Nexo LS18 - Manual (Page 70)

Page 70/103 S YSTEM ALIGNMENT GUIDELINES
9.3 Driving the LS18s from the AUX send
It is quite common to use the AUX send of a mixing desk to drive the Sub section of a PA system. This
gives the mixing engineer more flexibility to set the level of the subbass relative to the main PA, apply
special effects, or to use a different EQ on the Sub. However, it also raises some serious issues for the
performance & safety of the system (mostly time alignment).
At NEXO, great care is taken to design optimum phase alignment from one octave above to one octave
below the crossover frequency point. By doing so, drivers are working perfectly together and providing
the best efficiency possible. It is then up to the user to adjust the delay on the NX242 to match the
physical path difference of the different systems. It is thus possible to get a well adjusted system, even
without measuring instruments.
If LS18s are driven form an AUX output, NXAMPs are fed with two signals coming from different
sources. If those two sources (MAIN output & AUX send) are not exactly in phase, delay is introduced
into the crossover between the GEO S12 array and the LS18s. It is then mandatory to use proper
measurement tool to optimize phase response.
Why is it unlikely that AUX and MAIN outputs have the same phase?

Signal paths are likely to be different; any filter modifying the bandwidth and EQ of the
signal is also affecting the phase.
Example: a 24dB/oct high pass filter set at 15Hz is affecting amplitude of the signal by only
0.6dB at 30Hz, but the phase shift is 90°!! At 100Hz we can still measure 25° of phase shift.

Limiting bandwidth with a low pass filter can introduce a phase difference of up to 180°
(completely out of phase) at the cross over point.

If the signal is passing though any digital equipment, between 1.4ms and 2.2ms is being
added (around 70° phase shift at 100Hz) due solely to the converter delay! The additional
delay due to the processing itself (look ahead compressor, delay…) can be quite important
as well.
If both outputs are not measured in the actual configuration, it is very likely that phase alignment will not
be correct.
Consequences of badly aligned systems
Mis-aligned systems have lower efficiency: i. e. for the same SPL the system will have to be driven
harder, activating the displacement & temperature protection at lower output levels. Both sound quality
and reliability will decrease as the system is stressed.
Precautions & Checks
Before using the AUX of a mixing desk, ensure that MAIN and AUX outputs are in phase;
Always apply identical EQ or processing on both channels, so that the phase relationship will not be
altered;
Never add additional low pass filtering on the SUB or high pass filtering on the main system;
Inverting polarity on one channel should always result in a massive difference near the crossover point.
If that is not the case, the system is no longer aligned.