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7
MICROPHONE AMPLIFIERS
& LI MI TE RS
Mic / Line Inputs
The XLR 3 pin sockets used for the mic/line inputs are electronically balanced. They have the
following connections:
Pin 1: Screen
Pin 2: Phase
Pin 3: Non-phase
Mic Input Gain
Recessed pre-set potentiometers allow for adjustment of the gain of the microphone
inputs. This provides a gain range of +22dB to +67dB which enables the use of dynamic and
powered microphones. Connect the mic input and adjust the gain until the line output is at
the level that you need.
On On
1
2
On
Fig 7-4: Switch to Control LF Filter and Phantom Power.
Using Phantom Powered Microphones
For the mic input channel there are independent switches to provide phantom power at
+48V to the connected microphones. With phantom power selected, a voltage of +48V is
applied to pins 2 and 3 of the XLR connector to power the microphone, supplied through
6k8 resistors giving a current of 14mA. Phantom power is used when the switch is pointing
towards the arrow. The phantom power only applies when the input is set to mic mode.
Using the LF Filter
A switch also provides control for a high pass filter with low frequency roll off at 125Hz. The
filter is in when the switch is in the down position (towards the arrow). The filter only applies
when the input is set to mic mode.
Mic / Line Outputs
The XLR 3 pin plug output connector is electronically balanced and can be wired
unbalanced by grounding the non-phase signal, allowing you to feed balanced and
unbalanced equipment. It has the following connections:
Pin 1: Screen
Pin 2: Phase
Pin 3: Non-phase
The connector provides a line level output with an impedance of <50Ω in line mode and
150Ω in mic mode, with a maximum output level of -18dBu(mic) or +28 dBu(line).
46
1
Channel 1 Low Frequency (LF) Filter
2
Channel 1 Phantom Power
3
4
3
Channel 2 Phantom Power
4
Channel 2 Low Frequency
On
Redbox User Handbook No 3
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