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APPLICATION NOTE : DRIVING THE SUB FROM THE AUX SEND
Application Note : Driving the Sub 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 its subbass relative to the main PA, apply
special effects, use a different EQ on the Sub…However, it also rises some serious issues for the
performance & safety of the system (mostly time alignment).
What is the phase relation between the AUX and MAIN output of your Desk?
At NEXO, when we align systems, we take great care to have an optimum phase alignment from one
octave above to one octave below the crossover frequency point. By doing so, we ensure that both
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 you choose to drive the Sub from the AUX, you feed the NX242 with two signals coming from different
sources. If those two sources (MAIN output & AUX send) are not exactly in phase, you are introducing a
delay –without knowing it- into the crossover between your main system and your sub. Without the
proper measurement tools, you will never be able to tune the system as it should be.
Why it is unlikely the AUX and MAIN 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 only affecting amplitude of the signal by
0.6dB at 30Hz but the phase shift is 90°!! At 100Hz we can still measure 25° of phase shift
Should you want to restrict the bandwidth with a low pass filter, you 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 you are adding between 1.4ms and 2.2ms
(around 70° phase shift at 100Hz) due to the converter delay only! The additional delay due to
the processing itself (look ahead compressor, delay…) can be quite important
At the end of the day, if you have not measured both outputs in the actual configuration you can be 90%
sure that you won’t get the correct phase alignment that you would have had if the NX242 was fed by a
single source.
Consequences of badly aligned systems
Mis-aligned system have less efficiency: i.e. for the same SPL you will be obliged to drive the system
harder, causing displacement & temperature protection at lower SPL than a properly aligned system. The
sound quality will decrease. The reliability will decrease as the system is driven harder to achieve the
same levels. In certain situations you may even need more speakers to do the same job…
Consider the simple example of the AUX signal passing through a digital device (without processing) that
is adding a delay of 2ms due to its conversion time. The AUX is then sent to a CD12 subbass while the
MAIN is send to the S850 rig. The first graph display the phase around the crossover point (85Hz in this
case)
USER MANUAL LOAD2_22
DATE: 03/12/2004
