Balanced , Unbalanced , Passive , ActiveA normal 2-conductor line (from a tape deck or an electric
guitar) is usually referred to as unbalanced. This is where
the negative is "tied to Ground" or "Earthed". the reason for
this is so any extraneous signals or interference, drain to
earth. To achieve this the negative is usually a braided
shield wire surrounding the positive wire so that any
interference lays itself on the negative only and is not able
to reach the positive wire.
A 3-conductor Mic lead is a balanced line. this is where the
negative is the exact opposite of the positive because neither
are tied to ground and both come from opposite sides of the
microphone coil.
The main advantage is that any interference that manages to
get past the shield (which is still connected to earth) would
affect both the positive and negative wires to the same degree
and hence (because the two are opposite) cancel out.
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
amplifier!)
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.
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CROSSOVERS
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 exists
inside your home Hi Fi speakers or in a full-range PA speaker
box.
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 frequencies. 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
division.
BACK TO BASICS
I was asked to elaborate on the difference between the terms
ACTIVE and PASSIVE, and how they relate to Crossovers and
D.I. Boxes, but first I think it's a good idea to expound on the different types of PA
systems which are generally categorized according to how the
crossover is incorporated.
(Also in the future you can look forward to my explanation of
why microphone connectors need 3 pins.)
TYPES OF PUBLIC ADDRESS (P.A.) SOUND SYSTEMS:
Rough rule of thumb: 5 watts per person in audience
SIZE & TYPE No. CHANNELS POWER AUDIENCE SIZE
Vocal PA 6 - 12 ch 300 - 500 watts up to 100 people
Single 3-Way 12 -16 1000 - 2000 w around 200 people
Double 3-way 16 - 32 2000 - 5000 w 200 - 800 people
triple or quad
3,4 or 5 - way 24 - 32 4000 - 10 kw 500 - 4,000 people
Size and Type of system:
The Size refers to the number of sets of speakers on each
side:
The Type refers to how many different sorts of speakers are
used
For example:
a single 3-way may have: 1 W (bass) Bin, 1 mid bin & 1 Horn
(treble), on each side of the stage.
a double 4-way may have: 2 Subs (bass), 2 mids, 2 Horns & 2
sets of Rings (ultra-high) on each side.
There are many variations on this, ie;
a double 3-way might consist of a total of 4 Sub-lows and 4
Composite 2- way boxes. or a double 2-way might be 8 4560 Bins
(low) and 4 horns (hi)
The term ?-way refers to how the active crossover splits the sound
before the amplifiers feed the speakers.
A further example; you could have a system described as being
active 3-way/passive 4-way where an active 3-way crossover feeds
3 sets of amps driving a speaker system that contains subs, mids,
horns and rings, within which there is a passive crossover which
further splits the hi component into hi and ultra hi between the
horns & rings.
A Vocal P A is generally used for vocals only, ie: the room or
crowd is small enough that the instrument amps and drums are
loud enough without additional re-enforcement. Except the Kick
drum is usually also miked due to its poor position (close to
the floor) and its importance in most music forms.
Usually the speaker system would be a 15" speaker and 90° horn
combination, but this is not regarded as 2-way as there is no active
crossover involved and only 1 amp drives the speakers.
(the term 1-way is not used, the usual descriptive term is full-range
box as the speakers handle both low and hi) These speaker boxes
would contain a passive crossover which splits the sound low & hi
between the speaker and horn.
A system such as this could be described as passive 2-way.
CHANNELS:
Number of channels relates to the number of inputs of the
mixing desk. This is usually the maximum number of
microphones. In a full mic up you will consider every vocal,
number of instruments and size of drum kit. For example the
following is a typical mic list:
desk ch. instrument microphone
1 Kick drum AKG d112
2 snare drum Shure 57
3 hi hats condensor
4 rack tom Sennheiser 421
5 floor tom Sennheiser 421
6 over head condensor
7 bass guitar D I Box
8 bass guitar amp Shure 57
9 left guitar amp Shure 57
10 right guitar amp Shure 57
11 keyboard D I Box
12 sax bug
13 left vocal Shure 58
14 centre vocal Shure 58
15 right vocal Shure 58
16 delay return
aux1 left reverb return
aux2 right reverb return
POWER:
These figures are the most used and abused specifications that
you will come across. Since the advent of composite speakers
the conventions of classifying in terms of double, single,
3-way, 4-way etc have been going by the wayside. It seems very
clear cut to use a number to express the power of a system but
you must realize that the Dynamics of a system will be a
function of its Efficiency versus its Power. As yet there is
no clear measurement of efficiency. Surprisingly the more
modern composite systems are less efficient than the larger
old style speaker systems, so a 4 kw composite system may be
equal to a 3 kw double 3-way in terms of Sound Pressure Level.
S P L:
Sound pressure level is measured in deci-Bells. this is an
even worse can of worms as dB s are measured in a logarithmic
scale and the value of SPL measured in dB is also a function
of distance from the source.
PASSIVE / ACTIVE:
Another way of describing systems and the way in which the
sound is divided between the different types of speakers. A
Passive system might have one power amp driving a passive
speaker box containing a high frequency Horn, low frequency
Speaker and a Passive Crossover which divides the signal
within the speaker box. An Active system will incorporate an
Active crossover, which is an electronic processor, wired in
before a number of power amps enabling separate amps to be
used for each type of speaker.
CROSSOVERS (& 2-WAY SPEAKERS):
Capacitors pass high frequencies and resist low frequencies.
Inductors/Coils pass low frequencies and resist high
frequencies.
An RLC circuit has a resonant frequency, the resistance of the
speaker is a determining factor in the behaviour of a passive
crossover.
The crossover frequency is a point 3dB below where the
frequency attenuation drops away or comes back up. This is
because it is not possible to stop dead at a particular
frequency, the frequency attenuation curves away according to
a slope measured in deci-Bell per Octave. It is difficult to
measure the point where the frequency starts to curve away as
it starts as a gentle slope, it is easier to measure where it
has dropped by 3dB. This point is of significance to us as a
drop of 3dB is usually about half the sound power. If both
your high and low speakers use the same crossover point you
should have a fairly even response as both speakers are each
taking half the power for frequencies
surrounding the crossover point.
A steeper attenuation slope is more desirable as this means
that less of the low frequencies go to the high speaker and
less of the high frequencies go to the low speaker.
A steeper slope is achieved by a more complex crossover
circuit. For high powered speaker systems it is more desirable
to use an active crossover which is an electronic signal
processor operating at line level before the power amps
(separate amps for low and high frequencies).
Beware the 240V - 3 phase adaptor!
Many owner/operators employ one of these when 3 phase is
not available. It allows certain equipment, such as
dimmer racks, to operate from 3 power points. Logically
you could assume you could draw 45amps from 3 x 15amp
outlets.
Not so, 3 phase equipment is designed on the assumption
that each leg is out of phase by 120ø and so the neutral
never has to handle more that one third of the total
load. Now, if each leg is on the same phase, the current
loads on the neutral will add (instead of canceling
out) and therefore 3 x 15amp supplies will overload the
neutral return of a 3 x 32amp device by 50%!
My advice: Keep the total load below the single leg
capacity of your device.