VAC Sigma User manual

Valve Amplification Company
VAC SigmaPreamplifier
Instructions
Manual issued-3March2008
Revised2April2008
Pleaseread carefullybeforeinstalling
Copyright 2008 by ValveAmplificationCompany. All rightsreserved.

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WELCOME
Congratulations on your purchase of the VACSigmaPreamplifier,and welcometo the
family of VACowners!
Your VACis designed not to the latest fad but to substance,providingthe highest quality
of sound. Timespent familiarizingyourself with this manual will be well rewarded.
Itis ourgoal to provide instruments of the highest quality,be informativeto all of our
customers, and to provide responsive, effectivecustomer service.
Please feel free to contact us if we can be of assistance. We can be reached:
Telephone[phone]
Fax[phone]
E-mail [e-mail]
Website www.vac-amps.com
We lookforward to beingof service,even as you havemany years of enjoyment fromyour
VACSigmaPreamplifier.
Kevin M. Hayes
President, VAC
INTRODUCTION
The VACSigmaPreamplifier is an all new designderived fromthe VACPhi Beta,and
employs four twin triode vacuumtubes operatingin Class A.This unique designprovides
great speed,detail,punch,and musicality,all without global feedback.Its moderate gain
structure and low output impedance provides an excellent match with awide rangeof
popular power amplifiers.
The new highly shielded power transformer keeps significant e.m.f.fields away fromthe
audio circuit,yet allowsthesource of power to be located close to the audio circuit’s
demands,resultingis fast,clear response to audio transients and highresolution of inner
detail.Low noise rectifiers further improvethe purity of the DC supplies,thus preserving
both the power and delicacy of the music.Heater voltageis precisely regulated to improve
tube life.
The chassis is rigid and non-magnetic.
Atriode MM phono stageor triode MC/MM phono stageare optional.

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GENERAL PRECAUTIONS &SAFETY NOTES
1. DANGEROUSVOLTAGESEXISTWITHINTHE CHASSIS.DO NOT REMOVE
CHASSIS COVER UNTILUNIT HAS BEEN SWITCHED OFF AND
UNPLUGGED FROM THE WALL OUTLET (POWER) AND ALL
COMPONENTSFOR 15 MINUTES.TUBESBECOME HOT INUSE,AND CAN
CAUSEINJURY.DO NOT EXPOSECHASSIS TO MOISTURE.DO NOT PLUG
INTO ANACSOURCE UNTIL ALL CONNECTIONSARE COMPLETED AND
ALL COVERSARE INSTALLED.
2.Complete all signal connections before connectingthe ACcord to the wall outlet and
turningthe unit on.Similarly,always turn the amplifier off and removethe ACpower
cord before detachingcabling.Failure to observethis sequence can result in damage
to the amplifier, speakers or associated components.
3.Atturn on,all volumecontrols should be fullcounter-clockwise.Allow one (1)
minute for warmup before disengagingmute and commencingplayback.Note: Mute
is automatically engaged when switchingon units equipped with the remote control
option.
4.For protection of your speakers and power amplifier, always engage“Mute” before
switchingthe power off or on.Note: Mute is automatically engaged when switching
on.
5. DO NOT OPERATE THE UNITIFITIS WET.
6. THE CHASSIS CONTAINSVACUUM TUBES.DANGEROUSVOLTAGES
EXISTWITHINTHIS CHASSIS.NEVER REMOVE THE COVER UNLESS THE
UNITHAS BEEN SWITCHED OFF AND COMPLETELYDISCONNECTED
FOR ATLEAST15 MINUTES.THE TOP COVER MUSTALWAYSBE LEFT
SECURELYINPLACE.
7. Takecare to install your preamplifier securely in asturdy,well designed stand so that
it will not fall or tip over (this could cause serious, possibly fatal injury).

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INSTALLINGTHE SIGMA PREAMPLIFIER
Physical requirements:
1)Provide Adequate Ventilation.
2)The chassis cover will becomewarmduringnormal use.
3) This component must be installed in asecure, properly built (and ventilated) stand,
cabinet or enclosure.Itcould inflict serious injury or property damageif it falls, or
if the stand upon which it is placed tips over.This concern is particularly important
in households with small children who might try to climbfurniture.
Basic Hookup:
1)Connect line level inputs (CD,Tape,etc.)to the appropriate RCAinput jackson the
rear panel. With the exception of Phono (when fitted), all inputs are “line level”.
2)Connect phono cables to the rear panel RCAinput labeled “Phono.”Connect the
ground wire fromthe turntable or phono cable to the “Ground” terminal provided on
the rear panel.(See “Phono Section” for cartridgetypes appropriate for use.)
Connect output interconnects to your power amplifier.
3)Connect the ACpower cord to the IEC connector.For best performance,route the
power cord away fromsignal cables.
4)Before turningthe power switch to the “on” position,set output level to zero by
rotating the “Volume” control fully counterclockwise.Note: Mute is automatically
engaged when switchingon.
5)Allow the preamplifier to warmup for aminute before you unmute the unit; failure
to wait can cause atransient to be sent throughyour power amplifier that might cause
damageto your loudspeakers and amplifiers.
6)Always mute the preamplifier before switchingit off.The best practice is to also turn
off your power amplifier(s) before turningoff the preamplifier.
Aswith all highperformance audio products,the sonic characteristics of all VACproducts
will continue to improveduringthe first several hundred hours of use.

