QSC ABX Comparator User Manual page 8

In the calibration mode, the ABX Comparator will produce a 1 volt RMS, 1 kHz tone and send it to both channels
of both amplifiers in place of the audio signal input. At the same time, the digital voltmeter display will show
the output level from each amplifier channel, starting with Channel 1 of Amplifier A. In Calibration Mode,
however, the loudspeakers are muted, so you won't hear the test signal.
1. Turn off the audio source—the CD player, DAT, et al—until you complete the calibration process.
2. Adjust the gain control of Channel 1 of Amplifier A until the voltmeter reads a reasonable voltage. The
ideal voltage will depend on the amps under test and the loudspeakers used for the listening test. For
example, a more efficient pair of speakers won't require as much gain or power as a less efficient pair
will. For most systems a good starting point would be 20.0 to 40.0 volts.
NOTE: For better accuracy and easier adjustability, set the amplifier gain controls toward
the upper range of settings.
3. Once you have the gain set for Amplifier A, Channel 1, take note of the voltmeter reading. You'll want
to match this voltage with the three other amplifier channels. Press ENTER.
4. Now the ABX prompts you to set the gain for Amplifier A, Channel 2. Adjust the channel's gain control
until you get the same voltmeter reading as with Channel 1, within 0.1 volt. Press ENTER.
5. Repeat step 3 for Amplifier B, Channel 1, and again for Amplifier B, Channel 2.
NOTE: If one of the amplifiers under test has no gain controls, you'll have to match the other
amp to that one.
After you complete calibrating all four channels, the ABX will automatically proceed to the trial mode, which
is the session of actual listening trials. (However, if you are unsatisfied with or unsure about the calibration, you
can press the CALIBRATE button on the front panel to go back and repeat the procedure.)
READYING THE TEST SESSION
Start the test session with the listening judge in a comfortable listening position in front of the speakers. For
program material, use a familiar, high-quality recording, although there are many preferences and schools of
thought on what constitutes suitable music for audio performance evaluations. Acoustic music is often good for
revealing distortions in an amplifier, while heavy metal might tend to mask them. Synthesized bass might highlight
differences in the low end. Clean, up-front vocals that sound harsh through one amp indicate probable midrange
problems, particularly if they don't sound harsh through the other amp. Use your own judgment in selecting the
recording.
The ABX Comparator can run up to 25 trials in a session. Afterwards, the LCD display will show how many correct
answers the listening judge entered, out of how many trials, and the resulting percentage of correct answers.
You can also review the results of each trial in the session.
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