PASSIVE & ACTIVE
These words are commonly used to describe D.I Boxes, Crossovers
and Speaker Boxes.
Passive means that no power supply (240 volt, battery or plug
pack) is required. The audio signal is all that is required
for the device to operate.
Active means that the device requires electric power in addition
to the audio signal. (any amplifier is an Active device but the
word Active is never used as there is no such thing as a Passive
D. I. BOXES
Passive D.I. (Direct Input/Injection) Boxes usually consist of a
transformer where the primary coil is high-impedance (between
1000-5000 ohms) connected across the sheild & signal connectors
of the instrument socket. A second socket is also connected so
that the same instrument can still be connected to an amplifier.
It does not matter which socket is used for instrument or
amplifier as they are connected together in parallel.
The secondary coil is low impedance (200-500 ohms) (similar to
a microphone coil) connected across pins 2 & 3 of a cannon/XLR
socket. Pin 1 (the earth/sheild) is connected to the case of
the D. I. Box. This secondary/output of the D. I. Box is a
balanced signal, allowing it to run long distances (40 meters
of multi-core) with minimal interferance. There will be a drop
in signal level of the audio passing through the transformer
but most mixing desks have enough gain on their microphone
input channels to cope with this. A good quality transformer
should have no significant degradation of dynamics or tonality.
Active D. I. Boxes are wired in exactly the same way except the
transformer is replaced by a pre-amp with an un-balanced input
and a balanced (pins 2 & 3) output. The pre-amp is powered by a
battery or (now things get confusing) by phantom power, supplied
by the mixing desk, travelling (backwards) back up the multicore
& microphone lead into the D.I. via its output socket.
Phantom Power; At the mic input of the mixing desk, 48 volts DC
positive is applied to both pins 2 & 3, negative is applied to
pin 1 (sheild). The impact of this voltage on the audio signal
is cancelled out as it is a balanced line (pins 2 & 3 are “hot”
& “cold”, opposite to each other, and any current or
interferance which is the same will cancel out) The Active
device (our D. I. Box, condensor mic or shot-gun mic) will tap
the power it requires to run its pre-amp from its output socket.
Phantom Power is called that as it is effectivly invisible and
works like magic!
Personally I prefer to use passive D.I. Boxes as there are no
batteries to go flat and phantom power can create extremely
loud noises when D. I. Boxes are connected or disconnected or
if the mic leads are faulty.
Earth Lift switches on D.I.Boxes allow you to break the earth
connection between the earth used by the instrument amplifier
on stage and the earth used by the mixing desk. This is an
advantage as an earth loop, causing hum, can be created when
two devices (instrument/amp & mixer) are connected together
but draw electic power (with earth) from seperate power outlets.
Passive Crossovers are usually built into Passive speaker boxes.
Their purpose is to split the full range signal from a single
amplifier into treble for the horn (or tweeter) and bass for
the low speaker (or woofer). This is what normally exixts
inside your home HiFi speakers or in a full-range PA speaker
They consist of an RLC network (resistor, inductor, capacitor)
Passive components which divide the frequency spectrum as
inductors (coils) prefer to pass low frequencies but choke high
frequencies & capacitors prefer to pass high frequencies &
resist low frquencies. No additional power is required, just
the amplified signal as it reaches the speaker box.
Active Crossovers are connected before the (multiple)
amplifiers. You would require seperate amps for each of your
horns, mids & low speakers. Active cossovers split the audio
into (usually) Hi, Mid & Low at line level after the output
of the mixing desk, and before the inputs of your power amps.
They are defined as an active device since they require
electric power to run their internal pre-amps & tone filters.
The main advantages of an active system is more power amps
for a larger speaker system and more accurate frequency