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OPERATINGTHE SIGMA PREAMPLIFIER
Front Panel Controls:
Althoughmost of the front panel controls (Volume,Power,Mute) are self explanatory,VAC
has outfitted this unit with several advanced features which bear further description:
Phono Section MM (option): When fitted, input “L6" will be connected to adiscrete, pure
tube phono preamplifier,providing40 dB of gain,suitable for use with MM (moving
magnet) and many high-output MC (movingcoil) cartridges.Your VACdealer can
recommend the proper phono cartridge(s).
Phono Section MC (option): Asabove,this will be fitted to “L6",and provides
approximately 60 dB of gain.Itshould not be used with highoutput pickups,which may
generate excessivedistortion.Your VACdealers can recommend the proper phono
cartridge(s).Aswitch on the backpanel selects between MC (60 dB) and MM(40 dB)
mode.
Monitor: This switches between “Source” and “Tape”.Inthe “Source” position,input
selection is determined by the Source Selector Switch.Inthe “Tape” position,the Source
Selector is overridden,and the Tape inputs are activated.[Important: The Sigma
Preamplifier is outfitted with afull “tape loop”,includingaline level tape input and afixed
level recordingoutput.Inaddition to drivingthe recordinginputs of atape deck,this output
can supply signal to any typical line level input,enablingit to control asecond zone in multi
roominstallations.]
Cinema:This switches between “Source” and “Cinema”,afixed volumepass-through.In
the “Source” position,input selection is determined by the Source Selector Switch.The
“Cinema” position provides access to one of the SigmaPreamplifier’s most unusual feature.
When set in the “Source” position,input choice is governed by the Input Selector switch.
This is the normal setting,and enables the unit to be used as atypical preamplifier.Selecting
the “Cinema” position bypasses all preamplifier controls (includingvolume) and passes the
output fromyour surround sound processor (i.e., the front left- and right-channel outputs of
amultichannel processor) directly to your power amplifier.Inthis case,the users’ analogue
stereo sources would be connected,as usual,to the line,phono and tape inputs.A/V inputs
(DVD,Laserdisc,DSS,etc.)would be connected to the hometheater processor,and the
processor’s main outputs would be connected to the pair of rear panel RCAjackslabeled
“HomeTheater Direct”.When the control is set to the “Cinema” position,the Sigma
functions as apass-throughONLY,and is controlled by the external processor,preamplifier,
or other component connected to the “HomeTheater Direct” (a/k/a “Cine”) inputs.
Mute: This knob is springloaded; movingthe control clockwise will toggle between the

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“Mute” and “Operate” condition.When muted,ared glow will be observed in the LED.On
occasion the preamplifier may seemnot to haveresponded to aturn of the “Mute” knob; this
is due to “contact bounce” in the switch.Simply turn the knob again until the desired
condition is attained.
REMOTE CONTROL
The remote control allows for wireless adjustment of “Mute” and “Volume”.
The remote wand is powered by two “AAA”batteries.These should be of the alkaline type.
You will need to install themin the wand before usingthe remote.Since batteries can leak,
they should be removed fromthe wand if it is not goingto be used for an extended period
of time.
VolumeUp/Down: These buttons cause amotor to turn the volumecontrol.Motorized
controls offer superior sound,as well as direct visual indication of volumesetting.The front
panel control may also be used directly in the conventional way.
Mute: Pressingeither side of the “Mute” button will toggle between the“Mute” and
“Operate” condition.When muted,ared glow will be observed in the LED indicator.Mute
is automatically engaged when switchingon units equipped with the remote control option.
NOTE ON ABSOLUTE PHASE
Throughthe main RCAoutputs SigmaPreamplifier inverts absolute phase.There is some
debate about the audibility of absolute phase,and it generally is not controlled for by the
recordingindustry (i.e., about half of your recordingsmay haveinverted absolute phase).
Ifyou wish to correct your playbacksystemfor absolute phase,this is easily accomplished
in the connections to your loudspeakers.Assumingthat your source component(s),power
amplifier,and loudspeakers do not invert phase,you can correct for the inversion of the
SigmaPreamplifier by connectingthe positiveoutput terminals of your amplifier(s) to the
negativeterminals of your loudspeakers,and the negativeterminals of your amplifier(s) to
the positiveterminals of your loudspeakers. Sonically, this is absolutely transparent.
Throughthe XLRmain outputs,pin 2is positivepolarity (“pin 2hot”) and pin 3is negative
polarity. Note: Do not connect either pin to ground,as someXLR-to-RCAadapters might
do.

