Allen&Heath ML3000 USER GUIDE - Manual (Page 19)

ML3000 User Guide 19
Gain Structure
How the levels between the different signal stages
are set up is referred to as the gain structure. For
best performance it is important that the connected
source signals are matched to the ‘normal operating
level’ of the console. Similarly the levels of the
connected amplifiers and destination equipment
should be correctly matched to the console outputs.
If set too high then the signal peaks will be clipped
resulting in distortion, and if set too low then the
signal-to-noise performance will be degraded
resulting in excessive background hiss and noise.
Using the Meters. The ML3000 provides metering
of inputs and outputs. For best results operate the
console with the LED bar meters averaging around
‘0’ allowing the loudest moments to reach ‘+6’.
Reduce the gain if the red peak LEDs start to flash.
Note that the peak leds light 5dB before actual
clipping to warn that you are nearing distortion and
should reduce gain. The LED bar meters have a
peak response with fast attack and slow release so
that fast musical transients are accurately
displayed. The VU meters have a slower attack so
that the average levels are better displayed. Both
types of metering are useful in live sound mixing.
Matching a Source to the Console. Start by
turning down the channel fader and send levels to
prevent unexpected loud volumes reaching the
main speakers and monitors. Adjust the GAIN
control for an average ‘0’ reading on the channel
meter. Press PFL to listen to the signal using
headphones, local or wedge monitor, and to view its
level on the main LED and VU meters. Once the
gain is correctly set you can raise the levels to bring
the channel into the mix. Note that you may need
to adjust the gain if you make significant changes to
the EQ. Make sure that any equipment inserted
into the channel is set to operate around 0dBu line
level. It is best to first set the gain with inserted
signal processors such as compressors switched to
bypass.
Matching the Console to Destination Equipment.
The console produces a standard XLR output level
of 0dBu for a meter reading of ‘0’. It can produce a
maximum of +23dBu and is therefore well suited to
driving equipment operating at nominal 0dBu or
+4dBu while providing plenty of headroom. If you
are connecting directly to a sensitive power
amplifier it is advisable to turn down its input trim
control if the normal console level is too high.
Simply turning down the console output faders
degrades the output stage noise performance and
reduces the resolution of the fader movement. The
output faders are best operated around ‘-10’ to ‘0’
for loudest average volume required. This allows
plenty of additional headroom if you need it.
Terminology. The normal operating level is the
optimum signal level for best console performance,
indicated by ‘0’ meter readings and resulting in the
0dBu output level. The channels operate at 0dBu
and the mix stages at –2dBu for extended
headroom. Headroom is the extra level available
above normal to allow for loud peaks before the
signal becomes clipped resulting in audible
distortion. The signal-to-noise ratio (SNR) is the
difference measured in dB between normal level
and residual noise floor (hiss) produced by the
console electronics. The dynamic range is the
sum of headroom and SNR representing the
maximum signal range possible from quietest to
loudest.
Using the VCA Groups. Assigning a channel to
one or more VCA groups lets those group faders
control the level of its VCA element. Each fader
provides up to +10dB boost. Note that the channel
VCA allows a maximum combined fader boost of
+10dB. Any more is simply ignored. It is best to
operate the VCA group faders around their nominal
‘0’ position. You can also use a VCA group to
reduce the overall level of a hot mix without having
to adjust all the channel faders.
Final word… A little care with setting gain
structure throughout the signal chain will give you
the very best performance and most manageable
control of the mix.
INPUT GAIN INSERT
FADER
VCA
CHANNEL
0dBu
+10
+20
+30
-10
-20
-30
-40
-50
-60
L R LR C
MIX
FADER INSERT
DIR OUT
EQUALISER
-90
0dBu
+/-15dB +60/-10 dB
0dBu
PAN/BLEND
-3dB law
VCA GROUPS
OUTPUT
VU
OUTPUT
MIX CHANNEL
HEADROOM
signal-to-noise
+23dB
-90dB
+23dBu
0dBu
-90dBu
DYNAMIC RANGE
113dB
NOISE FLOOR
CLIPPING LEVEL
SNR -2dBu
MIX HEADROOM
+23dB