dbx DriveRack PA+ - Manual (Page 37)

Section A
Appendix
33
DriveRack
®
PA+
A.9 System Setup and Gain Structure
The DriveRack® PA+ offers a wide range of tools for sound system design and setup. These
tools can make your system more efficient and better sounding, but to get the best possible
sound it is important to use these tools properly. In the DriveRack PA+ we have included
a Setup Wizard tool to help in system setup. If you use the Setup Wizard to set up your
DriveRack PA+ it will automatically set the limiters for some amplifier selections. If your
amplifiers are not available in the Setup Wizard, you should choose the Custom setting.
The following section explains how to maximize system gain and how to use the limiters to
protect your amplifiers from clipping.
In traditional system design, the output of your console would be routed to a system EQ,
a compressor, and a crossover with output level control. From the crossover, there may be
additional filters that are employed to improve the response of your speakers. There may also
be limiters set up to keep your amplifiers from going into clipping and protect your speakers
from the hazards of a clipped signal. Your amplifiers play a vital role in system setup, because
they are last item in the chain before your speakers and offer the greatest amount of gain
(that is their job after all). If your amplifiers are incorrectly setup you will not be using your
system to its fullest potential and could be harming your speakers.
One thing that is critical to system setup is maximizing gain structure. Gain structure refers
to aligning the gain of each device so that they all clip at the same point, and the noise
floor of the entire system is at its absolute minimum. Quite often PA systems are setup with
the amplifier input controls turned all the way up in the incorrect assumption that this is the
only way to get the maximum output level. 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 over +18 dBu of output level before clipping. This means that with your
amps tuned all the way up you are sacrificing 12 dB of headroom, resulting in poorer noise
performance and the potential of system clipping. By adjusting the amplifier controls properly,
you can maximize your system performance.
A way to set up your amplifiers for maximum gain structure is to use the clip indicators of the
console and amplifiers themselves. Disconnect the output of the amplifier from the speakers.
Run a continuous signal (pink noise or sine waves) through your console. Turn up the output
of the console until it begins clipping.
If there is no clip indicator on the console then use the output meters; most reputable
console manufacturers use red LEDs at the top of the meters to show the onset of clipping.
Once the console is clipping, back the output gain down slightly until the clip indicator turns
off. Run this signal through the DriveRack PA+ and into the amplifiers with the crossovers
and output gain section in the DriveRack PA+ set for your particular speakers. Make sure that
the output limiters are turned off. Run this signal into the amplifiers and turn up the input