QSC 1100 User manual

1
Owner’s
Manual
®
A U D I O
Rev. C
Model 1100
Model 1200
Model 1400
Model 1700
▲
▲
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SeriesOne
*TD-000027-00*

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EXPLANATION OF
GRAPHICAL SYMBOLS
The lightning flash with arrowhead symbol, within an
equilateral triangle, is intended to alert the user to the
presenceofuninsulated“dangerousvoltage”withinthe
product’senclosurethatmaybeofsufficientmagnitude
to constitute a risk of electric shock to humans.
The exclamation point within an equilateral triangle is
intended to alert the users to the presence of important
operating and maintenance (servicing) instructions in
the literature accompanying the product.
EXPLICATION DES
SYMBOLES GRAPHIQUES
Lesymboleèclairavecpointdeflècheàl’intrérieurd’un
triangle équilatéral est utilisépour alerter l’utilisateur
de la présence àl’intérieur du coffret de “voltage
dangereux”non isoléd’ampleur suffisante pour
constituer un risque d’elétrocution.
Le point d’exclamation àl’intérieur d’un triangle
équilatéral est employépour alerter les utilisateurs de
la présence d’instructions importantes pour le
fonctionnement et l’entretien (service) dans le livret
d’instruction accompagnant l’appareil.
CAUTION: To reduce the rish of electric shock, do not remove
thecover. No user-serviceableparts inside. Referservicing to
qualified service personnel.
WARNING:Topreventfire or electricshock,donot expose this
equipment to rain or moisture.
CAUTION
RISK OF ELECTRIC SHOCK
DO NOT OPEN
ATTENTION: Pour enviter les risques de chock electrique, ne
pasenleverlecourvercle.Aucunentretiendepiecesinterieures
par l’usager. Confier l’entretien au personnel qualifie.
AVIS: Pour enviter les risques d’incendie ou d’electrocution,
n’exposez pas cet article a la pluie ou a l’humidite.
AVIS
RISQUE DE CHOCK ELECTRIQUE
NE PAS OUVRIR
SAFEGUARDS
Electrical energy can perform many useful functions. This unit has
been engineered and manufactured to assure your personal safety.
Improperusecanresultinpotentialelectricalshockorfirehazards.In
ordernottodefeat thesafeguards,observethefollowinginstructions
for its installation, use and servicing.
PRECAUTIONS
L’énergieélectriquepeutremplirdenombreusesfonctionsutiles.Cet
appariel a été conçu et réalisé pour assurer une sécurité personnelle
entiére. Une utilisation impropre peut entraîner des risques
d’électrocutionoud’incendie.Danslebutdenepasrendreinutilesles
mesures de sécurité, bien observer les instructions suivantes pour
l’installation, l’utilisation et l’entretien de l’appareil.
Series One Power Amplifier
Operation Manual
TABLE OF CONTENTS
I: INTRODUCTION
1.1 Welcome .................................................... 3
1.2 Warranty..................................................... 3
1.3 Overview of Amplifier ................................. 3
1.4 Specifications ............................................. 4
II: BASIC INSTRUCTIONS
2.1 Unpacking and Inspection.......................... 5
2.2 Important Precautions ................................ 5
2.3 Quick Instructions....................................... 6
III: OPERATION
3.1 Cooling ....................................................... 8
3.2 AC Requirements....................................... 9
3.3 Input Connections ...................................... 9
3.4 Octal Module Accessories........................ 12
3.5 Input Switches and Mono Bridging........... 12
3.6 Speaker Connections............................... 13
3.7 25 and 70 Volt Systems ........................... 15
3.8 Protection Features.................................. 15
3.9 Operational Troubleshooting.................... 16
IV: SPEAKER PROTECTION
4.1 Background .............................................. 18
4.2 DC Protection........................................... 18
4.3 Horn Driver Protection.............................. 18
4.4 Power Capacity ........................................ 19
4.5 Power Limiting.......................................... 19
4.6 User Responsibility .................................. 19
V: MAINTENANCE AND SERVICE
5.1 Cleaning ................................................... 20
5.2 Dust Removal........................................... 20
5.3 User Maintenance .................................... 20
5.4 Obtaining Service..................................... 20

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I. INTRODUCTION
1.1 WELCOME
Thank you for selecting a QSC Series One power amplifier for your audio system. Our goal is to ensure your complete
satisfaction with your amplifier purchase. Please do not hesitate to call your QSC Dealer or QSC Audio Products if you
have any service problems or questions not answered in this manual.
