OK. You need a PA system for your band or church group.
you're new to it all and see a variety of boxes with more buttons, indicators and knobs
than the flight control deck of a jet fighter. You don't want to buy the wrong thing and
it's a large amount of money we're talking about here. People around you are using words
like 'Impedance' and 'compressor' and acronyms like 'dB SPL'. Scary stuff...
So I'm going to demystify it for you and let you in on some
of the basics you have to know.
What is a PA?
PA is "Public Address", an amplification
system which takes a source signal (speech or music) at the input and makes it louder at
the output. In it's most basic form, a PA system is composed of 3 main components - a
sound source, an amplifier and a loudspeaker.
The sound source produces an electrical signal that
represents the sound. The sound source can be a microphone or a playback device such as a
The amplifier is used to increase the level of the electrical
signal from the sound source so that it can be heard at sufficient volume from the
Finally, the loudspeaker is the device which converts the
electrical signal from the amplifier into mechanical energy (sound).
Of course in the real world we often need to amplify more than one
source. The mixer is used to mix the sources together and use just the one amp and speaker
for all of them.
Let's now take a look at each of these components in more
Sound sources vary widely. They can be microphones for capturing sounds
from voices or instruments. They can be from a recorded media such as CD or cassette. Or,
in broadcast situations, they may be from a radio transmission or the telephone. Whatever
the source is we need to understand its properties so that we can connect it properly to
our PA system.
Microphones are essentially the reverse of a loudspeaker,
where the vibrations in the air cause movements in a small diaphram in the microphone.
Microphones usually produce a very weak signal which needs an extra stage of amplification
before it can be sent to an amplifier. This is called a pre-amplifier. These are found in
most mixers, but they can also exist as separate units in some cases. There are many
microphones available, but that is a subject for another article.
Line Level Sources refer to the group of devices whose output
is at a suitable level to be connected straight into the amplifier. These devices require
no pre-amplification, although it is still common to put them through a mixer first.
Devices in this category include CD players, cassette decks, keyboards and vamplifier
As we said earlier, amplifiers take a line-level signal and boost it to
the levels required to drive a loudspeaker. We refer to all these amplifiers as power
amplifiers as they handle the high powers required to drive loudspeakers. Here are
some of the facts you need to understand to ensure that the correct amp is chosen for your
Output power is the main parameter you will pay attention to when
selecting an amplifier. Output power is expressed in Watts - e.g. 500 Watts (500W). In the
case of a two channel amp (stereo), this is usually per channel. Care must be taken here
as the power delivered by an amplifier depends on the Impedance of the loudspeaker being
driven by the amplifier.
Impedance is a measure of the resistance offered by the loudspeaker and is
expressed in Ohms. The higher the impedance of the speaker, the less power will be
delivered by the amplifier (think of doing any kind of work - the more resistance to the
work, the less work that gets done). When reading the specification for an amplifier the
power will be stated for a specific Impedance - e.g. 500 Watts into 4 Ohms, 250 Watts into
8 Ohms. If only a single figure is quoted then assume this is for a 4 ohm load, which is
the industry standard. DO NOT DRIVE AN AMP AT LOWER THAN RATED Impedance - there will be
much wailing and gnashing of teeth as your amp and speakers burn out. As most loudspeakers
have a standard 8 Ohm load rating, this means you need to know a little about...
The loudspeaker is the final link in the chain. In essence it is a very
simple device, essentially a box or cabinet containing one or more drivers (speakers) that
produce the actual sound. Different combinations of drivers and cabinet construction
methods produce speakers with different characteristics, some made for volume, others made
for quality, some even made for price. Lets now take a look at the components of a
Basically the box in which the drivers are mounted. However, it plays an
important part of the overall sound of the loudspeaker.
Cabinets can be constructed from wood, fibreglass or plastic.
An effort is made to use materials which are as rigid as possible so it doesn't start
vibrating when the speaker is in use as this affects the sound. Bracing (struts for
structural reinforcement) aids with this goal in better cabinets.
