Input Connections; IQ Mixer / Multiplexer; AMB-5 Ambient Sensing Input- Page 16

Page 16

IQ Mixer / Multiplexer  Hardware Installation

Rev. 0

0.775 volts). Select the center position (L) for line
level signals up to +32 dBu. Slide it to the right (P)
to provide 44 VDC to mics requiring phantom
power.

Each input has a screwdriver-set, calibrated gain
control to compensate for different input source
levels. The slot on the control shaft points to the
gain setting. The settings are labelled for line-level
input. Add 25 dB to the scale if the inputs are
switched for microphone level signals.

Use a screwdriver to adjust the gain pot so that the
input signal level plus gain equals roughly 0 dBu.
You will need to know, or estimate, the level of the
input source. Setting the source signal level to ap-
proximately 0 dBu will provide 20 dBu of headroom
in the input preamp. Some recommended settings
follow in Figure 3.11:

Fig. 3.13  Unbalanced Audio Input Wiring

Figures 3.12-13 show the normal wiring for bal-
anced and unbalanced inputs. It is also possible to
parallel the inputs of multiple units

 to increase the

number of mixer outputs. For example, the inputs of
two 6x2 

MPX-6s can be paralleled to create a 6x4

mixer. This technique is described in Section 3.3.6.

Balanced sources should be wired as shown below
in Figure 3.12. Notice that the shield is not con-
nected to the chassis ground of the source if the
source is also connected to the AC ground (that is,
it has a grounded AC plug). This prevents unwanted
ground loops.

Unbalanced sources should be wired as shown
below in Figure 3.13. The examples in Figure 3.13
are grouped according to whether twin-lead
shielded wire or single-conductor coax (and twisted
pair) wire is used.

3.3.2 

AMB-5 Ambient Sensing Input

In addition to its automatic mixing capabilities, the
AMB-5 also has the ability to adjust the output level
of Channel 1 to the ambient sound level. (Remem-
ber, Channel 2 functions only as an 

MPX-6.) It does

this with its sensing input (input 6). This is an
extremely useful feature. With an 

AMB-5 the paging

level at a train station can be automatically adjusted
so pages can be heard over the roar of an incoming
train and yet quieted to an appropriate level during
periods of softer ambient sound levels.

The sensing input section of the back panel is
shown in Figure 3.14 below. It has the same fea-
tures as the other mic/line inputs: input gain control
and input level switch.

The obvious way to use the sensing input is to
connect a microphone (such as a Crown PZM-6D) to
it and locate the microphone so that it can accu-
rately receive the ambient sound level. 

Great care

must be taken in the placement of the ambient sens-
ing microphone so that it is not too close to the
loudspeakers being driven by the system. If it is too
close, the system could go into feedback oscillation.

Fig. 3.14  Sensing Input Section

+

–

L

M

P

0

-5

-10

-12

5

10

15

21

ADD 25
FOR MIC

SENSE
IN

+

2-wire line cord

(or battery power)

Output

INPUT

Floating

source

3-wire grounded line cord

(or other ground connection)

Shield not connected
at this end

INPUT

Grounded

source

Twin-lead shielded cable

Shield connected
to ground terminal

+

Output

+

2-wire line cord

(or battery power)

Output

INPUT

Floating

source

3-wire grounded line cord

(or other ground connection)

Input ground
terminal not used

INPUT

Grounded

source

Single-conductor coax

or twisted pair

Shield connected to both negative
(–) and ground input terminals

+

Output

+

–

+

–

+

–

+

–