AUSTRALIAN MONITOR AM1200 User manual

Operation Manual
AM1200

REFERSERVICINGTO
QUALIFIED PERSONNEL.
NO USER SERVICEABLE
PARTS INSIDE.
!WARNING!
DO NOT EXPOSE TO EITHER
RAIN OR MOISTURE
!CAUTION!
RISKOFELECTRICALSHOCK
DO NOT OPEN
The user should not attempt to service the unit. Only
qualified and knowledgeable personnel familiar with
the internal workings of the unit should attempt any
repair, servicing or authorized modification to the
unit. The unit does not contain any parts which the
usercanserviceorre-useinthisoranyotherproduct.
IMPORTANT!
Please read carefully.
Thisoperationmanualcontainsimportantinformationregarding
safetyprecautions,installation,performance,operationand
maintenance of your AM1200 power amplifier. You should
familiarize yourself with the contents of this
Safety Precautions
and Labelling
manual before operating your amplifier.
Therearpaneloftheunithasanumberofmarkingsand
internationally recognized symbols related to the
hazards and precautions that should be taken when
operatingMAINSconnectedequipment.
ThepresenceofaLIGHTNINGFLASHwithanarrowhead
containedwithintheboundariesofanequilateraltriangle
isintendedtoalerttheuserthatdangerousuninsulated
voltagesmayexistwithintheunit's enclosure. These
voltages may be of a sufficient magnitude as to
constitute the risk of an electrical shock.
This symbol is reinforced with the text:
ThepresenceofanEXCLAMATIONMARKcontained
within the boundaries of an equilateral triangle is
intended to alert the user that there is important
operatingandmaintenanceliteraturethataccompanies
the unit.
Theunitshouldnotbeoperatedinasituationwhereit
mayencountertheentryofwater,rain,oranyfluids.To
exposetheunittotheaboveconditionsmaymakethe
operationofthe unit hazardousandincrease the risk
of electrical shock.
If you are in need of special assistance and the
information you require is outside the scope of this
manual,pleasecontactyournearestserviceagentor
Australian Monitor direct:
THE TECHNICAL OFFICER
AUSTRALIANMONITOR
C/-AUDIOTELEXCOMMUNICATIONSPTYLTD
PRIVATEBAG149,
SILVERWATER. N.S.W. 1811
AUSTRALIA.
Phone Fax
Local [phone] (02) 9748-2537
International 61-2-9647-1411 61-2-9748-2537
Email [email protected]
Internet www.australianmonitor.com.au

Features:
- 4, 3 or 2 channel operation.
- Custom designed, 3RU heavy duty alloy chassis.
-Modularconstruction.
-Symmetrical layout- evenweight distribution.
-Linear,well-regulating,highcurrentpowersupply.
-Dualisolatedhighcurrentsecondaries.
-Highefficiencytoroidalmainstransformer.
-Continuoushighpowercapability.
- Lateral Mosfet Class AB output stage.
- Binding post and Neutrik "Speakon" output
connection.
Protection Features
-Suppressionof inrushcurrentat mains turn-on.
-Inputmuting at turn-on.
-Inputovervoltageprotection.
-Radio-frequencyinterferencesuppression.
-Thermalprotectionandindication.
-Short-circuitprotection and indication.
-Inputsignalstrapping(loopthrough)connectors.
-Activebalancedinputs.
-21 Positiondetented attenuators.
-Massiveheat-sink/heat-exchangers.
- Efficient front to back cooling.
- Dual, twin speed axial fans.
-Frontand rearcarryhandles.
-Frontand rear mountingpoints.
-High-quality,close-tolerancecomponentsused
throughout.
- Mains Circuit Breaker.
-IndependentDCsupplyrailfusesperchannel.
-Layout,grounding,decouplingandcomponentry
havebeenoptimizedtoprovidetheuserwith
stability, reliability and longevity.

Contents Page
1. Introduction 5
2. Controls,ConnectorsandIndicators 6
2.1FrontPanel 7
2.2RearPanel 9
3. Installation 11
4. Operation 13
5. Bridge Mode 15
6. TwoOhmorNotTwoOhm 16
7. Maintenance 17
8. Warranty 18
9. Specification 19
List of Illustrations Page
Figure 1. Block Diagram 5
Figure2.FrontPanelLayout 6
Figure3.RearPanelLayout 8
Figure 4. Case Dimensions 10
Figure 5. "Speakon" Connector Wiring 12
Figure 6. Bridge Mode Speaker Wiring 15

1. Introduction
Congratulations on choosing Australian Monitor for
yourprofessionalamplificationrequirements.
The design of your AM1200 Audio Power Amplifier
embracesalltheaspectsofawelldesignedunit.The
visual design, mechanical, electrical and sonic
parameters,alongwithour dedicatedmanufacturing
process, have all been optimized to provide a
professional tool that exhibits quality, reliability and
longevity.
TheAM1200 amplifiersare3unit(5.25")tall,19"wide
Figure 1 Amplifier Block Diagram
Introduction 5
rack mountable units.
Each channel of the amplifier comprises a balanced
active input with an attenuator driving a differential
classA drivestage whichin turn drives a fan-cooled,
class AB MOSFET output stage configured as a
sourcefollower.Theunitoperatesfromahighcurrent
capablelinearpowersupply.
Theseunitshavebeenspecificallydesignedtodeliver
their high power output with minimal distortion, and
providethecriticaldegreeofcontrolrequiredbyyour
speakers, at high duty cycles for extended periods.

