Crown MPX-6, SMX-6 cable, shielded, lead, Twin, Single, AMB-5 Ambient Sensing Input

IQ Mixer / Multiplexer Hardware Installation

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:

Floating	Shield connected
source	to ground terminal
Output +				cable
	2-wire line cord	INPUT
(or battery power)		+	–	shielded
Grounded	Shield not connected
source	at this end			lead-
Output +
	3-wire grounded line cord	INPUT		Twin
(or other ground connection)		+	–
Floating	Shield connected to both negative
source	(–) and ground input terminals
Output +
	2-wire line cord	INPUT	–	conductor- coax
(or battery power)		+			twistedpair
Grounded		Input ground
source		terminal not used		Single
Output +	3-wire grounded line cord	INPUT			or
(or other ground connection)		+	–

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-6scan 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-5Ambient Sensing Input

SENSE		ADD 25
IN	0	5 FOR MIC
	-5	10
	-10	15
	-12	21

M L P

+ –

Fig. 3.14 Sensing Input Section

In addition to its automatic mixing capabilities, the AMB-5also 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-5the 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.

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