Wheelock MX-8 Mixer Specifications download pdf (Page 20)

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What about the audio pair?

Asmentionedpreviously,self-ampliedspeakersusea4-wirehook-up:Apairforthe24VDCpowerandapairforthe

paging audio. What about the audio pair? Shouldn’t we worry about correct wire size for the audio wiring as well?

Pre-ampliedpagingaudioisaverylowpower,lowimpedance(usually600ohmsorless)electricalsignal.Forthis

reason, small gauge wire of #22-24 AWG works well, even for very long cable runs. To illustrate this further, consider that

telephone companies typically transmit low level voice audio over distances of two miles or more on #24 AWG wire.

Low power, low impedance pre-amp audio can generally be transmitted

over considerable distances using small (#22-24) wire gauge.

Shielded vs unshielded wire

Anentirebookcouldbewrittenonthissubject,solet’sboilitdowntoafewsimpleguidelines. Speaker lines from 70

voltcentralamplierscangenerallybeunshielded cable with little or no chance of noise pick-up. An exception would

be elevator shafts where an audio signal’s worst nightmare comes true. These are electrically high noise environments

and shielded wire or conduit should be used. Audio lines (wirescarryinganylowpowerpre-ampliersignal)should

always use shielded cable to avoid noise pickup. This included microphone wiring, amp-to-amp connection between

70voltampliers,andamplierconnectionstoFMtuners,tapedecks,CDplayers,etc.

Self amplied speakers are often installed in telephone paging systems with acceptable results using unshielded

telephone wire. Generally, this can be done with little or no noise pickup provided that the wiring doesn’t pass close

toelectricmotors,transformers,oruorescentlightingballasts.Ifindoubt,useshieldedcablefortheaudiopair.It’s

cheap insurance.

Step 4 - Other Considerations

Telephone System Interface

Nowthatweknowthebasicsofdesigningapagingsystem,intermsofspeakerplacement,efciencyandamplier

power, let’s explore how to connect all of this hardware to a telephone system.

Upto this point,we have beendiscussing apagingsystem asa“stand alone”system— asifwe hadallof these

components in place and we were going to use a single microphone to speak through it. This is often the case in small

retailstoresorperhapsabowlingalley,butinlargecomplexessuchasmanufacturingplants,hospitals,ofcebuildings,

airports, etc., it becomes very desirable to access the paging system via the inhouse telephone system thereby enabling

many users in many locations access to the paging system right from the nearest telephone. By taking advantage of

alltheextensionswithinthephonesystem,it’slikehavingaPAsystemwith20,50or100microphonesconnectedtoit;

all at different points in the building.

Telephone systems

Most commercial telephone systems in use today are basically variations of several types, namely electronic key systems

(EKSU’s), PBX systems, hybrid systems that are sort of a mixture of the two, and Centrex. First, let’s look at EKSU’s

and PBXsystems,because inthe eld,theyaresimilarfrom aninterface standpoint.Centrex systems are a little

different, so we’ll look at them separately.

Although they differ considerably in switching function, most EKSU’s and PBX systems have a built-in page output right

onthemainunitwhichisaccessedbypressingadedicatedkey(EKSU)orasetofnumbersonthetouch-tone™pad

(PBX).Ineithercase,thesystemhasnowconnectedyoutoalowpower(usually600ohm)audiomoduleinthemain

unit.That’sright,anaudiooutputthatyoucannowbringrightovertothe600ohmoutputonyourpagingamplieror

self-ampliedspeaker(s).

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Wheelock MX-8 Mixer Specifications Page 20