Allen&Heath GL2800 - Manual (Page 25)

GL2800 User Guide 25
GL2800 Applications and Operating Tips
The following is a collection of brief descriptions, application notes,
hints and tips to help the operator understand some of the technical
terms referred to, and to get the most out of the uniquely capable
GL2800 live sound mixing console. They are written as a concise
reference to spark your imagination to creatively and effectively deal
with the many challenges now faced in modern day mixing.
Gain Structure The term used to describe the gain (level)
matching of the signal through the audio system chain. Each item in
the equipment chain has its own optimum (‘nominal’) operating level
determined by its electrical circuits. If you overload it with a signal that
is too high then its output tries to produce more voltage than its power
rails can provide resulting in harsh sounding distortion as the audio
signal is ‘clipped’. If you work with a signal that is too low then you
are likely to hear the residual hiss of the circuits as the ‘noise floor’ is
amplified along with the signal. The optimum operating level is the
point at which the signal is high enough above the noise floor so that
the hiss is not heard (good ‘signal-to-noise’ ratio, the SNR), yet
provides enough space (‘headroom’) before clipping to allow for the
louder, dynamic musical moments without distortion. ‘Dynamic range’
specifies the maximum range between noise floor and clipping. It is
the sum of the SNR and headroom. The larger the figure the better…
less noise, more headroom, more forgiving to wide ranging signals.
A microphone produces a very tiny signal, for example -50dBu (a few
millivolts). This should be amplified by the channel preamp to the
optimum operating level of the console circuits, around 0dBu
(0.775V), well clear of the noise floor (typically less than -90dBu), and
with good headroom (typically clipping at greater than +20dBu). With
a channel routed at unity gain from preamp input to main output, the
GL2800 boasts a massive 116dB dynamic range. The signal can
pass through many stages which affect its level within the console; the
channel, its EQ, fader, pan, a group mix and its fader, on to the main
LR mix with its master fader, through the matrix, and of course any
inserted equipment. The output XLR provides the professional
standard +4dBu (1.23V) nominal level to the next stage in the audio
chain, typically a speaker processor such as limiter or crossover, or
straight to the amplifier. The amp boosts this signal to tens of volts to
move the speaker cones so producing the audible sound.
To get the best performance from your system, it is important that you
set up the gain structure correctly within the console and also within
and between the connected equipment. Ideally, each circuit should
be set to clip at the same time if the signal became too hot, in other
words each would have similar headroom relative to its nominal
operating level. The resulting dynamic range is the difference
between the highest noise floor and the lowest headroom through the
system. The performance is only as good as the weakest link.
The GL2800 provides comprehensive metering to check every
point within the console signal path. Use the channel meters and
PFL/AFL system to set the gains and mix levels to average around ‘0’
meter reading with usual peaks around ‘+6’. Reduce the gain if the
red ‘+16’ or ‘PK’ indicator lights.
Use the equipment meters to set each item within the system path to
operate at its nominal level. We advise that the speaker processor or
amplifier trims are set so that the console outputs can drive up to their
nominal ‘0’ level. It is a common mistake to set amplifier trims to
maximum sensitivity when this results in the console master faders
being operated at very low positions, with low meter readings, and
reduced dynamic range and therefore audible residual system hiss.
+1
+3
+6
-12
-16
-9
-6
-3
-1
-20
0
+9
-30
-40
-50
-60
-70
-80
-90
+12
+16
+20
SIGN
AL-
TO-
NOISE
RATIO
DYNA
MIC
R
A
NGE
HE
AD
RO
OM
CLIPPING
NOISE
NO
R
M
AL OP
E
R
ATI
N
G
R
A
NGE
SIGNAL
!
☺
A search on the Internet
can reveal a host of additional
information on setting correct
gain structure and many other
audio topics and educational
resources.