A DI potentially has several functions. Presenting the proper load to many devices is certainly one of them. However, DIs also step down the nominally
line level signals to a
mic level output, can provide isolation and convert unbalanced inputs like guitar outputs to a
balanced signal. So for the guitar example, a DI essentially provides the proper loads to the
instrument and
console, converts the nominal
line level signal to
mic level and adapts the unbalanced, 1/4" TS guitar signal to a balanced,
XLR output in order to interface the guitar to a standard mic input. The DI also provides isolation of the
instrument from the
system and many DIs offer
ground lift switches (which lift the signal
ground, not the
power ground) to potentially
address ground loops. Some DIs even provide stereo to
mono summing, not typically used for the guitar example but good for keyboards,
etc.
The
input impedance of the DI box is also sometimes a reason for selecting between either an active or
passive DI. Some electronics, such as
passive guitar and
bass pickups, like to see a very high
input impedance and active DIs often have input impedances several times that of most
passive DI boxes, so you often see active DIs used for instruments with
passive pickups. Active DI boxes are also often better for driving very long runs. On the flip side,
passive DI boxes can sometimes handle greater input levels and
transformer style
passive DIs sound more 'musical' when overdriven as you are saturating the
transformer rather than
clipping a
circuit.
However, there can be great differences in
transformer based
passive DI boxes and where quality is important, especially at low frequencies, the added cost of a high quality Jensen or Lundahl
transformer based product is usually a good investment.
