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X-Rack Mic Amp Module
A.4 Calibration Information
The X-Rack Mic Amp module is factory calibrated and should only need calibration if a potentiometer or
other component has been replaced or if it is suspected that there is a problem with calibration.
In each of the following instructions it is assumed that the lid of the X-Rack has been removed and that
power has been applied. It is also assumed that unless otherwise specified, all switches are released and
all front panel potentiometers are at unity or minimum position as appropriate. The required accuracy for
each adjustment will be specified along with the target value. All level and distortion measurements
should be made with audio-band 20Hz to 20kHz filters unless otherwise specified.
All presets are accessible from the top of the unit.
A.4.1 Microphone Input
Equipment Required: Calibrated audio oscillator and audio level meter
Test Signal: 50Hz sinewave @ –12dBu, common mode
Input and Output: Oscillator to Mic Input and Output to the audio level meter
Unit Setup: Set the Mic Gain to ‘36dB’ (mid-position)
CMRR Trim
Adjustment: Adjust VR7 (CMRR) for minimum level (normally < –40dBu)
A.4.2 Line Input
Equipment Required: Calibrated audio oscillator and audio level meter
Test Signal: 1kHz sinewave @ 0dBu
Input and Output: Oscillator to Line Input and Output to the audio level meter
Unit Setup: Set the Line Gain to indent (0dB) and select ‘LINE’
Level Trim
Adjustment: 1. Adjust VR6 (0dB) for 0dBu ±0.05dB.
A.4.3 Output Balance
Equipment Required: Calibrated audio oscillator, audio level meter and a ‘balance’ adaptor (see
below)
Test Signal: 1kHz sine wave at +24dBu
Input and Output: Oscillator to Line Input and Output to the audio level meter via the
‘balance’ adaptor
Unit Setup: Set the Line Gain to indent (0dB) and select ‘LINE’
Adjustment: Adjust VR8 (BAL) for minimum level (< 55dBr)
A.4.4 ‘Balance’ Adaptor
For the output balance adjustment, a ‘balance’
adaptor such as that illustrated here will be
required. This adaptor consists of a pair of close
tolerance resistors in an in-line cable and is
used to sum together a balanced output in
order to correctly adjust the level balance of the
measured output; perfect balance should result
in complete signal cancellation.
5K01**
5K01**
2
3
1
2
3
1
0V
+
–
0V
+
–
To measuring
equipment
From unit
under test
1
2
1
Resistor tolerance should ideally be 0.01%
Absolute level measured will depend upon the input
impedence of the measuring equipment.
1.
2.
Note