Your QSC Series One power amplifier can be readily operated by anyone familiar with professional audio systems. The
four models covered by this manual, the 1100, 1200, 1400 and 1700, have similar features and connections. Any
differences will be covered in the following detailed instruction sections of this manual. Because not all amplifier features
andcharacteristicsarestandardized,werecommendthatbothexperiencedandfirst-timeusersreviewthecontentsofthis
manual.
Please consult the table of contents for quick reference to sections of interest. We recommend that all users review the
“Basic Instructions”section of this manual before installing or operating the amplifier.
1.2 WARRANTY AND DISCLAIMERS
QSCAudioProducts,Inc. isnotliableforanydamagetospeakers,amplifiers,oranyotherequipmentthatiscaused
by negligence or improper installation and/or use of the Series One Amplifier.
Product Warranty
QSCAudioProducts,Inc.guaranteestheSeriesOneAmplifiertobefreefromdefectivematerialand/orworkmanship
foraperiod of three years fromdateofsale,andwill replace defective parts andrepairmalfunctioningproductsunder
this warranty when the defect occurs under normal installation and use—provided the unit is returned to our factory
via prepaid transportation with proof of purchase (sales receipt). This warranty provides that examination of the
returned product must disclose, in our judgement, a manufacturing defect. This warranty does not extend to any
product which has been subject to misuse, neglect, accident, improper installation, or where the date code has been
removed or defaced.
Warranty and Service Repair Instructions
1. Packthe product safelymakingsure to includeacopy of thesalesreceipt,your name,returnaddress,and phone
number. Mark the package: Attention Service Department.
2. Call QSC’s Service Department—1-800-772-2834—and obtain a “Return Authorization”(R.A.) number.Ship the
product prepaid to QSC Audio Products. We recommend UPS.
3. We will determine if the product is under warranty:
a. If it is, we will repair and ship it back to you at no charge.
b. If it is not, we will contact you and inform you of the charges. Upon your approval, we will repair the product
and ship it back freight and services charges collect (COD).
1.3 OVERVIEW OF AMPLIFIER
The basic Series One circuit design is the result of years of QSC product development, combined with a host of new
features,anumberofcircuitimprovements,andtheuseofpremiumcomponents. Theresultissuperbaudioperformance,
exceptional reliability, and a flexible interface system.
Tobeginwith,eachchannelhasabalancedbipolarpowersupplywhichassuresproperdynamicresponseduringprogram
peaks. We then combine a superior series of complementary power transistors with a modern high-performance
integrated circuit at the input, in order to deliver the required power with the minimum number of amplifying stages. This
ensures minimum signal degradation with maximum reliability and consistency. The circuit accepts balanced or
unbalanced input signals and provides the high internal gain needed for low overall distortion and wide frequency
response. Due to the circuit simplicity and the fact that we can use direct-mounted output transistors, Series One power
amplifiers offer higher power, smaller size and lower weight than older designs.