6 Controls & Connectors
Figure 2 Front Panel Layout
2. Controls,
Connectors
& Indicators

Front Panel
Figure 1 shows the panel layout of the AM1200. The
functionsofthecontrolsandindicatorsareasfollows:
1.Attenuator
Levelcontrol foryouramplifierisprovidedbya21
positiondetentedpotentiometerandindicatesgain
reduction in decibels from the 0 dB position
(maximumgain,noattenuation).
2. Status Indicator
ThisisadualcolourLEDwhichdisplaysthestatus
of the output stage and displays three levels of
operation.
Theselevelsare:
Below-20dB (unlit)
-20dBandabove (green)
1dBbelowactualclipping (red)
The LED will turn green once the output voltage
exceedsthe -20dBpoint (3.5volts).
The LED will change to red once the output ex-
ceeds the -1dB point before actual clipping of the
amplifier’soutputstage.Thethresholdofthe-1dB
point is referred to the amplifier supply rails and
alterswith changes in the mainssupply, changes
in the load and duty cycle fluctuations.
Theattackanddecaytime(ballistics),ofthestatus
circuit are those of a Peak Programme Meter
(P.P.M.)
NOTE:Theamplifierisnotdamagedbyrunninginto
clipping,butspeakersmaybe.Tomaximisethelife
of your speakers, try to keep clipping infrequent.
3. Fault Indicator
ThisamberLEDwillilluminatewhenafaultcondition
exists.
The fault detection circuit monitors the difference
betweendriveandoutputinyouramplifier.
Ifyouhaveashortonthespeakeroutput(orablown
negativerailfuse)theLEDwillflashbrightlyinsync
with the programme. This LED will also flash with
programmepeaksforgrossoverloadsoriftheload
is 2 ohms or less.
The circuit has two stages of operation:
1.Itwillprovideindication(e.g.grossoverload)but
does not affect the input signal (a faint flash).
2. It will indicate and mute the input signal (e.g.
shorted output) (brightly flashing or permanently
on).
4. Power Switch
PresstheswitchDOWNfor powerON and UP for
power OFF. At start-up (turn-on) the input to the
amplifier is muted by 30dB for approximately two
seconds.
5. On / Thermal Indicator
Whenswitching theamplifier on, thisred LEDwill
momentarilyflashRed,indicatingcorrectoperation
oftheMainsIn-RushCurrentSuppressioncircuit.
After establishment of the Inrush Current
SuppressioncircuittheLEDwill change to Green
indicatingtheunitisonandreceivingmainspower.
In the advent of a thermal overload this LED will
illuminateredindicatingthattheinternaloperating
temperature of one or all amplifier channels has
exceededasafelevelofoperationandthe amplifier
willbeshutdown.Thefanswillcontinuetorunand
oncetheamplifierhascooleddownsufficiently,the
amplifierwillstartupautomaticallyprovidingInrush
CurrentSuppressionandinputsignalmutinguntil
establishment of the amplifier after which it will
returntonormaloperatingmode.
NOTE:Youshouldalwaysensurethatthefangrille
iskeptcleanandfreefromthebuildupofdustand
lint.Thiswillensurelongeroperationofyouramplifier
and reduce the possibility of it prematurely going
intothermalshutdownmode.
6. Bridge Mode Indicator
This LED will illuminate yellow when the relative
pairsofchannelsoftheamplifierhavebeenselected
tooperateina "BRIDGE"modeofoperation.
Selection of the bridge mode is accomplished by
engagingtherearpanel"PUSHTOBRIDGE"Switch.
See section 5 (page 15) for more information on
Bridgemodeoperation.
Controls & Connectors 7

