Wheelock MX-8 Mixer Specifications download pdf (Page 5)

Step 1 - Select the Correct System

24voltselfampliedor70voltcentralamplied.Whichisbest?Theansweristhateithersystemisrightforcertain

applications. Here are a few guidelines:

Forsmallsystemsinvolving6-8speakers,24VDCselfampliedsystemsoffertheadvantagesoflowcost,easyinstallation

usingtelephone(quad)wire,individualvolumecontrolandbuilt-inredundancy(ifonefails,therestkeepworking).Self

ampliedspeakersarespeakerswiththeirownamplierattached.Theyusea4-wireconnection—a2-wireconnection

forthe24VDCthatpowerstheamplierandanother2-wirefortheaudio.

70voltdistributedsystemsaregenerallypreferablewithlargersystemsbecauseonecentralampliercanpoweragreat

manyspeakersthatcostmuchlessthanselfampliedspeakers.Thepricebreakoccursatabout6-8speakers,where

thecostofacentralamplierandlowercostspeakerswillbegintooffsetthecostofselfampliedspeakersand24volt

power supplies. The wiring is simpler: A single pair of wires.

70voltsystemsarethebetterchoicewhengoodsoundingbackgroundmusicisarequirement.Thespeakersandampli-

ersgenerallyhavemuchbetterfrequencyresponsethantheselfampliedsystems,whicharedesignedbasicallyfor

goodspeechreproduction,butnotforgoodmusicreproduction.Additionally,70voltampliersprovideselectiveinputs

and mixing capability for FM tuners, CD players, etc.

Wiring/Code Issues for Audio Systems

Inthepast,therehasbeenmuchdebateontheissueofwiringrequirementsforspeakersystems,

especially70voltaudio.Table11,chapternineoftheNationalElectricCode(NEC)1999version,establishesClass3

circuitratingfor70Vaudioaswellasfor24voltsDCsignalingcircuits.Thismeansthat70volt(centralamp)or24volt

(self-amplied)speakersystemsmayuseClass 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,1999edition)

page1183:“….Amplieroutputcircuitsratednotover70V,withopen-circuitvoltage(thismeansnospeakersconnected)

not over 100V, may use Class 3wiringassetforthinCodesection725-41(a)ofArticle725”.Unlesslocalcodesorlocal

authoritiesspecifyotherwise(yes,alwayscheckifindoubt),thisrulingbasicallysaysthatyoudon’thavetorunconduit

(atypicalClass1wiringmethod)forspeakercabling.

Actually,UL(Underwriter’sLaboratories)hasgreatlysimpliedthematterbyrequiringamplier

manufacturerstosilkscreentheirspeakeroutputsectionterminalswiththespecicclassofwiringmethodrequired,

a much simpler approach that makes it easy for anyone to understand. For 70 volt speaker lines, Class 3 wiring is

specied.For100Voltorgreaterspeakerlines,Class1wiringisspecied.

A word about 100 volt systems:100voltaudiosimplymeansthattheamplierdevelopsfulloutputpower,inwatts,

with 100 volts of potential across its speaker output terminals. This is a higher transmission voltage than 70V or 25V,

andtherearecertainadvantagestothis.Technically,thehigherthetransmissionvoltage,themoreefcientthesystem

becomes. Consider the relationship P=VA, where P=power in watts, V=voltage and A=current in amperes. This is a

variationofOhm’sLawwhichsaysthatforagivenamountofpower(inwatts)thehigherthevoltage the lower the

amperage.Thechoiceofwiresizeandlengthofrunisdirectlyrelatedtotheamountofcurrent(amperage)thatthe

wire has to carry. In high voltage, low amperage systems, wire runs can be longer and smaller in size than the wire

runsusedinlowervoltagesystems.100voltsystemsaremoreefcientthan70voltsystems,whichinturnaremore

efcientthan25voltsystems.

AccordingtotheNEC,25and70voltcentralampliedsystemscanbewiredusingclass3methods,forexample,the

commonpracticeofloosecablingdrapedacrossceilingtilesorothermethodsusingexposedcabling.Bystrictdenition,

100 volt audio would have to be wired according to class 1 wiring methods, which usually means conduit in commercial/

industrialinstallations.However,ifthebuilder/installerisalreadycommittedtoconduitforthespeakerwiring(almost

agivenforhospitals,factories,governmentbuildings)100voltaudioforthespeakerwiringbecomestheclearchoice

becauseofthepowertransmissionefciencyandlowerwiringcost.

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

andthepowerinvolved.ThemajorityofapplicationsintheUSare70volt.25voltisalsocommonlyused,mostlyin

schools. However, in order to make our discussion easier, the term 70 volt audio will be used throughout this book to

describeallcentralampliertypesystems.

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Wheelock MX-8 Mixer Specifications Page 5

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