In order to ensure that users get the full benefit of high performance amplification despite real-world hazards, our circuit
includes many operational and protective features. Complete protection is provided for open-circuit, short-circuit, and
mismatched loads; the amplifier will shut down temporarily if it over-heats, and AC circuit breakers protect against
excessive power levels. Our protection circuits are designed to ensure a minimum of false triggering and unwanted
interruptions and except for the AC breaker, which replaces the usual AC fuse, all protection systems will reset

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automatically as soon as safe operation is assured. An equally important muting circuit protects the loudspeakers from
unexpected damage, by muting the amp during turn-on and turn-off and by blocking DC faults, whether caused by the
amplifier or preceding components.
Thefrontpanelpresentsessentialuserinformationandisrecessedtopreventdamage. Thegreenpowerindicatorserves
as a pilot light and individual red clip indicators monitor overall performance of each channel. The AC switch and circuit
breaker are front mounted for convenience and quick resetting. Except for the Model 1100, the Gain controls and input
programmingswitcheshavebeenplacedonthebackoftheamplifiertoprotectthemfromdamage,inadvertentadjustment,
or tampering. The Gain controls, as well as a pair of headphone jacks, have been placed on the front panel of the Model
1100 to allow easy access in monitoring applications.
Theamplifiersusea separatebipolarpowersupplyfor eachchannel,forminimumcrosstalk,cross-distortion,and greater
reliability. A single dual-secondary transformer feeds isolated, separately fused rectifiers and filter capacitors for each
channel. Thus the remaining channel can keep running in case of breakdown.
In order to interface properly with a variety of pro-audio systems, we have included all of the popular input and speaker
connectors. BalancedorunbalancedinputscanbemadeusingXLRplugs,screwlugstothebarrierstrip,or1/4-inchphone
plugs (ring-tip-sleeve for balanced inputs). Speaker connections are made with 5-way binding posts.
The steel chassis, of the 1200, 1400 and 1700, is a 14 gauge, single piece design with integral rack mounting ears. The
1100 chassis features single piece aluminum extrusions that form the sides of the chassis and feature integral rack
mounting ears and heatsinks. The extra strength chassis and rugged mounting of internal parts contributes greatly to
QSC’s reputation for reliability by protecting the circuitry from years of road abuse.
All of these points are more fully explained in the following Sections.
1.4 SPECIFICATIONS
1100 1200 1400 1700
OUTPUT POWER (per channel)
Both Channels Driven
8 ohms, 20-20 kHz, 0.1% THD 50w 100w 200w 325w
4 ohms, 20-20 kHz, 0.1% THD 70w 150w 300w 500w
Bridged-Mono Operation
8 ohms, 20-20 kHz, 0.1% THD 140w 300w 600w 1000w
DYNAMIC HEADROOM dB
8 ohms 2.0 2.0 2.0 1.9
4 ohms 2.3 2.5 2.5 2.9
DISTORTION, THD @ 8 ohms
20-20 kHz, at rated power Less than 0.1%, 0.01% typical
SMPTE-IM at rated power Less than 0.025%
FREQUENCY RESPONSE 20-20 kHz, +0, -1.0 dB at 1 watt
DAMPING FACTOR @ 8 ohms Greater than 200
NOISE (A-weighted) 100 dB below rated power
VOLTAGE GAIN, dB 26 29 32 34
SENSITIVITY, V RMS 1.0 1.0 1.0 1.0
(for rated power, 8-ohms)
INPUT IMPEDANCE 10K unbalanced inverting
20K balanced or unbalanced non inverting
CROSSTALK -70dB, 20-20 kHz
CONTROLS AC Switch, Circuit Breaker, Gain Knobs, Input Programming DIP Switches,
and Bridging Switch
INDICATORS Power: Green LED Clip: Red LED
COOLING Convection Convection 2 speed fan 2 speed fan

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AMPLIFIER PROTECTION Full Short Circuit, Open Circuit, Over-Temp, Ultrasonic and
RF Protection. Stable into reactive or mismatched loads.
