In the beginning there was sound...!
G'day welcome to the fifth of our meandering series, following
the signals long journey from the stage to the speakers. This time
around we will take a look at what takes the signal out of the mics
and on to the next link in the audio chain
the various leads
The mixing desk - or any other bits of gear in the signal path
with adjustable gain - will amplify any 'noise' in the system by
the same amount as the audio signal running down the line. As well
as carrying the audio signals, the aim of any lead or circuitry
is to reduce the amount of 'noise' in the system.
All cable used for carrying signals (other than those connecting
the power amps to the speakers), should be screened. Screened cable
can always be recognised by its coaxial construction where one or
more inner conductors is enclosed in a 'screen'. The screen intercepts
any interference noise - and drain it away to earth before
it can affect the signals passing along the inner wires.
The screen itself may be formed from woven copper braiding, metal
foil wrapping, or it may even be made from conductive plastic. By
far, the most common type of coaxial cable used for audio is 'braided-screen'
cable as they offer excellent screening efficiency, combined with
When it comes to cabling, it pays to have the right connections.
The major downside of braided-screens are that they are a pain
in the arse and time-consuming to work with. To make a connection,
you either have to unpick the last half inch or so of the screen,
or part the strands enough so that you can pull the inner conductors
through the side of the screen, about half an inch from the end.
You then have to twist together the strands of screen so that they
can be soldered to the appropriate terminal in the plug. It is due
to their excellent screening properties that braided-screen cables
are useful in situations where long cable lengths are needed, but
where flexibility is essential. Eg: mic cabling. Braided-screen
cable is also used extensively in professional patch leads and instrument
Balanced or Unbalanced
and I don't mean psychotic!!
The simplest difference between Balanced and Unbalanced leads and
circuits is the number of conductors inside them.
'Balanced' circuits and leads offer the best method of minimising
external noise in a PA system.
A balanced lead carries the signal on two conductors + shield.
The signals on each conductor are the same level but opposite in
polarity, i.e. one is positive when the other is negative. (In technical
terms, it is 180 degrees out of polarity, Pin 2 provides a positive
voltage and pin 3 provides a negative voltage when the microphone
diaphragm receives a positive pressure wave that pushes the diaphragm
inward. Thus, pin 2 is "hot"; pin 3 is "cold".)
A balanced input amplifies only the difference between the two
signals and rejects any part of the signal which is the same in
both conductors. Any noise or hum that is picked up by a balanced
- 2 conductor cable - tends to be identical in both conductors and
is therefore rejected by the Balanced Input, while the equal but
opposite polarity original signals are amplified.
[Technically, balancing is known as 'common mode rejection' or
a differential input.]
The screen is there to intercept any external RF interference and
drain it away to earth before it can affect the audio signals passing
along the inner wires.
Inside Unbalanced Leads are one conductor and a braided Shield.
They have the HOT (+) signal running down the centre wire, and the
COLD (-) and the GROUND (Earth) running down the outside shield
together. They are unable to reject any electrical noise or hum
that has been picked up by the lead and amplify any signal present
in the conductor.
Unbalanced leads are extremely susceptible to picking up noise
and as such, they should only be used for short cable runs under
3 metres, eg. FX patch leads to and from the mixing desk. A long
length of Unbalanced lead can act as a giant aerial, picking up
all sorts of electrical noise and radio frequencies (RF), all of
which will be amplified the same amount as the audio signal running
down the line. Ever been to a gig when suddenly you hear Taxi radios
blaring through the PA? This is usually the result of RF picked
up by guitarists unbalanced pick-ups or leads.
If an unbalanced lead is being made up from a 'two-core-n-shield'
cable, one of the cores is not needed. Connect the spare core to
the screen at both ends of the cable or just cut off.
Combining Balanced and Unbalanced Circuits.
Sometimes it is necessary to run from a Balanced 3-pin connector
circuit to an Unbalanced 2-pin circuit, or vice-versa. For example,
when running leads in to and out of effects units, or from a guitar
jack output to a 3-pin XLR connector. (Technically we are 'interfacing'
Balanced to Unbalanced!)
This is done easily by wiring the HOT lead to the centre pin of
the guitar jack and twisting the SHIELD and COLD together and wiring
them together to the other leg of the guitar jack
Europe vs. USA/Japan - DANGER!!!.
On three-pin XLR connectors it is universally agreed that Pin 1
is Earth/Ground, but for a long time there was a dispute between
Europe vs. USA/Japan about whether Pin 2 or Pin 3 was Hot !!
This causes mega problems when mixing Balanced with Unbalanced
circuits. For example, if Pin 3 is being used as HOT, and #1 and
#2 wires are twisted together to form an Unbalanced line, and this
is plugged into a piece of gear that has Pin 2 HOT, the HOT signal
will run down the Earth/Ground braided shield - picking up massive
amounts of noise. NOT GOOD!!
Nowadays, the universal standard is Pin #2 hot. If
you are using older and new gear or are unsure what the wiring standard
is, look up the manufacturers manual or on the specification plates
on the back of the gear, they usually have a little diagram telling
us 'what's hot & what's not.'
No matter how good your cable, an audio link is only as good as
its connectors. Don't skimp on quality, you'll pay for it in the
long run with increased noise and crackle and general unreliability.
Make sure that any cable clamping system that may be fitted is used
properly, especially if the cables are going to be constantly plugged
and unplugged. Most of our hassles with leads happen as a result
of stress on the soldered connections inside the plug due to inadequate
or non-existent clamping. The clamp should always grip the outer
insulation of any type of cable, to take the mechanical stress.
(In particular watch the power plugs. In the event of gear failure,
if the earth lead has come loose, you become the path for the current
to take to ground!!!!) Regularly take the covers off your leads
and inspect the solder joints for cracking or 'dull' appearance.
If so, resolder, easy!!
When using off-the-shelf adapters to convert circuits
and connectors, signal loss will occur at every converter site as
a result of changes in resistance and capacitance from one connector
to another. Use sparingly!! Preferably, get off your butt, go and
buy the best quality connectors you can afford, and wire-up your
own. This way you can make something up that's usually far more
quiet, reliable and stronger/tougher than anything pre-made.
Last of all, it is in our own best long-term interests to avoid
having our leads in a big pile of 'spaghetti.' No matter what type
of cable and connectors you have, the less kinks 'n' twists and
general plugging and unplugging the better. Keeping cables tidy,
in the end, saves us time, $ and potential embarrassment at a gig!
Well thats about it for this link in the chain. Next time
we will be looking at the polarity and phase related aspects of
In the meantime, if you've got any audio questions give me a yell.
Send me an email or post a message on ROADTALKZ. [I'm not the font
of all knowledge, but if I don't know the answer to your questions,
I know enough tech guru's who will!!]
Until then, have-a-good-one!!