QSC ABX Comparator User manual

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ABXComparator
USER MANUAL
▼▼
▼▼
▼ ABX Comparator Double-blind listening test switcher
ABX COMPARATOR
CALIBRAT E TRIAL R E V I E W S C R O L L
POWER
ON
RC
ABX1
DAT A
U NI T A X U NI T B E NT E R

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ABXListeningTestComparator
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,theABXComparator’sfeaturesandfunctionsallowyoutoquicklysetupandconductsessions
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?
Comparisontestsbetweenamplifiersareoftenplaguedwiththeseunintendedbutveryrealinconsistenciesthat
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.
Slowswitchingbetween amplifiers Your“audiomemory”isshort-lived. Youwon’tcatchsmall 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
consciousmindsubliminallytobadconclusions,asina“ThisisaBrandAamp,andthisisaBrandB—
whichsoundsbettertoyou?”typeoftest.Thesolutionistohavethelistenercompletely 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
theotheris“B,”andthetwohavebeensetupsothattheyputoutpreciselythesameoutputlevels.“X”designates
theunknownfactor—is itAorisit B?Thus,everytime the ABXComparatorrandomlyselects “X,”whichcould
beeitherAorB,thepersonjudgingtheampswillcompareittoampAandampB(allthewhilepayingattention
to detect audible differences between the two). The challenge then is to identify X. And this is truly “double-
blind” 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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Onetrialis notenough;evenflipping acointodecide whichampisXwould yieldtherightresult abouthalfthe
time. No, it takes repeated trials, with X randomly selected each time, to get a statistically significant test. If
thelisteningjudgeanswerscorrectlyinonlyabout50%ofthetrials,heorshedidn’thearanyaudibledifference
betweentheamps.Butiffarmorethan50%oftheanswersarecorrect,thelistenerclearlydiscernedsomeaudible
difference.
Noticethatthedecisionmadeineachtrialisn’t“whichampsoundsbetter?”Instead,thegoalistoidentifywhether
theampssoundtotallyalike or different; deciding whichisbettersoundingisajobbest left for later. You can’t
judge whether one amp sounds better than another unless you determine that they actually sound different!
TheABXComparatornotonlyautomaticallyandrandomlyselectstheamplifiers,butalsotracksthechoicesthe
listeningjudgemakesineachtrial.Afterasequenceoftrials,theABXwillreporthowmanytrialsandhowmany
correct answers the judge made.
ABX COMPARATOR
CALIBRAT E TRIAL R E V I E W S C R O L L
POWER
ON
RC
ABX1
DATA
UNIT A X UNIT B EN TE R
II. DESCRIPTION
THE ABX COMPARATOR
Powerswitch
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.
Calibration
voltmeter display LCD display UNIT A button UNIT X button UNIT B button ENTER button
CALIBRATE button TRIAL button REVIEW button SCROLL button Infrared sensor
(for remote)
CH2—LINEOUT—CH1 CH2—LINEIN—CH1 CH2—LINEOUT—CH1
INPUT OUTPUT INPUT
RS232
B
B
A
M
P
CH1
CH2
CH1
CH2
A
A
A
M
P
P1=G
P2=+
P3=-
BA
SLAVE
QSCAUDIO
PRODUCTSINC.
REPLACE
FUSEWITH
SAMETYPE.
SEEUSER'S
MANUALFOR
RATING.
MADEIN
U.S.A.
Binding post signal terminals “Combo”signal connectors
RS232 output
to slave ABX
AC power
connection
RS232 to optional
computer
Front panel
Rear panel
Thefrontpanelfeaturesapowerswitch,adigitalvoltmeterdisplay(forcalibration),analphanumericLCDdisplay,
eight input and control buttons, and a female 8-pin DIN connector. During operation, menus, instructions, and
data appear on the LCD display.

