A p p l i c at i o n N ot e

Audio Cable Testingby MR-PRO
The Minirator MR-PRO generates audio test signals for commissioning and servicing of audio installations. Additionally itmeasures the impedance of connected devices, of 70V/100V
speaker systems and audio cables. This application notes describes the cable testing feature including verification of any
false pin1 connection to the cable shield.

The MR-PRO features a cable test function that quickly detects
bad or wrongly terminated XLR cables. The test is based on animpedance measurement.
1. Cable test for 1:1 wiring

MR-PRO measures cables
with pin 1 failures

The cable test function detects the 1:1 connection of XLR cables
by adding two different resistances at the cable end (1 kOhm and2 kOhm resistance). In case the impedance measurement finds
the expected resistance values then the MR-PRO indicates “OK”
for a proper 1:1 connection.
Measurement Guideline• Connect one cable end to the MR-PRO output connector andthe other end to the MR-PRO cable test input or alternatively
utilize the NTi Audio Cable Test Adapter for permanently installed cables.
• Select the MR-PRO measurement function “CABLETEST”. The
displayed test result shall be “OK” (= all pins are connected tothe same pin at the other end).

Cable Test Adapter

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MR-PRO output
2CableMR-PRO input orCable Test Adapter
R2 = 1k

1

R3 = 2k

3XLR
OutXLRIn

MR-PRO cable testing by measuring the impedance

Sample Test Results displayed on the MR-PRO Screen
The tested cable is defective, the pins
2 and 3 are crossed. Other error indications may appear depending on thecables fault.
The tested cable is OK.

MR-PRO test results

2. Cable test for pin1 detection
Imagine you assemble a standard 3-pin XLR cable, but accidentally solder the cable shield to pin 2 and the hot wire to pin 1 on
both sides. This cable is commonly analyzed by all cable testers as“OK”, as all pins are 1:1 connected, but the MR-PRO detects suchassembly problems.
How can a wrongly connected shield be detected?
A good XLR cable is a nearly perfect balanced device. For mostaudio cables the capacities between pin2 – pin1 and pin3 – pin1
will differ less than 5%. Differences higher than 10% are possible indications of a shielding fault. Differences exceeding 25%
confirm an unbalanced cable, caused by a wrongly assembled
shield.

Test results of a 5m audio cable with
pin1 failurewww.nti-audio.com

Measurement Guideline• Connect the cable to the MR-PRO output and leave the second
end open (=not connected).
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• Select the MR-PRO measurement function “IMPEDANCE”
and adjust test level = 0 dBu and frequency = 10 kHz.
• Select “RL” in the bottom line, showing the detailed impedance results of pin 2 and pin 3.
• Any impedance measurement result with a deviation of morethan 10% indicates already a possible pin 1 assembly problem.
Measurement Hints:
The MR-PRO supports impedance measurements up to 50 kOhm
with the individual impedances up to 25 kOhm at pin 2 and 3.
This measurement range ensures accurate test results according the generator specifications. This measurement range allows
detecting wrongly connected shields for audio cables longer than3 meters.
For shorter cables with an impedance above the measurement
range you may extend the cable under test with a good XLR-cableof approx. 2 meter length, thus reducing the total cable impedance into the MR-PRO measurement range.

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