8 Controls & Connectors
Figure 3 Rear Panel Layout

Rear Panel
7. Balanced Input
A female 3-pin XL type connector is provided on
eachinput:
Pin 1 = Signal Ground;
Pin2= Hot (non-invertingorinphase);
Pin3=Cold(invertingorreversephase).
7a.SignalStrapping
A male 3-pin XL type connector is provided on
channels A and D and is wired in parallel with the
female input XLR for strapping / looping signal
betweenamplifiers.
8. Bridge Switch
Pushing this switch in engages the BRIDGED/
MONO mode of operation for the relative pair of
channels. In this mode, your amplifier will only
acceptsignal applied tothe Channel-AinputXLR
(for channels A and B) or signal applied to the
Channel-DinputXLR(forchannelsDandC).
Channel-A will now control the level of the A/B
bridged pair and Channel-D will now control the
leveloftheD/Cbridgedpair.
Speaker termination should be sourced from the
redbinding-postoutputs.
9. SPEAKON Output Connector
TheNEUTRIK(NL4MP)4waySPEAKONconnector
isprovidedasthemainspeakeroutputtermination.
Thisemergingstandardofloudspeakertoamplifier
connection allows access to both channels of the
amplifier via the one connector for bi-amp
applications. Channel-A (or D) is considered the
dominantchannelandhasbothchannelswiredto
theSpeakonconnector.Seetheinstallationsection
ofthismanualfordetailedinformationonSpeakon
wiring.
9a.Binding Post Outputs
Binding posts (banana jacks) are provided for
speaker output termination with banana plugs or
barewire.Theredpostisusedaspositiveandthe
black post is used as negative.
10.MainsLead
Youramplifierissuppliedwithaheavydutymains
Controls & Connectors 9
lead(powercord)appropriatelyratedforthemains
supply voltage marked on the rear panel of your
amplifier.
The wires in the mains lead are coloured in
accordancewiththefollowingcode:
BROWN = ACTIVE;
BLUE = NEUTRAL;
GREENANDYELLOW = EARTH.
! CAUTION !
Your amplifier must always be earthed!
11.MainsCircuitBreaker
A “push to reset” thermal acting circuit breaker is
suppliedonyouramplifierprovidingoverallprotection
of your amplifier’s power supply and the
interconnectingmains.Thebreakerwillisolatethe
"active" mains conductor in the event of a high
current internal fault or in continued overload
conditions.
Thebreakerwillnotbeabletoberesetimmediately
after tripping. A cool down period of around 30
seconds is required before the breaker will reset
backintocircuit.Ifthebreaker"trips" immediately
afterareset,thenafaultmayhavedevelopedwithin
theamplifier.
12.D.C. Rail Fuses
YourAM1200amplifierisfittedwith8Ampfusesper
rail - per channel, as overload protection for the
outputstageof your amplifier.
These fuses are in series with the high current
supply railstotheamplifier'soutputstageandwill
blowwhen:
1) An internal fault exists;
2)Thereisasustainedoverload;
3) There is a sustained short circuit;
4)Sustainedloadfault.
Note:ThefrontpanelFaultIndicatorwillilluminate
and pulse in sync. with the program source if the
negativesupplyRailfuseblows.
DistortionwillresultandthefrontpanelStatusLED
will illuminate RED if the Positive Rail fuse blows.
Replace only with 8 Amp, 3AG fast acting type
fuses.

Figure 4 Dimensions
10 Installation

3.Installation
Mains Lead Wiring
WARNING
Your amplifier must be earthed at all times!
When you first receive your amplifier it may not have
a mains plug attached. You must ensure that an
appropriate plug is used and corresponds with the
amplifier’scurrent(ampere)requirementsandmeets
the approval of your local energy authority.
Please refer termination of this lead to qualified
personnel. Australian Monitor takes no
responsibility for any damage or harm resulting
from improper termination of this lead!
Thewiresinthemainsleadarecolouredinaccordance
withthefollowingcode:
GREEN AND YELLOW = EARTH
Connecttothe terminalmarkedwiththeletterE,with
theEARTHSYMBOLorcoloured GREEN.
BLUE = NEUTRAL
Connect to the terminal marked with the letter N (or
coloured WHITE in USA and Canada, or coloured
BLACK in the United Kingdom).
BROWN = ACTIVE (LIVE)
Connect to the terminal marked with the letter A or L
(orcoloured BLACK inUSAand Canadaorcoloured
RED in the United Kingdom).
Power Requirements
Mains Voltage Circuit Breaker Rating
100to 120volts 20 Amps
220to 240volts 10 Amps
PowerConsumptionatidle = 250 Watts
Consumptionatrated4ohmpower = 2300 Watts
Ensurethatyourmainsvoltageisthesameastherear
panelmainsvoltagemarker(+/-10%).
Mounting
Your amplifier is designed for standard 19" rack
mounting and occupies 3 EIA rack units (5.25"). The
mountingcentresare:
Vertical: 2.25" (57.15mm)
Horizontal: 18.15" (461.1mm)to18.62"(473.0mm).
The slots in the mounting flange will accept bolt
diameters up to 1/4" (6.35mm).
Werecommendthatyouprovideadditionalsupportfor
theamplifier,especiallyifroaduseis planned, as the
weightcanbend some racks otherwise.Thissupport
canbeprovidedbysecureshelving,supportrailsora
rearrackmountingstriptomatchupwiththerearrack
mountpointsprovidedonyouramplifier.
Cooling
Each pair of channels in your AM1200 amplifier is
cooled by an axial fan which draws cool air from the
frontof theunit and expelsthe heatedair via therear
oftheunit.Theseunitsoffertwospeedfanswhichrun
athalfspeed,switchingtofullspeedwhentheinternal
heatsinktemperatureexceeds600 C(1280F).
Anunrestrictedairflowintoandoutfromtheunitmust
be provided. Any restriction of the air flow will cause
heat to build up within the unit and possibly force the
unit into its thermal shutdown mode.
If the units are to be operated in an environment
wheretheairflowisrestrictedsuchassealedracksor
even when running 2 ohm loads, the cooling should
be supplemented by extra cooling fans to evacuate
the heated air and aid the flow of cool air through the
unit.
Input Wiring
IMPORTANT! Do not directly connect pin 1 on the
amplifier’s input or strapping XLR, to the amplifier’s
chassis, speaker ground or power ground!
NOTE: Input signal ground is not to be used as
a safety ground (earth).
Theinputtoyouramplifierisabalanced3-pinsystem
andrequiresallthreepinstobeconnected.Onlyhigh
quality twin-core shielded cable should be used.
When wiring for a balanced source the connector
goingtotheinputofyouramplifiershouldbewiredas
follows:
Pin 2 = HOT(InPhase-noninverting).
Pin 3 = COLD(ReversePhase-inverting).
Pin 1 = GROUND/SHIELD.
When wiring from an unbalanced source you must
ensurethatpin3isconnectedtopin1(inputground),
either by linking the pins in the input connector or by
the source equipment's output wiring.
When wiring for an unbalanced source:
Pin 2 = HOT(inphasewiththe amplifier’s output),
Pin 3 = GROUND/SHIELD(joinstopin1).
Pin 1 = GROUND/SHIELD
Installation 11