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To route the audio signals, the ABX Comparator uses passive circuitry comprising relay switches controlled by
theinternal microcontroller.Toavoid anypossiblecoloration oftheaudio andtoallow bidirectionalsignalflow
(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.
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
panelbuttons,it’sprobablymuchmoreconvenienttousetheremotefromacomfortablelistening
position.
The ABX can also operate without the remote control.
AX
Enter
B
III. SETTING UP AN AMPLIFIER TEST
Thediagramonthe following page shows how to wire theABXComparatorand 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:
• Apairof shieldedaudiocablesforconnecting the signalsourceoutputstothe ABX line-levelinputs.
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.
• AnotherpairofshieldedaudiocablesforconnectingtheABXtoAmplifierA’sinputs.Oneendofeach
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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• AnotherpairofshieldedaudiocablesforconnectingtheABXtoAmplifierB’sinputs.Oneendofeach
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.
• Apairoftwo-conductor speaker cables forconnectingAmplifierA’soutputstothe ABX. Prepare one
endofeach cablesoitis suitablefortheamplifier’s outputs,witheithera connectororstrippedwire
ends,asrequired. The other end must haveeithera dual banana plug or stripped wireends,suitable
for connecting to the ABX’s binding posts labeled “AMP A.”
• Anotherpairoftwo-conductorspeakercablesforconnectingAmplifierB’soutputstotheABX.Prepare
oneend of each cable so it is suitable for the amplifier’soutputs, with either a connector or stripped
wireends,asrequired.Theotherendmusthaveeitheradualbananaplugorstrippedwireends,suitable
for connecting to the ABX’s binding posts labeled “AMP B.”
• Onepairoftwo-conductorspeakercablesforconnectingtheABX’sbindingpostoutputs,labeled“AMP
X,” to the speakers. One end must have either a dual banana plug or stripped wire ends, suitable for
connecting to the ABX’s binding posts, and the other end must be suitable for connecting to the
speakers’ input connection.
CH2—LINEOUT—CH1 CH2—LINEIN—CH1 CH2—LINEOUT—CH1
INPUT OUTPUT INPUT
RS232
B
B
A
M
P
CH1
CH2
CH1
CH2
A
A
A
M
P
P1=G
P2=+
P3=-
BA
SLAVE
QSCAUDIO
PRODUCTSINC.
REPLACE
FUSEWITH
SAMETYPE.
SEEUSER'S
MANUALFOR
RATING.
MADEIN
U.S.A.
CD PLAYER
AMP A
AMP B
CH 1 in
CH 1 in
CH 2 in
CH 2 in
CH1 out
CH1 out
CH2 out
CH2 out
Speakers
Speakers
Computer (optional)
RS232link(optional)
ABX cabling setup for testing amplifiers

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CONNECTING TO “COMBO” CONNECTORS
ThesixNeutrik“Combo™”connectorsaccepteither3-pinmaleXLRor¼-inch(6.3mm)male
TRS connectors. Pin 2 and tip are +; pin 3 and ring are -; pin 1 and sleeve are shield.
CONNECTING TO BINDING POST TERMINALS
When setting up the ABX for amp testing, it is vitally important for safety reasons that you
connectthecablingbetweentheamplifieroutputs,theABX,andthespeakerscorrectly.Use
only fully insulated stranded wire.
1. First, strip back the insulation no more than ½ inch (13 mm).
2. Insert the wire fully into the opening so that no part of the conductor is exposed,
then tighten the barrel. Use a coin to tighten it, if necessary.
3. Alternately, you can use dual banana plugs.
Whensetting upatestbetweentwoline-leveldevices, thesebindingpostconnectorsserve
asthe returnfromthedevices undertestand thesendtothe amplifier.Seetheappendixfor
more details on testing line-level devices.
IV. OPERATION
TURNING THE ABX ON
Connect the ABX to AC power: 100, 120, or 220–240 volts. The internal power supply automatically adjusts to
the AC line voltage. Turn on the unit using the front panel switch.