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MAINTENANCE
Chassis Care:
AlthoughyourVACchassis and faceplate are extremely durable,their finish can be
scratched,chipped or damaged if improperly treated.Cleaningwith asoft,dampcloth while
the amplifier is switched off and unplugged should restore the finish to its original lustre.
Never clean this component with harsh or caustic cleansingagents.
When shippingyour unit,be certain to wrap the amplifier in the cloth,tissue or plastic wrap
in which it was originally packaged.Given the highgloss of the lacquered finish,makesure
there are no ridges or sharp surfaces in those parts of the cloth which contact the faceplate.
Use of acoarser cloth or failure to otherwise protect the faceplate may result in damageto
the finish.VACis not responsible forcosmetic damageresultingfromimproper care or
packing.
InstallingNew Tubes:
Original Equipment Replacementtubes are available fromVAC.These tubes ensure your
SigmaPreamplifier will meet or exceed all factory performance specifications.
DANGEROUSVOLTAGESEXISTWITH THIS COMPONENT.YOU MUSTFOLLOW
THESESTEPSTO PROTECT YOUR SAFETY. Before replacingtubes,all power must
be turned off.Unplugthe power cord fromthe wall,and removeall audio interconnect
cables.Wait 15 minutes before removingthe top cover.Install the new tubes fully firmly in
their sockets, takingcare to makesure that pin pattern corresponds to holes in tube socket.
Reinstall the top cover before reconnectingthe unit.
NEVER INSTALL OR OPERATE THIS UNITWITHOUT ALL COVERSINPLACE.
Inthe event that trouble is encountered,checkall signal,speaker and power connections.
Ifthe problempersists,follow all safety precautions stated earlier in this section,and check
that all tubes are correctly seated in their sockets.Ifpossible,try another tube.Ifthe problem
persists, please consult your VACdealer or contact the factory directly.

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GENERAL INFORMATION ABOUT TUBES
Itis atruth that each brand of tube sounds different in aparticular highresolution circuit.
This is because no two manufacturers makeatube type in quite the sameway,and the
central tendencies of the performance parameters will differ slightly with each maker.This
sonic variability may at first seemaliability,but further thought will reveal that it is an
advantage.The owner of atube amplifier can select those tubes which sound likethe real
thingin his/her specific system. VACcan recommend the best tubes for your amplifier.
The tube types used in this component should last for 8,000 to 15,000 hours of use (two to
ten years for most users).There are always occasional exceptions,longor short.Ifyou have
to replace atube before,say,3,000 hours,just replace the single tube.Ifyou haveto replace
atube at 8,000 hours, replace themall and keep those still functioningwell as spares
VACwill be happy to test tubes for concerned customers.
WARRANTY
Your equipment is warranted for aperiod of thirty (30) days fromthe date of purchase.
Inaddition,if the registration formis received by VACalongwith acopyof your sales
receipt froman authorized VACdealer within this thirty days,aservice contract will be
extended to cover your equipment for two (2) years (tubes excepted).
This warranty applies only to units sold in the United States of America throughauthorized
VACdealers,and operated within the U.S.by the original purchaser.Itcovers factory
service,and,in the continental U.S., standard return shipping.For warranty information
outside of the U.S.contact the importer of VACequipment for your country.Units sold
outside of the U.S.should still be registered with VAC.Itis the responsibility of the dealer
and/or customer to determine the suitability of this product for any particular application.
Your questions and comments are always welcome. Contact:
ValveAmplification Company, Inc.
1911 East Avenue North
Sarasota, FL34234
Telephone[phone]
Facsimile941 952 9691
E-mail [e-mail]

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VAC SIGMA PREAMPLIFIER SPECIFICATIONS (typical)
Type:Vacuumtube line stagewith hometheater (direct power
amplifier) mode.
Options:Remote control for volume&mute
Dedicated MM phono input
Dedicated MC phono input
Linestage preamplifier:
Gain:24 dB
Maximuminput signal:Essentially infinite (attenuation precedes line stage)
Main output:Invertingto RCA
Pin 2positive“hot” on XLR, pin 3negative
Tape output:Unity gain fromselector, non-inverting(archival correct)
Phono stage preamplifier (optional):
Gain:40 dB MM, 60 dB MC (measured at tape output)
Residual noise:3mVat output (S/N ratio approx. 69 dB MM)
Overload:117 mV@1kHz =8volts RMSoutput, MM
460 mV@10 kHz, MM
Frequency response:RIAA +/- .25 dB
Notes:Does not invert absolute phase.
Tubecompliment:
3x12AX7 (phono option) (located on circuit board at left side of chassis)
2x12AU7/6189A(line stage)
2x12AX7 (line stage)

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