connector for speaker output, use only the mating
NEUTRIKNL4FCin-lineconnector.Thisconnectoris
designedso thatboth channelscan be sourced from
asingle connector.
Four SPEAKON connectors are provided on the
amplifier. The Channel-A SPEAKON carries both
speaker output signals for Channel-A & Channel-B
outputs. The Channel-D SPEAKON carries both
speaker output signals for Channel-D & Channel-C
outputs.
(seeFigure5:SpeakonConnectorWiringDiagrams).
TheChannel-BandChannel-CSPEAKONcarryonly
theirownoutput.
Thisarrangementallowsyoutheoptionofconnecting
to the outputs separately or together. Connecting
through a single connector has the advantage of
minimising connections, preserving phasing and
simplified channel allocation, which is particularly
important when bi-amping or in bridge mode.
IMPORTANT
Do not overload your amplifier by connecting the
Channel-BorChannel-Coutputtwice!
Channel-AorDisusedasthe“dominant”channeland
whensourcingadualoutputfromChannel-AorDthe
followingstandardshouldbeused:
Channel-AorD=LeftorLow Frequencies.
Channel B or C = Right or High Frequencies.
When in bridge mode:
Pin 1+ = Bridge Output Positive
Pin2+ =Bridge OutputNegative.
NOTE:In-lineXLRconnectorsoftenhaveatermination
lugthatconnectsdirectlytothechassisoftheconnector.
Do not link this lug to pin 1 at the amplifier’s input as
it will defeat the amplifier’s input grounding scheme.
Thislugisoftenreferredtoasa "drain”andisusedto
provide a termination to the chassis for shielding
purposes when a floating signal ground is required
between the source and destination, or when
disconnectingthesignalgroundisrequiredtoreduce
earthloopnoise,ornoiseinducedintosignalgrounds
fromstray magnetic fields.
OutputWiring
Whenwiringtoyourspeakersalwaysusethelargest
gaugewireyourconnectorwillaccept.Thelongerthe
speakerleadthegreaterthelosseswillbe,resultingin
reduced power and less damping at the load. We
recommendusingaheavydutytwocoreflex(fourcore
flexif bi-amping)10 to 12gauge (2mm2to2.5mm2or
50/0.25orequivalent)asaminimum.
Binding Post Outputs
When terminating to the 4 mm binding post (banana
jack) output connectors, banana plugs or bare wires
canbeused.Theredterminalispositiveandtheblack
terminal is negative (ground).
If running in BRIDGE mode, only the red binding
postsareused.ChannelsAorDprovidethepositive
output to the load and channel B or C provide the
negativeoutput to theload.
SPEAKON Outputs
When using the NEUTRIK SPEAKON (NL4MP)
12 Installation
SINGLE CONNECTION BRIDGED CONNECTION
DUAL CONNECTION
Figure 5. "Speakon" Connector Wiring Diagram
BI-AMP CONNECTION