In the first few seconds after being turned on, the ABX’s LCD display
willfirstdisplaythe unitmodelandthenitsfirmwareversioninforma-
tion. Then it will ask you for the type of test you wish to do.
For a power amp test, press the Abutton, then proceed to the calibration procedure below.
For a test of line-level devices, press the Bbutton and see the appendix for the test procedure.
PRE-SESSION: CALIBRATION
TheABXwillautomaticallyentercalibrationmodeafteryouselectapoweramptestfromthe“SelectTestType”
screen.Youcan also forcetheABXintocalibration modebypressingtheCALIBRATEbutton onthefrontpanel.

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Inthecalibrationmode,theABXComparatorwillproducea1voltRMS,1kHztoneandsendittobothchannels
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. AdjustthegaincontrolofChannel1ofAmplifierAuntilthevoltmeterreadsareasonablevoltage.The
idealvoltagewilldependontheampsundertestandtheloudspeakersusedforthelisteningtest.For
example,amoreefficientpairofspeakerswon’trequireasmuchgainorpowerasalessefficientpair
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. OnceyouhavethegainsetforAmplifierA,Channel1,takenoteofthevoltmeterreading.You’llwant
to match this voltage with the three other amplifier channels. Press ENTER.
4. NowtheABXpromptsyoutosetthegainforAmplifierA,Channel2.Adjustthechannel’sgaincontrol
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:Ifoneoftheamplifiersundertesthasnogaincontrols,you’llhavetomatchtheother
amp to that one.
After you complete calibrating all four channels, the ABX will automatically proceed to the trial mode, which
isthesessionofactuallisteningtrials.(However,ifyouareunsatisfiedwithorunsureaboutthecalibration,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
thoughtonwhatconstitutessuitablemusicforaudioperformanceevaluations.Acousticmusicisoftengoodfor
revealingdistortionsinanamplifier,whileheavymetalmighttendtomaskthem.Synthesizedbassmighthighlight
differencesinthelowend.Clean,up-frontvocalsthatsoundharshthroughoneampindicateprobablemidrange
problems,particularlyif theydon’tsoundharsh through theotheramp.Use yourownjudgmentinselecting the
recording.
TheABXComparatorcanrunupto25trialsinasession.Afterwards,theLCDdisplaywillshowhowmanycorrect
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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THE TEST SESSION BEGINS
NOTE: To preserve objectivity in the test, don’t tell the listening judge before or during the trials
which amplifier is A and which is B. Otherwise, if he or she has even a slight prejudice toward or
against one amp, they may try to pay attention to “does Amp X sound like Brand Z” instead of to
whether the amps sound alike or different.
Instruct the listening judge:
1. Each trial starts with the X, or unknown, amplifier, which could be either Amp A or Amp B. Compare
its sound with that of Amp A, by pressing the Abutton, and to Amp B, by pressing theBbutton. You
can switch to X by pressing the Xbutton. You may freely switch back and forth among Amp A, Amp
B, and Amp X as much as you’d like before you enter your choice. Listen carefully. Does X sound like
A, or like B?
2. Toregisteryourchoicewith the ABX Comparator, press the button—either Aor B—oftheamplifier
that you believe is X, the unknown. Then press ENTER.
3. TheABXwillnottellyouifyouwererightorwrong;instead,itwillimmediatelygoontothenexttrial,
randomly selecting which amp will now be X. In the this new trial, X might be the same ampilfier as
intheprecedingtrial, or it might betheotheramp.Don’tletyourprevious evaluations influence your
current one.
Repeat this procedure for each trial.
The ABX unit switches only the amplifier outputs to the speakers, and not the audio signal to the amps. This
prevents the listener from getting visual cues from the amplifiers’ front panel LEDs or meters.
