Allen&Heath GL2400 - Manual (Page 30)

30 GL2400 User Guide
Aux-Fed Subs Driving the PA system sub bass speakers with
their own mix has two main advantages. First, the mix is much cleaner
because only sources that generate the low frequencies, for example
kick drum, bass guitar and keyboards, are sent to the subs. Low
frequency bleed from these sources into open microphones such as
vocals and snare is eliminated from the PA much more effectively than
using just the channel filters. Second, the sub bass amplifier is only
working with the sources needed. It is not wasting power reproducing
unnecessary pickup.
The sub can be separately driven in several ways, for
example using a group, a mono or ‘C’ bus, or using an aux.
Using an aux send has become a popular method on the
smaller consoles. Simply feed the LR to a stereo crossover
to drive the top speakers, and a post-fade aux send to a
mono crossover to drive the sub speakers. Refer to the
speaker manufacturer for recommended crossover
frequency, usually within the range 80Hz to 120Hz. Align the
system so that the normal unity gain position of the channel
fader and aux send result in the correct speaker balance.
For sources you wish to send to the sub, turn up the channel
aux send to its unity 3 o’clock position. Avoid the temptation
to ride the aux send to create more or less sub as this may
upset the natural speaker balance or cause problems for
listeners in different parts of the room.
One problem with using the aux in this way is that it
becomes difficult to balance the top and sub speakers if you
need to adjust the overall volume of the PA. Typically the LR
(tops) use fader masters, and the aux is on a rotary master.
The GL2400 overcomes this with an innovative mode
switch above the master faders that routes the AUX6 mix
master through the M fader and on to the XLR output. This
switch is recessed for protection against accidental
operation. The result is you get three master faders aligned
for simultaneous control, individual mutes, AFL and meters,
and three properly balanced XLR outputs. Note that the
AUX6 rotary master does not affect this sub feed. It does
however independently affect the AUX6 jack output. This
gives you a further advantage that another sub can be
separately controlled using the rotary master, for example to
supplement the stage monitor mix.
Creating a separate C mix As with the aux-fed sub mix
described above, you can set the mode switch to create an
independent mix feeding a dedicated centre or mono speaker. The
channel AUX6 sends become the routing to this mix while the M fader
with XLR output becomes the master, correctly aligned with the L and
R faders. Set the channel aux sends to unity 3 o’clock position, and
release the LR routing switches if you do not want the signal in the LR
speakers as well. The GL2400 gives you the choice of either
mixing the sum of L and R to the M output (mode switch up), or an
independent AUX6 mix to M (mode switch pressed).
Other M Output Applications Apart from the aux-fed subs
and C mix described above, other examples include L+R sum mono
fill, additional mono zone feed, mono recording, mono PA with LR
providing two subgroups to M or a stereo recording… and more.
SUB
TOP TOP
SUB
R
AUX6 MODE
AUX 6 (POST-FADE)
GL2400
STEREO CROSSOVER
MONO CROSSOVER
M L
L C R
OUT
M