4. Operation
IMPORTANT
All signal source equipment should be adequately
earthed. This not only ensures your safety but
everybody else's as well. Faults can and do occur in
mains connected equipment where the chassis can
become “live” if it is not properly earthed. In these
instancesthefaultina“floating”(un-grounded)pieceof
equipment will look for the shortest path to ground
which could possibly be your amplifier's input. If the
faultcurrentislargeenoughitwilldestroytheinputto
your amplifier and look for the next available path,
which may be you!
Before making any connections to your amplifier
observethefollowing:
Ensurethemainsvoltagesupplymatchesthelabel
ontherear panelofyour amplifier (+/-10%).
Ensure that the power switch is OFF (UP)
Ensure that all system grounds (earths) are
connectedfrom acommon point.Avoid powering
equipment within a system from multiple power
sourcesthatmaybeseparatedbylargedistances.
Checkthecontinuityofallinterconnectingleadsto
your amplifier, ensure that there are no open or
short circuited conductors.
Ensure that the power handling of your load
(speakers) can adequately cope with the power
outputoftheamplifier.
Very Important
DuetothehighpowerabilityoftheAM1200 youneed
to be aware that certain precautions need to be
followedtoensurelongevityofyouramplifier:
-Neverturnyouramplifieronunlessallconnections
(inputsandspeakers)havebeenmade!
- Neverplug in a signal lead after the amplifier has
beenturnedon!Turntheunitofffirst.
- Never drive the output into clipping if the load is
open circuit or there is no speaker load connected!
PoweringUp
REMEMBER
Theamplifiershouldbethelastpieceofequipmentthat
youturnon and thefirst piece of equipmentthatyou
turnoff.
We recommend turning the attenuators on your
amplifier down when turning the unit on.
When you power up your amplifier, your amplifier
goesthroughanestablishmentperiodbeforeitwill
acceptsignal.TheInrushCurrentSuppression(ICS)
circuitisin operationforthe first 0.5seconds.This
limitsthemainscurrenttoprevent"nuisancetripping"
of circuit breakers.
During this period the THERMAL LED will flash red
whilst the mains voltage gradually charges up the
power supply. You will then hear a relay “click”,
indicatingmainsisnowdirectlyappliedtotheamplifier.
While the ICS circuit operates there is also a 30dB
muteonthe signal input.Aftertwosecondsthismute
will release, allowing any applied signal to pass
unattenuated.
When switching the amplifier off, wait a couple of
secondsbeforeswitchingtheunitonagain.Thisallows
the ICS circuit to reset.
Level Matching
Thenormaloperating positionforthe attenuator is
the"0dB"position(fullyclockwise,noattenuation).
In this position the amplifier operates at full gain.
Turningtheattenuatorback(anticlockwise)reduces
the input sensitivity by the amount marked on the
attenuatorscale(dial).
NOTE: If full power output is required you should
operateyouramplifierwiththefrontpanelattenuator
abovethe-15dB position,otherwiseclippingofthe
input circuitry and its resultant distortion will occur
beforefulloutputpowerisachieved.
Sensitivity
Youramplifierisalineardeviceoperatingwithafixed
input to output voltage gain (less attenuation). The
maximum output voltage swing is determined by the
applied mains voltage, load, load type and the duty
cycle of the applied signal.
Thevoltagegainfactorofyouramplifieris:37timesor
31.4dB.
The input sensitivity for your amplifier when the
attenuator is at the “0” dB attenuation position (fully
clockwise) is nominally:
+3.0dBu(1.10voltsin)forratedpowerintoa8ohm
load.
+1.6dBu(0.93voltin)forratedpowerintoa4ohm
load.
Operation 13
1.
2.
3.
4.
5.

producesavoltagedropduetothewire’sresistance.
Thisvoltagedifferencebetweentheampearthand
source equipment earth appears to the amplifier's
input as a signal and is amplified as hum.
Therearethreethingsyoucandotoavoidearthloop
problems:
Ensureyourmainspowerfortheaudiosystemis
“quiet” i.e. without equipment on it such as air-
conditioning,refrigerationorlightingwhichmay
generate noise in the earth circuit.
Ensure all equipment within the system shares
a common ground/ safety earth point. This will
reducethepossibilityofcirculatingearthcurrents
astheequipmentwillbereferencedtothesame
groundpotential.
Ensure that balanced signal leads going to the
amplifierareconnectedtoearthatoneendonly.
SignalGroundLifting
Whenpropersystemhook-uphasbeencarriedout,
youmaystillhavesomehumorhumrelatednoise.
Thismaybeduetoanyofthepreviouslymentioned
gremlins.
Oneofthemosteffectivewaystoreduceearthloop
orelectromagneticallyinducedhumistodisconnect
input signal ground at the input connector of your
amplifier.Thiseffectivelybreakstheearthlooppath
or open circuits the input ground path so no
electromagnetically induced currents can flow
throughtheamplifier.
NOTE: If the input signal ground is lifted you must
ensureadequateshieldingoftheinputwiring.Ifthe
signalsourceequipmentdoesnotprovideadequate
shielding(i.e.adefinitiveconnectiontoground)you
mustdisconnecttheshieldfromtheinputconnector's
ground pin (Pin-1) and reconnect it to the "drain"
contactontheinputconnector.Thiswillensurethe
shield on your input wiring actually goes to the
amplifier chassis and subsequently to earth.
DONOTCONNECTPIN-1DIRECTLYTOTHE
DRAINCONNECTION.
You will defeat the amplifiers internal grounding
scheme and possibly cause instability to the
amplifier.
This should only be done when the amplifier is
operated from a balanced signal source.
NOTE:Bewaryofquasi-balanced outputs,these
areoftennomorethanfloatingunbalancedoutputs.
14 Operation
1.
2.
Each channel of your amplifier has a nominal
balancedinputimpedanceof18kOhms(@1kHz)
andshouldnotpresentadifficultloadforanysignal
source.
Your signal source (i.e. the equipment feeding the
amplifier) should have an output impedance of
600Ohmsorlowertoavoidunwantedhighfrequency
loss in the cabling.
Inputoverload occursat+20.5dBu(8.25volts).
See the specification section for more detailed
information.
Hum Problems
Mostequipmentisdesignedforminimumhumwhen
used under ideal conditions. When connected to
otherequipment,andtosafetyearthinanelectrically
noisy environment however, problems will often
occur.
Thethree"E"sofhumandhumrelatednoisewhich
can plague your audio system are:
a) Electrostatic radiation,
b)Electromagneticradiation,and
c) Earth loops
Electrostatic radiation capacitively couples to
system elements causing an interference voltage
that mainly affects higher impedance paths, such
asamplifierinputs.Thesourceisgenerallyanearby
highvoltagesuchasamainsleadoraspeakerlead.
Theproblemcanusuallybereducedbymovingthe
offending lead away, or by providing additional
electrostatic shielding (i.e. an earthed conductor
which forms a barrier to the field).
Electromagnetic radiation induces interference
currents into system elements that mainly effect
lowerimpedancepaths. Radiotransmittersorstray
magneticfields from mains transformers are often
the cause of this problem. It is generally more
difficult to eliminate this kind of interference, but
again, moving the source away or providing a
magnetic shield (i.e. a steel shield) should help.
Earth loops can arise from the interfacing of the
variouspiecesofequipmentandtheirconnections
to safety earth.
Thisis by far the most common causeof hum,and
it occurswhensourceequipmentandtheamplifier
are plugged into different points along the safety
earth where the safety earth wiring has a current
flowing in it. The current flowing through the wire
3.

