User's Manual

20
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your system to its fullest potential and could potentially cause damage to your loudspeakers. When you select your amplifiers
in the Setup Wizard, the PA2 will automatically set the limiter thresholds and gain structure between the PA2 and amplifiers. If
your amplifiers are not available in the Setup Wizard, you should choose the NOT LISTED option. The following section explains
how you would go about manually optimizing the system’s gain structure and set the built-in limiters to protect your amplifiers
from clipping.
Gain structure refers to aligning the gain of each device so that the input circuits of all devices clip at the same time – this
allows you to know exactly how much headroom you have in the entire system by simply looking at the mixer’s main output
meter and optimizes the noise floor of the entire sound system. Quite often PA systems are setup with the amplifier input
attenuator controls turned all the way up, in the incorrect assumption that this is the only way to get the maximum level out of
the sound system. Setting up your amplifiers in such a manner can help prevent someone from raising your amp attenuators
and damaging the system (this is sometimes required for permanent install applications and requires the output gain be
reduced in the device feeding the amplifier), however, your noise floor will likely suffer in doing so.
Amplifiers are fixed gain devices, turning down the amplifier input attenuators does not change the potential output of the
amplifier – it only requires more input voltage to get full output power. Many amplifiers will clip with an input level greater than
+6 dBu when the input attenuators are turned all the way up. Most mixing consoles can deliver well over +18 dBu of output
level before clipping. This means that with your amps turned all the way up, you are sacrificing 12 dB of headroom, resulting
in poorer noise performance and the potential risk of clipping the amplifier. By adjusting the amplifier controls properly, you
can maximize your system’s performance and protect your loudspeakers. The following diagram illustrates the previous
example and shows how it can be easily remedied by simply lowering the input attenuators on the amplifier to apply 12 dB
of attenuation, effectively lowering the signal level entering the amplifier by 12 dB and fitting the signal within the operational
headroom constraints of the amp.
Before
Mixer master output faders
set to unity gain (performance level)
Mixer Max Output
Level: +18 dBu
Amplier Max Input
Level: +6 dBu
Amplier attenuators
turned all the way up
0
Amp clips 12 dB
before mixer!
12 dB dierence
+18 dBu
+18 dBu
+6 dBu
After
+18 dBu
+18 dBu
Mixer master output faders
set to unity gain (performance level)
Mixer Max Output
Level: +18 dBu
Amplier Max Input
Level: +18 dBu
Amplier attenuators lowered
to apply 12 dB of attenuation
0
Both amp and mixer
now clip at +18 dBu
One way to set up your gain structure is to play pink noise through the entire system and adjust each gain stage in the signal
chain in succession using the clip indicators on each device. If there is no clip indicator on your mixer then use the output
meters; most reputable console manufacturers use red LEDs at the top of the meters to show the onset of clipping.