Transcript gain - Bek Varcoe

Introduction to Live
Inputs
and
Gain
Sound
Structure
• Gain structure
• Balanced vs Unbalanced
• Mic or line level?
FOH- "Front of
House"
A front of house operator in live
audio controls the Left and Right
outputs of the console to the
speakers that the audience hears.
Some of the time, a band's
foldback or monitor mix is
controlled from the same console
as well.
This is done by
taking the various
inputs from the stage
(a DI or mic input)
and connecting them
to the console with
XLR connectors,
usually using a
multicore.
Multicores replace a large
number of XLR leads running
to the FOH, which would be
very messy.
XLR Trademarked name for circular 3-pin connectors
What does
XLR "XLR"
stand for?
developed by Cannon (now owned
by ITT).
was
originally nothing more than Cannon's part designation
for the connector, which meant "X Series," "Latch," and
"Rubber." In fact, you'll also sometimes see these
connectors referred to as "Cannon" connectors. XLR has
since evolved into a generic industry term, and many
manufacturers now make this style connector. In audio
work, XLR connectors are normally used for transmitting
Gain
Structure
Typically the first thing an engineer
will do once everything is set up
and plugged in onstage is do a line
check, and set gain levels.
They start with the initial point that
signal can be be controlled, at the
gain pot at the top of each
channel.
The PFL button can be engaged at
this point to give a more detailed
reading on a large meter.
Gain Structure
How much an electronic circuit
amplifies a signal is called its "gain".
In most specs or references you will
see gain expressed as a decibel
value (i.e. 6 dB of gain).
Occasionally, you may sometimes see
gain expressed as a straight numeric
ratio (a voltage gain of 4 or a power
gain of 2).
In a console, pre-fade listen is a very
useful tool for listening to signals
Gain
Structure
individually
without
disturbing the
general mix.
PFL generally sends a signal to
monitor outputs regardless of the
setting of that channel's fader, and
simultaneously mutes the other
channels.
In other words, PFL allows you to solo
a channel even if the fader is pulled all
So when setting gain levels, a good
procedure to follow is to
Gain Structure
• Hit PFL
• wind up the gain as the musician is playing until
you have a good solid 0 average on the main
meter
• Disengage PFL and slowly push the fader up until
it sounds right/is loud enough
Do it this way until it becomes
automatic - guarantees you never
Unity Gain
A device or setting which
does not change signal level
(amplify or attenuate the
signal) is said to be at "unity
gain."
In practice, unity gain is often a
desired setting for maintaining
gain staging, and for optimizing
operating levels and signal to
noise ratios. Correct gain
settings at all points in the
signal chain is very important
to a good mix. Mixers call it
good "gain structure".
• Amplify: to increase the
level of a signal
• Attenuate: to decrease
the level of a signal
• Check if the pad switch is in,
Can't see any signal?
• Check you are looking at the right channel,
• Check that the mic/DI is actually plugged in,
• Check that the multicore is plugged in correctly
at the back of the console.
Still nothing? Follow the chain and eliminate one
thing at a time: swap the mic, then the lead,
then change channels on the stage box, then
change channels on the desk. Do it in logical
sequence so it's easy to remember what
you've tried. Sometimes this is crucial when
the pressure is on and people are screaming at
If you can see the signal on PFL but can't hear
it, that means you check a different end of the
chain, ie: post channel.
• Is the channel bussed to LR?
• Are the outputs of the console connected?
• Do they run to the graphics?
• Are the graphics switched on?
• Are the outputs of the graphics connected to
the amps and are the amps on and up?
Signal leads carry
voltage
XLR leads carry voltage which then hits the
preamp in the desk, which we control with the
gain pot. They carry AC voltage, or alternating
current which flows two ways/ has two polarities,
and we call them positive and negative. The
XLR lead also has an Earth or ground
conductor, which carries interference and
passes it down to Ground.
Balanced vs.
Unbalanced
So, the term balanced refers AC electrical signal
having two "legs" independent of ground. One leg
is generally considered positive (+) and the other
negative (-) in voltage and current flow with
respect to ground. Unlike unbalanced audio lines
there is no "signal" carried in the shield or ground
connection (unless there is a fault.)The main
benefit is that any noise that gets induced into the
line will be common to both the positive and
negative sides and thus canceled when it arrives
at its destination, assuming the destination is
balanced.
Balanced vs.
This phenomenon is called "Common Mode
Rejection" and
basically just means that any
Unbalanced
signals common to both the positive and
negative legs of balanced lines get canceled.
This happens because when the receiving
device looks at the signal the common noise
actually shows up as out of phase with itself,
and gets cancelled.
Balanced lines are generally much better for
long cable runs due to their ability to reject
induced noises. XLR and TRS type cables are
designed to transmit balanced audio from one
balanced device to another. A standard 1/4-inch
These 6.5 jack plugs come in unbalanced and
balanced.
Mic Level vs Line
Level
In order to set the correct gain coming in to your
console, you might want to think about whether
your input source is at mic level or "line level".
Mic Level
The level (or voltage) of signal generated by a
microphone. Typically around 2 millivolts.
Compare this with the two normal line levels
(1.23 volts and .316 volts), and it becomes
apparent just how much amplification is going
on in a microphone preamp, and why it is
essential that preamps be of as high quality as
possible!
Literally, the average voltage of an electronic
audio signal. While technically any voltage over
25 millivolts RMS is considered line level, in the
modern audio world we narrow the scope a bit to
the two line level references in use today:
Line Level
Balanced "pro" gear runs at around +4 dBm (1.23
volts), while unbalanced "semi-pro" gear operates
at approximately .316 volts (-10 dBV). "Pro" and
"semi-pro" may be almost meaningless terms
anymore, but the two operating levels must still be
dealt with.
The important thing is to match the levels of the
gear you are using so that -10 equipment isn't
+4dBm
-10dBm