Bridge Mode 15
5. Bridge Mode
The term BRIDGE is used when two independent
amplifierchannelsareusedtodrivethesameload.The
load is in series (a bridge) between the two amplifier
channels.
Channel A is used as the "dominant" channel and its
output is in phase with the input signal, and channel
B has its phase reversed so it is exactly 1800out of
phase with the input signal.
As two amplifiers with a phase difference of 1800are
now driving the load you will now have double the
voltageintotheload.Thismeansyouwillnowhavefour
timesthepowerintothatload.Theoutputcannowbe
considered as an active balanced output.
AcommonuseofanamplifierinBRIDGEmodeisfor
driving70volt&100voltdistributionlines.InBRIDGE
mode,TheAM1200canproduceover70voltswithline
impedancesgreaterthan8ohmsandover80voltswith
lineimpedancesgreaterthan16 ohms.
Equallytheunitscanbeusedinbridgemodetoprovide
thecorrectvoltage/powerrequirementsforanapplica-
bleload.
Asshown inFigure6, therearethree stepsinsetting
Figure 6 Bridge Mode Speaker Connection
upyouramplifierforrunningitinBRIDGEmode.
Whilst the amplifier is off,
1.Connect thesignal sourceto the Channel-A (or D)
femaleinputXLR.TheChannel-A(orD)attenuator
becomesthelevelcontrolforbothchannels.
2. Engage the “push to bridge” switch.
3. Connect your load between the red binding post
output terminals, where the positive side of the
load is connected to the channel A (or D) output
(marked BRIDGE+) and the negative side of the
load goes to the channel B (or C) output (marked
BRIDGE-). There are no further connections re-
quired.
YoucanalsosourcetheoutputfromtheChannel-A(or
D)SPEAKONoutputconnectorwhereChannel-A(or
D)willbeonthepinmarked 1+ and Channel-B (orC)
will be on the pin marked 2+.
NOTE:Youshouldcheckaftermarketmanufactured
Speakon interconnecting speaker leads before con-
nectingthemtoyouramplifier.Someleadsaremanu-
factured for specific purposes, or specific use, and
may have pins shorted inside the connector. Any
speakonleadwithshortedpinswillobviouslyshortthe
outputofyouramplifier(eithertoground, or output to
output-becareful).

