VAC CPA1 Mk III User manual

CPA1Mk III &CLA1Mk III Instructions
Please read carefullybeforeinstalling
The VAC CPA1/CLA1MkIII Preamplifiersaredesigned forthe mostexactingapplications.The two
modelsaresimilarexceptforthe omission ofthe phonograph preamplifierinthe CLA1. All audio
circuitsareClass Atriode vacuumtube types.Inaddition,all directaudiosignalpaths arehard-
wired for best performance.
GeneralPrecautions
1.DONOTOPEN POWER SUPPLYORPREAMPLIFIER CHASSIS-THERE ARE NO
USER SERVICEABLEPARTSINSIDE.DONOTREMOVE TUBE CAGEUNTILUNITHAS
BEEN SWITCHED OFF FOR15 MINUTES.TUBESBECOMEHOTINUSE.DONOT
EXPOSE EITHER CHASSISTOMOISTURE.DONOTPLUGINTOAN AC SOURCE
UNTILALL CONNECTIONS ARE COMPLETED.
2.Completeall connections before connectingthe AC cordtothe wall outlet, particularlythe
6pincablefromthe powersupplytothe preamplifierchassis.Similarly,alwaysremovethe
AC powercordbeforedetachingthe 6pinDC powercable.Failuretoobservethis
sequence will result indamagetothe power supply.
3.Atturnon,all volumecontrolsshouldbe full counter-clockwise and the mode switch should
be set toMUTE. Allow1minutefor warm up beforeselectingSTEREO.
4.Forprotection ofyourspeakersand poweramplifier, alwaysswitch preamptoMUTEbefore
switchingthe power supplyoff or on.
5. Allowthe systemtositswitched off foratleast10 minutes beforedisconnectingthe preamp
from itspower supply.
Operation - Line StageSection
The line stagecan be operated intwobasicmodes,Activeand Passive-Buffered.The activemode
maybe ofhighgainormoderategain.These three options areselected byarotaryswitch on the
top ofthe chassison the right side. Mutethe preampbefore operatingthisswitch .
Inthe Passive-Buffered mode the amplifying triode (V4)isbypassed,and the controlsarefed
directlytoaClass A impedance translating Class Atriode tube (V5). Since the outputisbuffered,
thereareno gross cableinteraction problemsofthe type encountered withpassivecontrolcenters;
since thereisno gainstage,thismode has ‘unitygain’.The VAC Passive-Buffered mode is
capableofdrivinglongcableruns andthe preamplifiermaybe located somedistance fromthe

power amplifiers.
Since the Passive-Buffered mode has zerogain,you mayneed toturnthe volumecontrolswell
clockwise...the exactposition depends on the outputlevelfromyoursource,and the sensitivityof
yourpoweramplifiersand loudspeakers.Manyaudiophiles havethe beliefthatitisnevergood to
run volumecontrolswide open. Withacircuit ofthistype, however, it isactuallypreferabletodo
so.Don't worryaboutwherethe controlisset, justbe concerned withhowitsounds.The Passive-
Buffered mode does not invert absolutephase withrespect tothe line inputs.
The Activemodes add aClass Aamplifyingtriode tube toeach channel,and causesthe main
outputstohaveinverted absolutepolaritywithrespecttothe line inputs.Tocorrectforthisyou may
invert the polaritybetween yourpoweramplifierand loudspeakers.Inmostsystems,the Active
mode retrieves moreambient and reverbinformation.
Operation - Phono Stage
Beginbyselectingthe appropriatephono stagegainusingthe controlon the top ofthe chassisat
the right rear. The highgainposition isappropriatefor most movingcoil cartridges. The medium
gainmode isappropriateforsomehighoutputmovingcoilsand moderateoutputmovingmagnets.
The lowgainposition isforuse withveryhighoutputcartridges.Ifindoubt, feelfree toexperiment
and use the settingsthat sound best. Mutethe preampbefore switchinggain .
The phono stagedeliversan inverted signalwithrespecttoline inputs.Thereforeyou maywish to
invert the connections at your phono cartridgeifitsdesignpermits.
Cartridgeload impedance and capacitance areadjusted bytworotaryswitches on the backofthe
unit. Start at47kohmsand 150 pf.Listen forabit, and then reduce the impedance to22k.
Continue until you find the bestimpedance setting.Next, tryraisingand loweringthe capacitance.
When you havefound the bestcapacitance setting,listen toafewotherimpedance settingsto
verifyyour earlier choice.
Ingeneral,highercapacitances smoothhighfrequencyresonances.Lowerimpedances havea
similar effect, and can also improvesubjectivedetail.
Often the load parametersyou choose will differfromthose recommended bythe cartridge
manufacturer. Thismayhappen because ofdynamicload characteristics,which aretypicallybetter
behaved withvacuum tube circuits. Use the settingsthat sound best.
The availableload parametersareapproximately:
Impedance Capacitance
47,000 ohms0picofarads
22,000100
10,000120
1,000150
100180
220

Grounding
Noise and hummayoften be eliminated byconnectingawirefromthe bindingposton the power
supplytothe ground poston the preamplifier. The othertraditionalmethods ofcuringground loops
mayalso be tried.Someformsofnoise maybe temporaryand associated withwarmup ofthe
preamplifierand/orpoweramplifier. Determine correctgroundingafterall componentsarefully
warmed up. Sound qualitymayalso be affected bypower cordpolarityofyour components.
Tubes Compliment
V16DJ8/6922/E88CCphono input - shouldbe tested for lownoise
V212AX7A/E83CCphono
V312AX7A/E83CCphono
V412AU7A/E82CCline amplifier (activemodes) &tape buffer
V512AX7A/E83CCoutput impedance buffer
Phono noise ismostlikelycaused byV1.Line stagenoise (activemode)ismostlikelycaused by
V4.
(Historicalnote: The earlier CPA1and CPA1a used a12AT7inthe V1position. The CPA1MkII
and MkIII use a6DJ8 as indicated above.)
The V4position has no impact on the sound ofthe preamplifier inthe PassiveBuffered mode.
Please notethat powercords will haveaprofound influence on the characterofreproduced sound
inspiteofthe CPA1'sextensivepowersupplyfilteringand regulation,as will floatingthe ground
conductor.