SPEAKER PROTECTION DC Load Fault Protection
3 Second Turn-on, Instant-off Muting
COMPLEMENTARY OUTPUT DEVICES 4 8 16 32
POWER SUPPLY SingleTransformerwithIndependentIsolatedSecondaryWindings,Fault
Fuses, Rectifiers, and Filter Capacitors
DIMENSIONS
Faceplate Width Standard 19" Rack Mounting
Faceplate Height 1.75" 5.25" 5.25" 7.0"
Chassis Depth 8.7" 9.5" 9.5" 11.75"
WEIGHTShipping, Lbs 15 28 37 57
Net, Lbs 12 24 34 55
II: BASIC INSTRUCTIONS
2.1 UNPACKING AND INSPECTION
All QSC Series One amplifiers are fully tested and inspected before they leave the factory. Despite the protective carton
and rugged amplifier design, it is possible for shipping abuse to damage the amplifier. Check for obvious carton damage
while unpacking the unit. After removing the amp from the box, rotate it in all directions to check for loose parts inside.
Please save the carton for return shipment, if required. QSC does not warranty against damage caused by sending
amplifiers back in the wrong carton.
If shipping damage is evident, notify the transportation company immediately. Only the consignee can file a claim with
the carrier for shipping damage. QSC will cooperate fully in such an event. Be sure to save the carton for the shipper to
inspect.
2.2 IMPORTANT PRECAUTIONS
2.21 Thepowermustbe OFF when making any connections. If you connect plugs withthepoweron, especially in dry
environments, static sparks or bad cables can cause pops or hums which can damage speakers.
2.22 When first powering up the amp, have the amplifier Gain controls all the way off, in case of defective cables or
hookups. Turn the Gain controls up gradually until normal operation is verified.
2.23 Checkthe AC voltageprintedon the serialnumberlabelto ensureyouramplifieris properlyconfiguredforthe AC
voltage supplied in your area before connecting the AC plug.
CONNECTIONTOAVOLTAGESOURCEOTHERTHANTHEONESPECIFIEDWILLIMMEDIATELYDAMAGE
THE AMP, AND VOIDS THE WARRANTY.
2.24 Never connect the speaker terminals (red binding posts) for two channels together on any power amplifier. The
two channels will fight each other and possibly fail. Do not connect the speaker ground terminals (black binding
posts)tochassisorsignalgrounds,astheresultantgroundloopcouldcauseultrasonicoscillations.Inotherwords,
keep all speaker wiring separate for each channel, and separate from input wiring.
2.25 Do not remove the amplifier cover, as there are dangerous voltages inside. Do not expose to rain or moisture.
Referallservicingtoqualifiedpersonnel. Thewarrantywillbevoidiftheampistamperedwithbynon-QSCrepair
centers or personnel.
2.26 The QSC Warranty does not cover tampering by unqualified personnel, or repairs made at non-QSC repair
centers. Please call the factory for Service Center information and locations.

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2.27 Highvoltagescanbepresentonthespeakerterminals.Alwaysconnectspeakerterminalswiththepoweroff,and
use heavy gauge cable with no frayed strands or damaged insulation.
2.28 Please be aware that power amplifiers have high power circuitry inside with potential for fire and shock hazard;
neverpluginadamagedamplifieruntiltheconditionoftheinternalinsulationischecked. Ifacircuitbreakerblows
quicklywhenturningtheampon,the amplifierisdefectiveandshouldnotberestarteduntiltheamplifierhasbeen
repaired. Failure to observe these precautions could lead to fire or shock hazard.
2.29 Power amplifiers are inherently heavy and may become hot after use; provide adequate support and be careful
how you hold the amplifier when handling it.
2.3 QUICK INSTRUCTIONS
2.31 StereoOperation Theseinstructionscoverthenormaluseoftheamplifierintwo-channelorstereoapplications.
See Section 3 for detailed installation instructions and special cases.
2.32 AC Power Connect the AC cord to a standard GROUNDED outlet only. The amplifier will operate satisfactorily
overa+/-10%rangeofvoltages,butfullratedperformancewillbemetonlyattheratedvoltage. Failuretoproperly
ground the amplifier may result in unwanted hum and noise and will create a potential shock hazard.