The way the cabinet is put together is also very important.
The dimensions, volume and construction techniques all affect the sound. Most cabinets
will have a hole, known as a Port, that will allow the air to move freely in and out of
the cabinet as the driver cones move. This extends the bass response of the cabinet. True
cabinet design can be very scientific these days, often computer assisted. A good cabinet
design can make a cheaper set of drivers sound good, but the reverse is also true - a good
set of speakers can sound terrible when slapped haphazardly in a chipboard box.
Drivers are the components that produce the sounds. They are generally
comprised of a coil of wire sitting within a magnetic field. As electric current flows
through the coil of wire it moves within the magnetic field. By attaching a diaphram to
the coil, we can make a volume of air move at the same time and produce sound waves.
A single driver can be capable of producing the full range of
audible sounds (you do get "full range" drivers in the smaller sizes). In
practice, however, multiple drivers are more common in a loudspeaker for efficiency
reasons. A two-way system would have a low-to-mid range frequency driver and a
high-frequency driver too. One of the reasons for separating these out is that high
frequencies are more directional. High frequency drivers are normally mounted along with a
horn in order to control the direction. If you take a look at the specification for a
loudspeaker, you should find a Dispersion property. This indicates the angle at which the
high frequencies leave the loudspeaker. Narrower dispersion angles (30° - 60°) are more
suited to long-throw speaker applications whereas wider angles (90° - 100°) are better
When a loudspeaker has more than one driver we need a way to split the
frequencies so that the low frequencies go to the low frequency driver and the high
frequencies to the high frequency driver, etc. This is done using a Crossover.
A crossover splits an incoming audio signal into its
component frequency bands. Most two-way or three-way loudspeakers have an inbuilt
crossover network that does this job. This is known as a "passive" crossover (as
it needs no additional power). More advanced systems use an external crossover known as an
active crossover. This works slightly differently as it is inserted before the amplifier.
In this case, you will require additional amplifier channels to run each individual
driver. This technique is referred to as bi-amping or tri-amping (depending on whether the
sound is split into two or three frequencies). Such systems tend to sound cleaner, more
detailed and controlled with each driver being run by its own dedicated amplifier channel.
The down side is the expense as more amplifiers are required as well as the
Impedance is a measure of the load that the speaker represents to the
amplifier, and is measured in Ohms. Most speakers have an Impedance of either 4 Ohm, 8 Ohm
or 16 Ohm, with 8 Ohm being the most common by far.
But what happens when you want to connect two or more
loudspeakers to the same amplifier channel? Does the impedance change? Yes,
depending on whether they are connected in series or parallel. When this happens the
following maths applies:
So, with parallel loudspeakers, the impedence decreases and it increases with
To deal with more than two loads you just keep on adding
terms to the right. So two 8 Ohm speakers in parallel would give a total of 4 Ohm and
three 8 Ohm loads would give you a total load of 2.67 Ohm. Two 8 Ohm speakers in series
would give a total of 18 Ohm and three would give you a total load of 24 Ohm.
While having too high an impedance will do no damage to an amplifier, be
careful not to make the load too small. Most amplifiers have a minimum load that they can
drive, and going below that minimum causes the amp to try and put out more power than it
is rated for, and it may then distort. And distortion kills loudpeakers more often than
too much power.
The Mixer (A.K.A: "Mixing Desk",
Of course in the real world we often need to amplify more than one source. One
option is to have more amps and speakers, but this starts getting unwieldy and expensive
as the number of sources rises. The other solution is to mix the sources together and use
just the one amp and speaker for all of them. Enter the mixer.
In its simplest form a mixer has several inputs and a single
output where the combined signal comes out. To give us a little more control, individual
level (volume) controls are fitted to the inputs and a master level control to the output.
This then allows us to not only control the overall output level from the unit but also
the balance between the individual inputs. Most mixers have more functionality than this
and we need these extra functions when setting up a PA system.