In a passive (non-powered) DI box, a transformer does the impedance
conversion and ground isolation.
With a passive DI, the input and
output impedances can vary greatly based upon the impedances of the
devices that are being connected through the DI's transformer. I
personally have mainly dealt with guitars and mixers, but there's also:
keyboards, amplifier preamp outputs, outboard gear such as modelers and
preamps, A/D D/A converters, computer interfaces and other I/O...
tons of other balanced and unbalanced gear that might need to be used with a
So, a major limitation with a passive DI in some of those
situations would be an inconsistency of the input and output impedances
dependant upon the gear itself (not the DI).
In an active DI the non-powered transformer is replaced with a powered
electronic circuit. Almost all active DI's can be powered by
either a 9v battery or phantom power from the mixer. Many active
DI's can also take power from a wall-wart-type power supply.
DI's, active DI's have a very consistent I/O impedance on both the hi-Z/unbalanced and lo-Z/balanced sides.
When using the Ground Lift
function with many active DI's, the phantom power return is also cut.
With units like this, make sure a fresh battery is there or you could be
stuck trying to figure out why your signal suddenly cut out part-way
through a gig!