dbx DriveRack VENU360 Loudspeaker Management System - Manual (Page 55)

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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 at least 12 dB of headroom,
resulting in poorer noise performance and the potential risk of clipping the amplifier and damaging loudspeakers. By adjusting
the amplifier controls properly, you can maximize your system’s noise 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 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
Amplifier Max Input
Level: +6 dBu
Amplifier attenuators
turned all the way up
0
Amp clips 12 dB
before mixer!
12 dB difference
+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
Amplifier Max Input
Level: +18 dBu
Amplifier attenuators lowered
to apply 12 dB of attenuation
0
Both amp and mixer
now clip at +18 dBu
One way to calibrate the 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.
To calibrate the system’s gain structure and VENU360 limiters:
WARNING! Although it is highly unlikely that you are using tube amplifiers, since they are not practical for sound
reinforcement use, please note that most tube amplifiers can be damaged if operated without a load (the speaker)
connected. Therefore, do not perform the following procedure if using tube amplifiers unless you have verified they can be
operated without a load connected. This is not an issue with modern solid state amplifier designs.
1� You will need to send a pink noise signal through the entire system to perform this calibration procedure. Although
the VENU360 does have a built-in signal generator capable of generating pink noise, this will not help with optimizing
the gain structure between the mixer and VENU360 as the mixer’s output meters need to be active and available for
reference. Some mixers have a built-in pink noise generator, which will work. You can also check your smartphone’s app
store, as there are many audio apps now available which have a built-in pink noise generator, or search online for a pink
noise sample and burn it to a CD or load it into your portable music player or smartphone.
TIP: The VENU360’s built-in signal generator can be used for calibrating gain structure between the VENU360
and amplifiers. In many cases, calibrating the gain structure between the mixer and VENU360 may not be absolutely
necessary as you can figure out where to set the VENU360’s analog input clip levels and input Router/Mixer gains
based on the mixer’s max output level spec alone. For example, if your mixer’s maximum output level spec is +26 dBu,
the VENU360’s analog input clip levels (located in the Utility menu) can be set to +28 dBu then the input Router/
Mixer gains set to +2 dB. The mixer outputs and VENU360 inputs will now both clip at the same level (+26 dBu).
2� Once you have your pink noise signal, power down the sound system, disconnect all the loudspeakers from the
amplifiers, and turn all amplifier attenuators all the way down.
3� Now, turn the mixer, VENU360, and amplifiers back on. Go into the VENU360’s limiters and ensure they are all turned
off, OVER EASY is turned off, AUTO is turned on, PEAK STOP PLUS is turned on with OVERSHOOT set to 3, and their