2.33 FloatingChassisGround ThereisnoprovisionforliftingsignalgroundrelativetochassisgroundonSeriesOne
amplifiers. For safety reasons do not lift the ground pin on the AC cord. Electronic balanced inputs are provided
for hum rejection. Use balanced input cables to avoid hum, interference and ground loops.
2.34 Input Programming Switches 8-pole mini DIP switches are located on the rear panel—see the rear panel
illustration for details. They come factory set for normal stereo operation (switches 1,2 and 7,8 up). See section
3.5 for other cases.
2.35 Octal Socket For normal operation, nothing should be plugged into the octal socket. It comes from the factory
with a protective label to prevent corrosion of the pins.
2.36 Input Connections The input polarity is as follows:
1/4-inch plug: tip is “minus”or inverting input
ring is “plus”or non-inverting input
barrel is ground, as always
XLR plug: pin 1 is ground, as always
pin 2 is “plus”or non-inverting input
pin 3 is “minus”or inverting input (AES Standard)
Barrier Strip: “GND”is circuit ground
“+”is “plus”or non-inverting input
“-”is “minus”or inverting input
Whenmakingunbalanced connections, the barrelofanordinarytwo-wire1/4 inch plug willgroundthe“plus”side
of the balanced input; XLR plugs will need to have the unused pin grounded inside the plug, and the installer will
need to ground the unused screw on the barrier strip. It is still possible to use the balanced inputs to reject hum
with an unbalanced signal. See Section 3.3 for details.
2.37 SpeakerConnections Bananaplugs,spadelugs,orbarewireendscanbeconnectedtothe5-waybindingposts.
Be sure to observe correct polarity (red/black terminals) for each speaker to insure that all speakers move in the
same direction.
The1100featuresapairofstandard1/4"headphonejacksonthefront panel. Theyare connectedtothespeaker
outputs of the amplifier through a resistive pad that prevents excessive power levels from damaging the
headphones.
2.38 Power Up Startwith the gain controls off until properoperationisverified. Uponturningontheswitch, the power
LED should come on green. After three seconds, the muting circuit should release and turn on the sound. The
amp should now be working, and the Gain controls can be advanced. In case of difficulty consult Section 3.9.

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2.39 Operation and Indicators Note that the Gain controls are calibrated in dB. As you turn the Gain down, it takes
moreinputsignaltoreachfullpower,sotheGainshouldbekeptintheupper12dBofitsrangeforfullpoweroutput
from normal signal sources. The “0 dB”reference indicates that the amplifier gain is at 26 dB when the knob is
adjusted to this position.
Note that full amplifier output power may be achieved even if the gain controls are not turned fully up. However,
a larger input signal will be required to produce full rated output.
LED(LightEmittingDiode)indicatorsmonitor the operation of the amplifier. A green LED serves as the pilot light
to indicate that power is on. Each channel has a red “Clip”indicator that will show any distortion in the amplifier.
Uponpowerup,thesemaynotflashsymmetrically.Thisdoesnotnecessarilyindicatethattheremaybeaproblem.
The muting circuit blocks the sound for three seconds after turn-on and immediately after turn-off. This prevents
turn-on and turn-off thumps and transients from reaching the speakers.
Please refer to Section 3 for more detailed instructions.
1100 FRONT VIEW
AC BREAKER AC SWITCH GAIN CONTROLS
HEADPHONE
JACKS
PILOT LIGHT
CLIP INDICATORS
1100 BACK VIEW
AC CORD
BARRIER STRIP INPUTS
1/4 INCH INPUTS
INPUT
PROGRAMMING
SWITCHES
OCTAL SOCKET
XLR INPUT
SPEAKER
BINDING POSTS

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III: OPERATION
3.1 COOLING
3.11 The Model 1100 uses side-mounted, convection-cooled heat sinks for power transistor cooling. When rack
mounted, temperature rise should not be excessive for eight-ohm loads at normal listening levels, but operation
into lower impedances at high powers may cause heat build-up unless good access to cool air is provided at the
sides of the rack. If side clearance and/or louvers cannot be provided, some space should be provided over and
under the amplifier for air circulation.
