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 NXAMP4x1 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 NXAMP4x1 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 NXAMP4x1 was fed by a single source.
Consequences of badly aligned systems
Mis-aligned systems 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…
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