6. Two Ohm or
Not Two Ohm
NOTE: Ensure adequate ventilation and monitor the
FAULTindicatorstoguardagainstthermalshutdown
whendrivingtwoohmloads.
A preamble.
Theloadthataloudspeakerpresentstoanamplifieris
very complex and at different frequencies can be
inductive, capacitive, resistive, or a combination of
these(reactive).Withthecomplexinteractionofthese
attributes,whichalterfromloudspeakertoloudspeaker,
adefinitive loadfor an amplifierdoes notreally exist.
Loudspeakers operating within an enclosure are
specified with a nominal impedance. This nominal
impedanceisonlyaroughguidetotheloaditpresents
toanamplifier.
Asanexample,aloudspeakerwithanominalimpedance
of say 8 ohms, may have an impedance of over 50
ohms at resonance (bass frequencies), drop to less
than6ohmsaftertheresonancepeak(throughitsmid
bandarea)andthenincreasetoover16ohmsforhigher
frequencies.
A4ohmloadmakesanamplifierwork"harder"thanan
8ohmloadatthesamevoltage,asdoublethecurrent
isrequired.
Thoughvariousloudspeakersmaybemarkedwiththe
samenominalimpedance,someloadsaremoredifficult
thanothers.
Bassfrequenciesusuallyexhibithigherimpedances
and require higher voltages to achieve the desired
result. They also reflect higher energy back to the
amplifiersimplyduetotheamountofconeexcursion
involvedatlowerfrequencies.
The Mid frequency band usually offers the lowest
impedancesandthehighestdutycyclesrequiringboth
highvoltageandhighcurrent.
TheHighfrequencyregionusuallyoffersamoderate
impedance and usually does not need much voltage
but the instantaneous current demand can be much
greaterthan youthink.
As well as this burden on the amplifier, the transient
waveformsfoundinactualusecandemandalotmore
current than the "steady-state" sinewaves used in
most amplifier bench tests.
ThepoweroutputofyourAM1200amplifierquotedon
the specification sheet is derived from a voltage
excursionintoaresistiveloadforasinewaveatagiven
frequency. Though this method is in line with the
variousstandardsthatexist,itonlygivesanindication
to the maximum voltage swing (before clipping) for a
givenload.Thismethodofratingpowerdoesnotgive
anindicationofthecurrent(Ampere)capabilityofthe
amplifier, nor does it show the amplifier’s ability to
sustainhighenergywaveforms.
YourAM1200amplifierhasbeenspecificallydesigned
tobe ableto deliver more than twice the current than
that shown on the specification sheet to cope with
difficultloadsand/orhighenergywaveforms.
Thisextracurrentreserveistheresultofoverengineering
andistheheadroomtheamplifierutilizestocontrolthe
loudspeakeranddealwiththe“reactiveenergy”from
theloudspeaker loadthat hasto bedissipated within
theamplifier.
YourAM1200 amplifierisabletodrive2ohmloadsor
operate in BRIDGE mode into 4 ohms. The operator
mustbeawarethatwhendriving2ohmloadsorbridged
4 ohm loads that the currents running in the output
stageareverylargeandwillcause greater heat build
upwithin theamplifierthanhigherimpedanceloads.
The Front Panel FAULT Indicators can be used to
provideanindicationofthe"difficulty"oftheloadand
willgivetheoperatoranindicationoftheheatbuildup
in the output stage.
Ifthefaultindicatorsflashwiththe"clip"LEDsordonot
illuminate until well into clipping then the load can be
consideredas normal or easy.
If the fault indicator starts to flash before the "clip"
LEDs then the load should be considered complex
and/ordifficult.
For the more complex and/or difficult loads, the
illumination of the "fault" LED on programme peaks
shouldbeinterpretedastheoutputlevellimit.Driving
the output continuously past this point could result in
muting of the output stage, breakers tripping or
prematurethermalshutdown.
The fault detection circuit is also thermally
compensated,andfaultindicationwilloccurearlier
when the unit is hot. If the "fault" LED continually
lights earlier than normal, then the unit is heating
up.Ifthesignallevelisnotreducedtocompensate
for the heating of the unit then thermal shutdown
may occur.
16 Two Ohm or Not Two Ohm

7.Maintenance
Your AM1200 amplifier will need minimal
maintenance. No internal adjustments need to be
madetotheunittomaintainoptimumperformance.
Toprovideyearsofunhinderedoperationwesuggest
amaintenanceinspectionbecarriedoutonaregular
basis, say every 12 months or so.
Fans
Due to the openness of the air path through your
AM1200 amplifier, very little dust should settle
withintheamplifier.Theunithasbeendesignedso
thatanydust and/or foreignparticlesthat dosettle
withintheamplifierwillnotundulyhinderthecooling
of the unit.
Themeshgrilleinfrontofthefanswillacttolimitthe
amount of dust and lint entering the unit. You will
find in time that there will be a build up of dust and
lintonthegrillewhichmaystarttohindertheairflow
throughtheunit.Youshouldperiodicallyremovethe
dust and keep the grille clean.
Overtime,dustmaybuildupontheleadingedgeof
thefanbladesand reduce theircoolingefficiency.
Thetimetakenforthistohappenwilldependonthe
environmentandtheamount of use.
The fan blades are accessible once the lids are
removedandcanbeeasilycleaned.Youneedonly
hold the fan rotor still and wipe the dust off the
blades. Many users stall the fan and use
Maintenance 17
compressedairto blow thedustoffthe fan blades.
It is important to note that the fan blades must be
heldstillwhilstblowingairoverthebladesotherwise
you may burn out the bearings in the fan.
NOTE:
Make sure the unit is off and is unplugged from the
mains.Givethemainfiltercapacitorstimetodischarge
beforeremovinglidsandinspectingthefans.
Fuses
Thereareeight(8)railfusesprovidedonthebackpanel
of the unit. These rail fuses are in series with the
positiveandnegativeoutput supply toeachamplifier
channel and provide overall protection for the output
stage. If the amplifier is subjected to heavy use such
asshortcircuits,2ohmorbridged4ohmloads,these
fuseswilleventuallyfatigueandmayrequirereplacing
to ensure they do not fail at an inconvenient time.
Youshouldreplacethefuseiftheelementissagging
ordiscoloured.Onlyeverreplacewiththesametype
fuseandcurrentrating.
Whenchecking for afailed fuse,donot relyon visual
inspection alone. You should use an ohmmeter to
check continuity.
NOTE:
Makesuretheunitisoffandisunpluggedfromthe
mains and give the main filter capacitors time to
dischargebeforereplacingfuses.
Only competent or qualified persons should
attemptanyserviceormaintenanceofyouramplifier!

