ABX Comparator
USER MANUAL
O N
POW E R
D ATA
U N I TA X U N I TB E N T E R
C A L I B R AT E T R I A L R E V I E W S C R O L L
ABX1
ABXCOMPARATOR
R C
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ABX Comparator
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Double-blind listening test switcher
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ABX Listening Test Comparator
I. INTRODUCTION
The ABX Comparator is a signal routing and measuring device that permits simple and accurate comparative
listening tests of two or more power amplifiers or even line-level audio components. Using advanced
microcontrollers, the ABX Comparator’s features and functions allow you to quickly set up and conduct sessions
of critical listening test trials for performance evaluations.
Key features include:
• A built-in test tone generator and digital AC voltmeter for fast calibration of amplifier levels.
• High-speed relays for fast switching between amplifiers under test.
• A single pair of speaker outputs, for true accuracy.
WHY USE AN ABX COMPARATOR?
Comparison tests between amplifiers are often plagued with these unintended but very real inconsistencies that
lead to inaccurate auditions:
Unequal input sensitivities This leads to one amplifier being louder than another. Even experienced
critical listeners often confuse differences in volume with differences in audio quality.
Different speakers used with each amplifier The difference in frequency response between two
loudspeakers, even of identical models, can often be several dB. This can easily mask differences in
amplifier audio quality.
Slow switching between amplifiers Your “audio memory” is short-lived. You won’t catch small sonic
differences between amplifiers unless switching between them is quiet and fast (under 50 ms).
Unconscious bias Even when you try to be truly unbiased, your unconscious mind often leads your
conscious mind subliminally to bad conclusions, as in a “This is a Brand A amp, and this is a Brand B—
which sounds better to you?” type of test. The solution is to have the listener completely unaware of
which amp is which, even in the inital audition. This is called double-blind testing, and it’s the only
way to get a truly unbiased result.
The ABX Comparator has been specifically designed to eliminate all these errors.
THE ABX TESTING PROCEDURE
The letters “ABX” describe the listening factors used in double-blind testing of two amps. One amp is “A” and
the other is “B,” and the two have been set up so that they put out precisely the same output levels. “X” designates
the unknown factor—is it A or is it B? Thus, every time the ABX Comparator randomly selects “X,” which could
be either A or B, the person judging the amps will compare it to amp A and amp B (all the while paying attention
to detect audible differences between the two). The challenge then is to identify X. And this is truly “doubleblind” testing, so the listening judge doesn’t even know which amp being tested is A and which is B, but must
nevertheless decide whether X is A or B.
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One trial is not enough; even flipping a coin to decide which amp is X would yield the right result about half the
time. No, it takes repeated trials, with X randomly selected each time, to get a statistically significant test. If
the listening judge answers correctly in only about 50% of the trials, he or she didn’t hear any audible difference
between the amps. But if far more than 50% of the answers are correct, the listener clearly discerned some audible
difference.
Notice that the decision made in each trial isn’t “which amp sounds better?” Instead, the goal is to identify whether
the amps sound totally alike or different; deciding which is better sounding is a job best left for later. You can’t
judge whether one amp sounds better than another unless you determine that they actually sound different!
The ABX Comparator not only automatically and randomly selects the amplifiers, but also tracks the choices the
listening judge makes in each trial. After a sequence of trials, the ABX will report how many trials and how many
correct answers the judge made.
II. DESCRIPTION
THE ABX COMPARATOR
Power switch
voltmeter display LCD display UNIT A button UNIT X button UNIT B button ENTER button
O N
POW E R
Front panel
D AT A
CALIBRATE button TRIAL button REVIEW button SCROLL button Infrared sensor
U N I T A X U N I T B E N T E R
C A L I B R AT E T R I A L R E V I E W S C R O L L
ABX1
ABX COMPARATOR
The front panel features a power switch, a digital voltmeter display (for calibration), an alphanumeric LCD display,
eight input and control buttons, and a female 8-pin DIN connector. During operation, menus, instructions, and
data appear on the LCD display.
Calibration
Binding post signal terminals “Combo”signal connectors
B
A
M
P
B
CH1
INPUT OUTPUT INPUT
CH2
CH1
CH2
CH2—LINE OUT—CH1 CH2—LINE IN—CH1 CH2—LINE OUT—CH1
A
A
M
P
P 1=G
A
P 2=+
P 3=-
BA
Rear panel
RS232 to optional
computer
RS232
QSC AUDIO
PRODUCTS INC.
SLAVE
RS232 output
to slave ABX
(for remote)
REPLACE
FUSE WITH
SAME TYPE.
SEE USER'S
MANUAL FOR
RATING.
MADE IN
U.S.A.
R C
AC power
connection
The rear panel has input and output connectors, a 9-pin RS-232 port for connecting to an optional computer,
and an additional RS-232 port for slaving one or more additional ABX units.
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To route the audio signals, the ABX Comparator uses passive circuitry comprising relay switches controlled by
the internal microcontroller. To avoid any possible coloration of the audio and to allow bidirectional signal flow
(because testing of line-level devices uses a different hookup configuration than testing amplifiers does), no
active circuitry is used in the test signal path.
AX
THE REMOTE CONTROL
The handheld remote control, shown at left, communicates with the ABX via encoded infrared
light. The remote duplicates four of the ABX’s front panel buttons: UNIT A, UNIT X, UNIT B,
and ENTER. Although the listener could enter his or her choices for each trial using the front
B
panel buttons, it’s probably much more convenient to use the remote from a comfortable listening
position.
Enter
The ABX can also operate without the remote control.
III. SETTING UP AN AMPLIFIER TEST
The diagram on the following page shows how to wire the ABX Comparator and the other components needed
for a listening test of two 2-channel amplifiers.
In addition to the ABX unit, you will need the following:
• A high-quality line-level stereo signal source, such as a CD player.
• Two 2-channel amplifiers, designated A and B, to be tested.
• One pair of high-quality speakers, with a power handling capacity suitable for the amplifiers and
intended listening level.
You will also need the following cables:
• A pair of shielded audio cables for connecting the signal source outputs to the ABX line-level inputs.
One end of each cable must have a connector suitable for the source device, and the other end must
have either a male 3-pin XLR or a male ¼-inch (6.3 mm) TRS connector for the ABX unit.
NOTE: For best results, the signal source should have balanced outputs, and all line-level
signal cabling should be balanced as well.
• Another pair of shielded audio cables for connecting the ABX to Amplifier A’s inputs. One end of each
cable must have a connector suitable for the amplifier’s inputs, and the other end must have either
a male 3-pin XLR or a male ¼-inch (6.3 mm) TRS connector for the ABX unit.
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