System Setup And Gain Structure - dbx DriveRack PA User Manual

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DriveRack PA
Appendix A
A.8 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 Wizard setup
tool to help in system setup. If you use the Wizard to set up your DriveRack PA it will auto-
matically set the limiters for some amplifier selections. If your amplifiers are not available in
the Wizard, you should choose the Custom setting. The following section explains how to max-
imize 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 com-
pressor, and a crossover with output level control. From the crossover, there may be addition-
al filters that are employed to improve the response of your speakers. There may also be lim-
iters 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 ampli-
fier 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 amplifi-
er 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 deliv-
er 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 – many test CDs are available that have these
types of signals) through your console. Turn up the output of the console until it begins clip-
ping. 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
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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 attenu-
ators until the amplifier clip indicator begins to turn on. Turn the attenuators down slightly, so
the clip indicator no longer is on. You have just maximized the gain through your system. This
amplifier setting should give you maximum gain without clipping, another way of say this is
that when the output of your console is clipping you will also be at the clip point of your ampli-
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fiers.
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DriveRack ® PA User Manual
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