voltage level for the mixer and prevents the instrument from becoming loaded down with too low an impedance, which could cause tonal shifts and distortion. It also allows the signal to be transmitted over long lengths of cable.
They are used in live sound to get a signal out of an instrument on stage out to the mixing board, which can be as much as 500 feet away. They are also used in recording studios in order to plug an electronic instrument directly into a mixer or DAW with low impedance inputs without using a regular instrument amplifier.
A simple direct box consists only of a small transformer, but more sophisticated designs employ electronic gain stages that more closely resemble the input section of a modern instrument amp. They may also have some combination of ground lift switches, equalisation switches, level matching switches, isolated line outputs, and more.
Active or Passive DI Box?
The most obvious difference is that passive DI boxes don’t need to be powered. Active DI boxes require a power source, usually batteries or phantom power over an XLR jack. But there is more to it than that. Passive DI boxes are basically “just” transformers; they contain a transformer to facilitate impedance matching and balancing and very little else — the strength and impedance of the signal hitting the transformer depends on the source. This could be a disadvantage if your source signal is particularly low-level or low impedance, since the DI can’t do anything to “improve” the signal. However there are also advantages to passive designs: no noise added by active circuitry, no power required, and in some cases, a desirable coloration from the transformer.
An active DI box includes a preamplifier to provide a stronger signal and higher input impedance. This means that it can handle lower-level signals and still sound good. And, active DIs can be designed to be extremely quiet, hi-fi, and transparent tonally.
These days, there are excellent passive and active DI boxes. Many studios and sound companies have an assortment so that the best choice can be matched with each source.
How do you Choose?
There are so many options available; the only way to choose is to trial a potential DI in store with your own instrument, or instruments. Ensure that the output of the DI is connected to a low impedance input ( e.g. in Microphone input of a stage or recording mixer) normally with a balanced cable fitted with XLR connectors. Check that you get the same sonic quality as you can with your amplifier. Guitars can lose much of their high frequencies if the DI is not impedance-matching efficiently.