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X-Rack Mic Amp Module
A. The Mic Amp Module
A.1 Connection
The rear panel of the module carries the Mic (‘IN’) and Line input (‘LINE’) XLRs along
with a single output (‘OUT’) XLR. The Line input and output operate at a nominal
level of +4dBu although the gain of the Line input can be varied by a front panel
control. Also contained on the front panel is an additional mono Jack socket for a high
impedance Instrument input.
A.2 Operation
The X-Rack Mic module contains three seperate input amplifiers; a Microphone
amplifier, an Instrument input and a Line input, any one of which may be selected
at any one time. A set of High and Low Pass filters are also provided.
A.2.1 MIC Section
Normally, the Microphone input on the rear of the module will be selected (‘LINE’
and ‘INST’ switches released); pressing the ‘INST’ switch selects the mono jack
instrument input on the front of the module. This is a very high impedance
unbalanced input intended to be used with guitar pickups etc. To help alleviate
‘hum’, a ground lift (‘GND LFT’) switch has been provided which places a 1k2Ω
impedance in series with the sleeve of this connector and audio ground in the
module. The gain of these inputs is continuously variable between +12dB and
+75dB.
The impedance of the Microphone input can be varied between ≈1k2Ω and ≈10kΩ
by selecting the ‘IMP IN’ switch and adjusting the ‘Z’ control. This allows the
connection of line level signals to the Microphone input if required, and provides an
alternative input impedance for some dynamic microphones.
The ‘PAD’ switch reduces the signal level of both the Microphone or Instrument
inputs by 20dB. Phantom power, for microphones requiring this, can be switched on
using the ‘+48V’ switch. Please note that X-Rack units prior to serial number XRK0110
are not normally enabled for +48V. However, a field retrofit kit (629620XR) is available for
these units. Any in-warranty X-Rack units can be upgraded free-of-charge; for units out of
warranty a charge will be made. Please contact your local distributor to order this kit, if
required.
Note. Please note that connecting a microphone to the X-Rack Mic module with phantom power
switched on is not advised as it may cause damage to either the microphone or the input
stage of the X-Rack module. Take care not to connect line level sources (keyboards etc.) to the
microphone input with phantom power switched on as this may damage the output stage of
the connected unit.
A.2.2 LINE Section
The Line input on the rear of the module is selected by pressing the ‘LINE’ switch. The gain of this input
can be varied by ±20dB from the nominal 0dB.
The Ø (Phase) switch reverses the phase of the selected input.
2
1
SEL
XR621
MIC
INST
IMP
+48V
PAD
LINE
LINE
Ø
HF
IN
IN
IN
REC
REC
L
R
GND
LIFT
INSTRUMENT
OUTPUT
SIGNAL
LF
Z
LO HI
dB
+75 +12
+20 -20
dB
KHz
9 6
3 50
30 4
30 600
60
160 300
IN
OUT
LINE
1
2