About Microphony
Everyamplifyingdevice,be ittube ortransistor, produces someoutputwithmechanicalstimulation
(microphony). Infact, even cables possess microphony.Acceptabilityisnotaquestion ofwhether
thischaracteristicispresent but rather the degree towhich it ispresent.
Inthe VAC preamplifier, everytube (exceptpossiblyV5)will emitasound throughthe speakers
ifit isdirectlystruck. Thisisanormalsituation. The exact amountofsound depends on the tube
type, itslocation inthe circuit, and slight random variations intube manufactureand condition.
The amountofsound emitted when the tube isstruckisirrelevant. The tube and circuitcard
assemblyareshockmounted fromthe mainchassis,and as such areisolated fromdirectcontact.
When the chassisitself isstrucksharplytheremaybe eitherno sound oraslightsound emitted
fromthe speaker, butitshoulddieawayrapidly.Again,thisisanormalresult, and not
representativeofthe type ofmechanicaloracousticstimulation encountered when the unitis
properlyinstalled and playingmusic.However, ifthe sound ringson stronglywhen the chassisis
struck, the offendingtube shouldbe replaced.
It isbestnottosubjectthe tubes totappingorthe chassistorepeated mechanicaljars,as thismay
damagethe precision ofthe tubes and cause them tobecomemoremicrophonic.
About Noise
Everyamplifyingdevice,be ittube ortransistor, produces someamountof noise.If fact, even a
resistorproduces noise inthe presence ofcurrentflow.Acceptabilityisnotaquestion ofwhether
thischaracteristicispresent but rather the degree towhich it ispresent.
Inthe phono stage,the mostlikelysource ofnoise istube V1.Thisisbecause anynoise produced
byV1isamplified byall of the followingtubes.Withnormallownoise tubes,the subjectivelevel
ofelectronicnoise will be approximately1/3ofthe groovenoise,referenced toa.3mVoutputMC
cartridgeand agood directtodisc recordingplayed atloud levels.Electronicnoise will be
insignificant withnormalrecordingsat normallevels.
Inthe line stageactivemodes,the mostlikelysource ofnoise isV4.The audibilityofnoise
produced inthe line stageitselfisnotaffected bythe volumecontrolsetting,and isdetermined by
the sensitivityofthe poweramplifierand the efficiency ofthe speakers.Withaspeakerof
extremelyhighefficiency(forexample,aKlipschorn)the preamplifiershouldbe setinthe passive-
buffered mode toeliminatethe noise ofamplifyingtube V4.Noise shouldbe insignificantineither
mode withspeakersinthe typicalefficiencyrangeof80 to93 dB/1watt.
Toconfirm which tube isnoisy,you mayswap the tubes between the channels.Forexample,
exchangeV4L and V4R.Ifthe noise changes channelsthe V4tube inthe noisychannelshouldbe
replaced.Besuretomutethe preamplifier, turnitoff,and allowthe tubes tocoolbeforemaking
swaps.

CPA1MkIII / CLA1MkIII NOMINALSPECIFICATIONS
FrequencyResponse:
Phono, RIAA +/-.15 dBfrom 20 Hzto20,000 Hz
Line,7.5Hzto180 kHz+0/-0.5dBinto20,000 ohmload and 1metershielded interconnect
1.5Hzto380 kHz+0/-3.0dBinto20,000 ohmload and 1metershielded interconnect
Tape:2.0Hzto380 kHz+0/-3.0dBinto20,000 ohmload and 1metershielded
interconnect
Phono StageGain- CPA1only(1kHzat line stageinput):
Highgainmode:65 dB
Medium gainmode:55 dB
Lowgainmode:39 dB
Phono Overload:
Highgainmode:3.8mV@1kHz, 36 mV@20 kHz, .45 mV@20 Hz
Medium gainmode:12.2mV@1kHz, 110 mV@20 kHz, 1.4mV@20 Hz
Lowgainmode:88 mV@1kHz, 820 mV@20 kHz, 9.8mV@20 Hz
Line StageGain(1kHz):
Passivebuffered mode:- .75 dB
Medium gainactivemode:+ 9.0dB
Highgainactivemode:+18.5dB
Line Output vs. Distortion (1kHzinto20,000 ohm load):
1.5V@.1%THD
5.0V@.2%THD
11 V@1.5.%THD (overload)
AbsolutePhase:
Line stage, Passivemode isnon-invertingfrom line inputs
Line stage, Activemodes isinvertingfrom line inputs
Tape output isnon-invertingfrom line inputs
Phono stagedeliversan inverted signaltoline stageand tape buffer
The abovecommentsaretrue forthe optionalbalanced outputswithrespecttothe EIA
standard
Tube Compliment:
CPA1 CLA1 Type
62E83CC/ECC83/12AX7A
20E88CC/ECC88/6DJ8/6922 (lownoise for MC)
22E82CC/ECC82/12AU7A
AC Power Requirements(US market):
110 VAC to126 VAC
Alternativeinputvoltages selectableatthe fuse holder(adjustfuse value accordinglyas
well)