dbx DriveRack 260 User Manual page 82

DriveRack
Appendix A ®
Appendix
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 – many test CDs are available that have
these types of signals) 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 260 and into the amplifiers with the crossovers and output
gain section in the DriveRack 260 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 attenuators 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 amplifiers.
Once you have found the clip point of your amplifiers, you can mark this position and turn
the amplifiers back up to the point where they are clipping. You can now use the output
limiters in the DriveRack 260 to protect the amplifier from clipping no matter what you do at
the console. With the amplifiers clipping, now go to the Limiter page of the DriveRack 260
and turn the limiter for each output band on. Make sure that the threshold is all the way up
to +20dB. Now slowly reduce the threshold until the clip indicator of the amplifier turns off.
You have successfully set the output limiter up and the amplifier will not be able to go into
clipping no matter what the input is. You should now proceed to the next output band and
go through the same process. If you are using a sine wave to create clipping in your amplifier
you should make sure that the sine wave is within the frequency range of the output band that
your are working with. Once all the limiters are set up you can now turn the amplifiers back
down to the gain maximized position. The Wizard will automatically set your limiters based on
your choice of amplifier. Because of component variances within the amplifier circuitry, you
may need to adjust the limiter settings by a few dB after running the Wizard to ensure that
they are set correctly. Once all the limiters are set up and the gain is maximized, reduce the
output of your console and reconnect your speakers. Now for the fun part, sit back and send
your favorite music through the system. If your system is not loud enough, you may want to
consider an amplifier or amplifiers with greater output power. Most loudspeaker manufacturers
recommend an amplifier that can provide 1.5 to 2 times the rated RMS power of the speaker.
If this setting is too loud when your speakers are reconnected you can turn down the amplifier
input attenuators even more.
74 DriveRack® User Manual