ENDING AND REVIEWING THE TEST SESSION
Youcan end the test session at any time by pressing the REVIEW button on the front panel ofthe ABX, or you
canjust have the evaluator perform all 25 trials. Remember, the more trials you conduct, the more statistically
significant your results will be.
If you press the REVIEW button, the ABX will ask you whether you want to enter review mode. Press Aif you
do, but if you want to continue with more listening trials, press B, then go ahead and do some more trials.
In review mode, the LCD display will show how many times the listening evaluator identified X correctly, out
ofhowmanytrials,andthepercentage.Toviewtheresultofeachtrial,
presstheSCROLLbutton.EachtimeyoupresstheSCROLLbutton,the
display will step to the result of the next trial.
OBTAINING STATISTICALLY SIGNIFICANT RESULTS
Givenonlytwo possibilities in eachtrial,arandomselection of AandB,suchaseach selection beingbasedon
coin toss—or even just selecting the same one every time—would tend to yield correct results about 50% of
the time. In a small sample, i.e., a small number of trials, the actual results may be wildly different from 50%,

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but over an increasing number of trials will almost certainly tend to approach 50%. For example, the chances
ofacoincomingupheadsontwosuccessivetossesis1in4;onthreesuccessivetosses,1in8;onfoursuccessive
tosses,1 in 16. Getting tosses of heads ten times in a row islikely only one time in 1024. In other words, large
samples tend to smooth out the aberrations of randomness that occur.
Therefore, not only do you need to conduct a fair number of trials in a session, but the results have to be
substantially better than 50% to mean that there is an audible difference between the units under test.
The very minimum number of trials you should do in a session is ten. Out of ten trials, if the listener chose the
correct amplifier seven times, he or she might have heard some difference between the two, but it’s really too
inconclusive. Eight correct choices would indicate a probable audible difference between the amplifiers, with
about a 95% level of confidence. Nine out of ten would be an even stronger indication. Ten out of ten would
almostcertainly indicatethelistener heard somesonicdifferencesbetweentheamps,since there’s onlya1in
1024 chance it could randomly.
What we’re really after in these statistics is a high degree of confidence that the results show real conditions
and not random occurrences. For example, a series of 25 trials has 33,554,432 possible combinations of right/
wrong answers, ranging from 0 correct/25 wrong, up to 25 correct/0 wrong. There are 5,200,300 possible
combinations of 12 correct/13 wrong, and an identical number of possible 13 correct/12 wrong. There is only
onecombinationof 25 correct/0 wrong, and whileitis possible that a random sequence ofresponsescould be
right25outof25times,thereisonlya1in33,554,432chanceofithappening.Therefore,wecansaythatthere
isa33,554,431/33,554,432(99.999997%)chanceofit not happening;thatwouldalsobeourlevelofconfidence
in the results: 99.999997%.
We won’t be quite as picky with the listening tests; a 95% minimum level of confidence will be good enough.
That is, there should be less than a 5% chance that the results can be attributable to chance.
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The table on this page lists a recommended range of trials, and the minimum number of correct responses
necessary to reach a 95% or better level of confidence.
Asyouincreasethenumberoftrials,yourdatabecomesmoredependable.Noticethatyouneed8of10butonly
17 of 25 to get the same degree of confidence.
So,ifyour listener correctly “guesses” theidentityof “X” at least theminimumnumber of times in thesession
oftrials,youcanconfidently estimate that there is an audible difference between the amplifiersundertest.Of
course, that may mean that one amp sounds worse than the other—maybe even the amp you favor, if you do
have an interest in one brand or model over the other. On the other hand, if the listener gets fewer than the
minimumnumbercorrect,itdoesn’tnecessarilymeanthattheampsareaudiblyindistinguishable,althoughthey
may be. It more accurately means only that you can’t confidently say there’s an audible difference.
It’s not a good idea to try more than 25 trials in the same sitting with the same listener. After a while, listener
fatigue sets in and it gets harder for him or her to concentrate and judge the sound quality.