Allen&Heath GL2800 - Manual (Page 32)

32 GL2800 User Guide
Mix and 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, keyboards and
sound effects 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 subs can be separately driven in several ways. A common
method is to use the independent M mix. Route all sources to L-R.
Route the sources you want in the subs to both L-R and M. Feed the
LR to a stereo crossover to drive the top speakers, and M 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. The PA overall level is adjusted by moving
the L, R and M (sub) master faders together.
Another method that has become popular is to use an Aux mix to
drive the subs. This is referred to as ‘aux-fed subs’. 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 GL2800 overcomes this with the
monitor mode ‘REV’ switch that swaps the rotary aux master with the
related fader group master. This switch is recessed for protection
against accidental operation. This puts the aux (sub) master on to a
fader next to the PA tops master faders. The result is you get all
master faders aligned for simultaneous control, individual mutes and
meters, and three properly balanced XLR outputs. We recommend
that you use Aux 8 for the sub feed. This gives you 4 master faders
next to each other, for example, Subs, L, R and C.
Other M Output Applications Apart from the main PA C mix
or sub-bass speaker mix, other examples include L+R sum mono fill,
additional mono zone feed, mono recording, mono PA with LR
providing two more subgroups to M, engineers PAFL wedge feed…
and more.
Using the channel Direct Outputs
Recording - Default factory setting is pre-fade so that your live fader
mixing does not affect the recording. Use the channel recessed PRE-
EQ switches to select either post-EQ or pre-insert, pre-EQ for your
recording. Post-EQ lets you use the EQ for source correction affecting
the recording too. Select pre-EQ if you do not want your compressor
and EQ to affect the recording.
Monitor mixing - You could also patch DIR OUT into a matrix EXT IN
to add a ‘more me’ channel into a monitor send created from the
groups and LR. If preferred you can reconfigure the direct outputs to
post-fade by setting internal jumpers.
Single effects - Use post for feeding an effects processor with one
source only, for example a dedicated reverb on lead vocals or snare.
GRP8
AUX8
AFL
0 0
0
5
5
30
20
10
10
M
LISTEN
R L
AUX9-10
10
10
20
30
5
5
0
00
10
10
20
30
5
5
0
00
10
10
20
30
5
5
0
00
M R L 8
MUTE MUTE
SIG 0 +6 PK!
SIG 0 +6 PK!
SIG 0 +6 PK!
SIG 0 +6 PK!
0dB OO
L-R TO M
GRP8 TO LR
GRP8 TO M
MUTE
STEREO
+10 OO
9-10 AUX
+10 OO
8 AUX
AFL
REV REV
L
PAN
R
=
L R C AUX8
TOPS SUBS
GRP8
0 0
30
20
10 10
20
30
00
10
20
30
00
10
20
30
00
M R L 8
L R SUBS
TOPS
(WEDGE)
M
OUT
OUT
DIRECT