Specifications
microphone features: it will
not
sense ambient room noise,
gating will
not
be direction-sensitive, and gating threshold
adjustment
will
be necessary. Figure 23 describes the re-
quired circuit. Note that
[I
]
microphone-on sensitivity and
effective gating threshold are adjusted by potentiometer
R5
[level adjustment-but not gating adjustment-is available
using the Channel Gain control]; [2] circuit power is supplied
by the mixer; [3] a metal enclosure must be used for
shielding, and
[4]
transformer lead P2 is not used.
CONTROLLING NON-GATED MICROPHONES
FIGURE
22
-
0
a-
-
-
OPERATING HINTS
Phasing
Proper microphone cable phasing is essential to AMS opera-
tion. If pins 2 and 3 are reversed in a cable for a conventional
balanced, low-impedance, microphone-mixer setup, the
microphone will still function properly [although with revers-
ed polarity]. But if pins 2 and
3
are reversed in an AMS
cable, sounds from the
rear
of the microphone will gate it
on. The AMS mixer perceives the microphone's rear as the
front and vice versa, and the microphone gates on only if the
rear becomes the voice entry. In
all
AMS installations, cable
conductors that start out as pins 2 and 3 should end up as
pins 2 and
3,
respectively.
Microphone Muting
To install an at-the-microphone muting switch for an AMS
channel
without
using the logic terminals, the required cir-
cuit is slightly different from that of
a
conventional
microphone and mixer. Figure 24 shows the required
corn-
ponents. All resistors are 1/4-watt, and the capacitor is a
metallized polyester film non-polarized type [CDE
MMWA05W5-20, Sprague 431 P505X9R51. A low
leakage current capacitor like this is needed to avoid
-
n
0
0
0
-
undesirable switching clicks. The switch must be a snap-
action type, such as a toggle switch.
The circuit must be housed in a metal enclosure for shielding
purposes. The enclosure is grounded to pin 1 of the
microphone cable, but must be insulated from accidental se-
cond grounding through the enclosure mounting surface.
This avoids the formation of a ground loop in the microphone
circuit.
-
A
0
0
0
Grounding
In AMS wiring, avoidance of ground loops is of the utmost
importance. Good grounding practices must be followed
when using extension cables, junction boxes, and cable
snakes.
Each microphone cable shield must be connected to ground
only
at the mixer. For instance, if the shielding shell of an
XLR connector is connected to pin 1, and the connector is
plugged into a grounded junction box, the cable shield
becomes tied to ground at the junction box
and
the mixer.
The result may be a considerable amount of hum and rf in-
terference. Good noise rejection is virtually guaranteed by
elimination of ground loops.
-
3
0
0
0
-
MIC MlXE
R
510fl,5%
f
GATE OUT
LOGIC GROUND
MUTE
IN
OVERRIDE IN
MICROPHONE MUTING
FIGURE
24
INTERNAL WIRING MODIFICATIONS
This section describes the internal wiring modifications
previously mentioned.
Refer to the specific application or
logic function for detailed information on the purpose of each
modification.
NON-AMS MICROPHONE GATING CIRCUIT
FIGURE
23
-
R1
7%
11
C3
02
154F
LO
z
NlC.
IN
C
4
R6
J2
R2
1
OOK
I)
*
-
INPUT
TRANSFORMER
PR
p13
1
MARY
rRy
mas2
Parts
Q2: 2N5087
R1: 75K
I14
watt
R2:
100K
Ih
watt
R3: 33K watt
R4:
4.7K
'1.1
watt
R5: 20K Pot. Audio Taper
R6: 4.7K
'14
watt
P
2
C1: .22uF 1OV
C2: 2.2uF 16V
C3: 15uF 16V
PI-P3
75
OHMS
P3
s
2
P
2
C4: 2.2uF 16V
J1: Female XLR
DC
RESISTANCE
(f
20%)
J2: Male XLR
(31: 2N5210 TI
:
Shure A95UF or 90A8032
S1-S2
4300
OHMS