dbx DriveRack PA2 - Manual (Page 23)

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subs, you can perform the same test between the low driver in the main speaker and sub to ensure polarity for all drivers are
matched. You will also want to make sure that all drivers/speakers reference absolute polarity. This means that all drivers would
push outwards then inwards during each cycle. If the absolute polarity is reversed, the drivers would move in the opposite order
(inwards then outwards). If using subs, you can use the subs for absolute polarity reference – just make sure the low driver in
the mains matches the sub polarity then match the high driver to the low driver in the mains. If you’re using bi-amped speakers
with no subs, more extensive testing may be necessary to determine each driver’s absolute polarity.
Once polarity is matched for all drivers in the system, you’re ready to optimize the crossover filter types and driver alignment
delays. The “polarity” test mentioned in the previous paragraph can also be used for determining which crossover filter types
work best with your speakers and if driver alignment delay is necessary. Play the selected sine tone through the system (once
again, the sine tone frequency should match the set crossover frequency of the drivers being tested) and invert the polarity on
one of the two drivers being tested – as previously mentioned, this test works best if the sound pressure levels of both drivers
are matched. Adjust the driver’s levels until the most phase cancellation is achieved. Try adjusting the driver alignment delay for
each driver to see if any further phase cancellation can be achieved – set it for the most phase cancellation. Note that most
applications won’t require any driver alignment delay. For more information on setting driver alignment delays, see ‘Delay’ on
page 39.
Once you’ve dialed in the driver alignment delay, perform this same “polarity” test while auditioning different combinations of
filter types in the PA2’s crossover to find the combination of settings which provide the most phase cancellation. When done,
make sure to switch the polarity back to the proper setting so the drivers will again be in phase.
Changes to all these settings can be difficult to judge when heard in the context of the whole system, but by zeroing in on
the specific crossover frequency, matching levels, and inverting polarity on one of the drivers, you are free to experiment
with different settings to determine the best settings for your loudspeaker system. Adjust each of these parameters until the
most signal cancellation is achieved then switch the polarity back so the drivers are once again in phase. Maximizing these
parameters using this test will allow your system to work in harmony and improve its frequency and phase response.
3. Gain Structure & Limiters
Now that the crossover settings are optimized and the loudspeaker drivers are in phase, it’s time to calibrate the gain structure
of the system. This will provide ample headroom for all system components in the signal chain and optimize your system’s
signal-to-noise ratio.
Your amplifiers play a vital role in system setup, because they are the last devices in the signal chain before your loudspeakers
and offer the greatest amount of gain (that is their job after all). If your amplifiers are setup incorrectly you will not be using
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