PILOT LIGHT
AC
BREAKER CLIP
INDICATORS
AC
SWITCH
MODEL 1200/1400 FRONT
OCTAL
SOCKETS
GAIN CONTROLS
XLR INPUTS 1/4 INCH INPUTS
BARRIER STRIP
INPUTS
MONO BRIDGE SWITCH AC CORD
SPEAKER CONNECTIONS
INPUT
PROGRAMMING
SWITCHES
MODEL 1200/1400 BACK, SIMILAR TO 1700

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3.12 The Model 1200 uses an internal heat radiator, top ventilation slots, and the faceplate for cooling. The faceplate
will get quite warm in severe usage, but this heatsink system will serve adequately when the amp is used in the
open air.
Ifthe amps are mounted in tightly confined racks, we recommend that a fanbe located so that it blows air into the
rear of the rack. If the back of the rack is sealed, the airflow will be guided into the rear vents of each amp and
out through the faceplate vents. This will provide the same flow-through cooling as the 1400 and 1700 have. A
normal sized “muffin”or computer fan should provide enough cooling air to service several amps. In addition, an
optional fan kit for the Model 1200 is available from QSC which can be installed by qualified service personnel.
3.13 TheModel1400and1700amplifiersfeatureHighTurbulenceFlow-ThroughCooling. Aninternal,two-speedfan
forces air across a light-weight, high-turbulence heat radiator. Air flow is from the rear, so that warm air is
exhausted to the front rather than baking the rest of the rack. This prevents the recirculation of heated air and
reduces internal rack temperatures. Be sure that plenty of inlet space is allowed in the rack for free air flow.
Dustfiltersarenotprovidedon the amplifiers since small filters tend to clog quickly and reduce airflow. However,
dust can be a problem and it should be removed from the insides of the amplifiers by using a compressed air jet
through the vents several times a year. If amps are used in a fixed installation, where removal is not convenient,
consider an additional fan pack in the bottom of the rack with a large external dust filter. The additional fan will
maintain air flow, and the external filter can be removed and cleaned from the front of the amp rack.
When installing the 1400 or 1700 in the same rack with passively cooled amps or fan cooled amps that exhaust
into the rack, locate the 1400 or 1700 on the bottom. This will assure the coolest air for all amps.
3.14 Thermalcut-outTheamplifiershouldnormallyrunalittlewarmtothetouch,andunderhigh-poweroperationmay
get quite warm, especially around the heat sinks. If the heat sink temperature becomes excessive, internal
thermostats for each channel will remove power temporarily. (See Section 3.8)
3.2 AC REQUIREMENTS
3.21 AC tolerances Series One amplifiers are designed for safe operation at AC voltages l0% higher than rated;
however, temperature rise and transformer hum may increase somewhat. Operation on lower-than-normal AC
voltagesis not harmful to theamp, but performance will be progressivelylost. For voltages down to 75%of rated
voltage, no effect other than loss of peak power should be noticed. If voltage declines further, short-circuit
protection (current limiting cutback) may be experienced during heavy peaks into low impedance loads. The
muting circuit may not come on at less than 70% of rated voltage, but once on, it should stay on down to about
30% of rated voltage. There should be no sub-audio or DC transients caused by fluctuating AC voltages; you
should suspect poorly regulated preceding components if thumps or voice-coil excursions are observed during
peaks.
