Step 1 - Select the Correct System
24voltselfampliedor70voltcentralamplied.Whichisbest?Theansweristhateithersystemisrightforcertain
applications. Here are a few guidelines:
Forsmallsystemsinvolving6-8speakers,24VDCselfampliedsystemsoffertheadvantagesoflowcost,easyinstallation
usingtelephone(quad)wire,individualvolumecontrolandbuilt-inredundancy(ifonefails,therestkeepworking).Self
ampliedspeakersarespeakerswiththeirownamplierattached.Theyusea4-wireconnection—a2-wireconnection
forthe24VDCthatpowerstheamplierandanother2-wirefortheaudio.
70voltdistributedsystemsaregenerallypreferablewithlargersystemsbecauseonecentralampliercanpoweragreat
manyspeakersthatcostmuchlessthanselfampliedspeakers.Thepricebreakoccursatabout6-8speakers,where
thecostofacentralamplierandlowercostspeakerswillbegintooffsetthecostofselfampliedspeakersand24volt
power supplies. The wiring is simpler: A single pair of wires.
70voltsystemsarethebetterchoicewhengoodsoundingbackgroundmusicisarequirement.Thespeakersandampli-
ersgenerallyhavemuchbetterfrequencyresponsethantheselfampliedsystems,whicharedesignedbasicallyfor
goodspeechreproduction,butnotforgoodmusicreproduction.Additionally,70voltampliersprovideselectiveinputs
and mixing capability for FM tuners, CD players, etc.
Wiring/Code Issues for Audio Systems
Inthepast,therehasbeenmuchdebateontheissueofwiringrequirementsforspeakersystems,
especially70voltaudio.Table11,chapternineoftheNationalElectricCode(NEC)1999version,establishesClass3
circuitratingfor70Vaudioaswellasfor24voltsDCsignalingcircuits.Thismeansthat70volt(centralamp)or24volt
(self-amplied)speakersystemsmayuseClass 3 wiring methods, for example the common practice of loose cabling
draped across ceiling tiles. Further, according to the National Electric Code Handbook (McGraw-Hill,1999edition)
page1183:“….Amplieroutputcircuitsratednotover70V,withopen-circuitvoltage(thismeansnospeakersconnected)
not over 100V, may use Class 3wiringassetforthinCodesection725-41(a)ofArticle725”.Unlesslocalcodesorlocal
authoritiesspecifyotherwise(yes,alwayscheckifindoubt),thisrulingbasicallysaysthatyoudon’thavetorunconduit
(atypicalClass1wiringmethod)forspeakercabling.
Actually,UL(Underwriter’sLaboratories)hasgreatlysimpliedthematterbyrequiringamplier
manufacturerstosilkscreentheirspeakeroutputsectionterminalswiththespecicclassofwiringmethodrequired,
a much simpler approach that makes it easy for anyone to understand. For 70 volt speaker lines, Class 3 wiring is
specied.For100Voltorgreaterspeakerlines,Class1wiringisspecied.
A word about 100 volt systems:100voltaudiosimplymeansthattheamplierdevelopsfulloutputpower,inwatts,
with 100 volts of potential across its speaker output terminals. This is a higher transmission voltage than 70V or 25V,
andtherearecertainadvantagestothis.Technically,thehigherthetransmissionvoltage,themoreefcientthesystem
becomes. Consider the relationship P=VA, where P=power in watts, V=voltage and A=current in amperes. This is a
variationofOhm’sLawwhichsaysthatforagivenamountofpower(inwatts)thehigherthevoltage the lower the
amperage.Thechoiceofwiresizeandlengthofrunisdirectlyrelatedtotheamountofcurrent(amperage)thatthe
wire has to carry. In high voltage, low amperage systems, wire runs can be longer and smaller in size than the wire
runsusedinlowervoltagesystems.100voltsystemsaremoreefcientthan70voltsystems,whichinturnaremore
efcientthan25voltsystems.
AccordingtotheNEC,25and70voltcentralampliedsystemscanbewiredusingclass3methods,forexample,the
commonpracticeofloosecablingdrapedacrossceilingtilesorothermethodsusingexposedcabling.Bystrictdenition,
100 volt audio would have to be wired according to class 1 wiring methods, which usually means conduit in commercial/
industrialinstallations.However,ifthebuilder/installerisalreadycommittedtoconduitforthespeakerwiring(almost
agivenforhospitals,factories,governmentbuildings)100voltaudioforthespeakerwiringbecomestheclearchoice
becauseofthepowertransmissionefciencyandlowerwiringcost.
100 volt audio is not new: It is widely used around the world, especially in Europe. High power stadium applications in
this country have traditionally used even higher voltages, such as 140 volt and 200 volt, because of the length of runs
andthepowerinvolved.ThemajorityofapplicationsintheUSare70volt.25voltisalsocommonlyused,mostlyin
schools. However, in order to make our discussion easier, the term 70 volt audio will be used throughout this book to
describeallcentralampliertypesystems.
Comments to this Manuals