8.Warranty
AustralianMonitorwarrantstheoriginalpurchaserof
each AM1200 amplifier (purchased at an authorised
Australian Monitor dealer) that it will be free from
defects in materials and workmanship for a period of
two(2)years fromtheoriginal date ofpurchase.
AustralianMonitor will,at itsoption, repair orreplace
any unit or component covered by this warranty
which becomes defective or malfunctions under
normal use and service during the period of this
warranty, at no charge for parts or labour to the
original owner.
This warranty does not cover thermal problems
due to obstructed airflow, or defects or malfunc-
tions resulting from accidents, misuse, abuse,
operation with the incorrect AC mains voltage,
connection to faulty equipment, modification or
alterationwithoutpriorfactoryapprovalorservice
by unauthorised personnel.
It is the owner's responsibility to ensure that normal
maintenance inspections are carried out at regular
intervalsasrecommendedinthemaintenancesection
of this manual. Australian Monitor reserves the right
to refuse warranty service where the owner fails to
18 Warranty
AM-SERIES2YEARWARRANTYREGISTRATION
IMPORTANT
Please complete this card and return it immediately after unpacking the product.
This card is to be sent DIRECTLY to Australian Monitor.
NOTE! Warranty is effective ONLY upon receipt of this card.
COMPANY
NAME MODEL
ADDRESS SERIAL No
DATE PURCHASED
CITY DEALER
STATE CODE COUNTRY
Ensure that you fill out and send your warranty registration card.
Use this copy to record a duplicate of the details.
take reasonable care in use and maintenance of the
amplifier.
Tovalidatethiswarranty,theoriginalpurchasermust
complete and mail the warranty registration card
directly to Australian Monitor within fourteen (14)
days of purchase.
Toobtainwarrantyservice,the equipment shouldbe
shippedtoanauthorisedAustralianMonitordealeror
direct to Australian Monitor. Freight to Australian
Monitor is at the owner's expense.
Unitswithadefacedserialnumberwillnotbeaccepted
for warranty service. Any evidence of alteration,
erasure or forgery of the purchase receipt will also
void this warranty.
Australian Monitor accepts no liability for any
consequential damages, whether direct or
indirect, arising from the use or misuse of its
products.
Australian Monitor reserves the right to alter its
designsandspecificationsat any timewithoutnotice
or obligation to previous purchasers.

Specification 19
Test conditions
Inputsource = 600ohm, Balancedandground referenced(CMRR test hadfloating ground &50 ohm
source). Mains regulated to 240 volts / 50hz. All measurements taken at binding post output terminals.
Standardproductionunitsmeasured.Nocompensationapplied.
AM1200Specifications
General
TheAM1200willdeliverorexceed-200wattsRMSintoan8ohmload-300wattsRMSintoa4ohmload-600
wattsRMSbridgedintoan8ohmload*forasingle,pair,orallfourchannelsbeingdrivencontinuously(withless
than0.05% IMDand THDfrom 1watt to rated power) from20Hz to20kHz.
Output Condition Output Power
E.I.A. Power @ 1 kHz, <0.05 % THD+N. 8ohm 4ohm 2ohm
Singlechanneldriven. 220W 370W 450W
Allchannelsdriven. 200W 320W -
Bridge mode @ 1 kHz, <0.05 % THD+N. 16ohm 8ohm 4ohm
Single pair (C & D only) 410W 670W 800W
Both pairs 400W 640W -
Dynamic Power @ 1 kHz, at onset of clipping,
20 cycles @ 0dB, 480 cycles @ -20 dB.
Singlechanneldrivenre 4ohm 410 Watts
Bridged pair driven re 8 ohm 750 Watts
Bridged pair driven re 4 ohm 1000 Watts
Distortion (0.5 dB below clipping re 4 ohms)
THD+N (@ 1 kHz) <0.003%
IMDSMPTE (60 Hz & 7 kHz 4:1) <0.015%
IMDDIM30 (3.15kHz square & 15 kHz) <0.008%
Output Impedance @ 1 kHz <0.011ohms
Damping Factor @ 1 kHz re 8 ohms >720:1
Output Rise Time <2.2µS
(80%,leadingedgeof20kHzsquarewave)
Slew Rate >60V per µS
(leadingedge,20kHzsquarewave@clipping)
Frequency Response
20 Hz - 20 kHz < -0.3, -0.15 dB
-3dBpoints 5 Hz - 140 kHz
Input Impedance
Lineto Line(Balanced) 18k ohms
InputSensitivity(nominally)
Forrated powerre 8ohm 1.10 Vrms (+3.0 dBu)
Forrated powerre 4ohm 0.92 Vrms (+1.5 dBu)
VoltageGain 37 times (31.4 dB)
Input CMRR
@ 1 kHz (re 8 ohm rating) >90 dB
Signal / Noise ratio
"A"weighted (re8 ohm rating) >101dB
Crosstalk
@ 1 kHz (re 8 ohm rating) >90 dB
Weight Net 57.2 lb (26kg), Shipping 63.8lb (29kg)
Dimensions HxWxD
excludinghandles 133 x 483 x 375 (mm)
includinghandles 133 x 483 x 460 (mm)
(19 inch EIA rack mounting, 3 units high)

2000
www.australianmonitor.com.au
Distributed by:
Audio Telex Communications Pty Ltd
ACN001345482
www.audiotelex.com.au
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