3.22 ACsupply Inordertomaintainfullratedpower, poweramplifiersrequirewell-regulatedACvoltageoftheproper
rating. Thisis notalwayseasy toassurewhen largebanksof ampsareused. The problemisfurther complicated
by the fact that virtually all practical amplifier power supplies use peak rectification of the AC waveform. This
meanspowerisdrawnonlyfromthetipsoftheACsinewave. Whenmanyampsareused,orthereisanexcessive
length of inadequate gauge AC wiring to the amps, these tips can be seriously eroded without a major effect on
measured RMS voltage.
Bulk-power devices, such as lamps, on the same circuit may not be greatly affected, but other electronic
components, which normally use the same type of rectification, may be seriously affected during high-power
peaks. Thisisespeciallytrueofsensitivedeviceslikecomputers,videogear,etc. Thisiswhypowerampsshould
have their own electrical circuit if possible.
3.3 INPUT CONNECTIONS
(See illustrations in Section 2.3 for location)
3.31 Input Labeling All input functions are located on the rear panel of the amplifier and are labeled.
3.32 Input Jacks 1/4-inch ring-tip-sleeve, female XLR, and three-circuit barrier strip terminal blocks are provided for
input connections.
3.33 Input Circuit An electronic balanced input is standard. This uses matched, 20 kΩresistive dividers and the
differential input of a high performance 5532 op-amp to accept balanced input signals and reject common-mode
signals. For best performance in the balanced-input mode, the source should have equal impedances for both
signalconductors,sothattheloadingeffectoneachlegwillbethesameforcommonmode(noise)signals. Minor

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mismatcheswillresultinslightlossofcommon-moderejection,butwillstillhavemuchgreaternoiserejectionthan
unbalanced inputs.
3.34 BalancedInputs Forproperbalanced-lineoperation,thecableshieldshouldbeconnectedatthepoweramplifier
end only and kept separate from both signal conductors. The cable shield should be connected to the barrel of
a1/4-inchplug,topin1of an XLR plug, or to the “GND”terminal of the barrier strip. Balanced-line cables contain
two signal conductors, a “plus”polarity, often called “high”or “hot”, and a “minus”polarity, often called “low”or
“return”. The“plus”conductorshouldgoto the ring of a 1/4-inch plug, to pin 2 of an XLRplugortothe“plus”input
of the barrier strip, for the amplifier to reproduce the signal in the same polarity (non-inverting operation). This
conforms to the international standard for XLR connections. The “minus”conductor goes to the tip of a 1/4-inch
plug, to pin 3 of an XLR connector, or to the “minus”terminal of the barrier strip.
Amp Input
Figure 3.34a 1/4-inch RTS Balanced Output Connection
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Amp Input
Figure 3.34b 3-Pin XLR Balanced Output Connection
3.35 Unbalancedinputs Sincetheinputsignalrespondsto thedifferencebetweentheplus andminussignals,ifonly
asingle-ended, unbalancedsignalisavailable,theunusedinputterminalmustbegrounded foroperationwithout
lossofgain. Theability to reject cable-induced hum and noiseislost,butthismaynot be needed in well-shielded
environments with short distances between audio components.
For unbalanced signals, the barrel of an ordinary two-conductor (mono) 1/4-inch plug will ground the sleeve
terminalwhenpushedallthewayintothe1/4-inchjack,sonospecialwiringisrequired. ForXLRplugs,thesignal
conductor should be connected to pin 2, and pin three should be connected, inside the plug , to pin 1 (ground).
On the barrier strip, the “minus”terminal is tied to the adjacent “GND”terminal, and the signal conductor should
be connected to the “plus”terminal. In all cases, of course, the shield goes to the ground terminals. The tip of an
unbalanced 1/4-inch plug has been made negative (inverting) because it is far more stable in systems which are
subject to complex ground loops.
Amp Input
Figure 3.35a 1/4-inch RTS Unbalanced Output Connection
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Amp Input
Figure 3.35b 3-Pin XLR Unbalanced Output Connection