Mackie FRS1700 Owner's Manual

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Mackie FRS1700 Owner's Manual Manual pdf 15 page image

15Owner’s ManualOwner’s ManualThe recommended conductor gauges are listed forAWG (American Wire Gauge) and Metric WG (MetricWire Gauge). Note that smaller AWG numbers = largerconductors and smaller Metric WG numbers = smallerconductors. The Metric WG is equal to ten times thenominal conductor diameter in millimeters.Longer Lengths&ORCABLELENGTHSOVERFEETMATOHMS ANDOVERFEETMATOHMS THECONDUCTORSIZESneeded for less than 0.5 dB power losses are rarelypractical for physical and cost reasons.As a practical compromise for these situations therecommended conductor gauge is 10 AWG or 25 metric.Speaker ImpedancesA speaker’s impedance varies with frequency.&OREXAMPLE ITMAYBEOHMSAT(Z ANDOHMSAT(Z7HATYOUNEEDTOKNOWISTHEAVERAGE (or nominal) impedance across the speaker’s frequencyrange. This will be printed somewhere on the cabinet,or in the specification section of the missing manual.If you’re just dealing with one speaker per channel,then make sure that the average impedance is greaterthan or equal to 2 ohms.In bridged-mono mode, make sure that your speakerimpedance is greater than or equal to 4 ohms.If you’re driving an assortment of speakers, you haveto make sure that the total impedance does not go belowthese same levels. There are two basic ways of linkingMULTIPLESPEAKERSSERIESANDPARALLEL4HEFOLLOWINGsections show how to make the connections and how toWORKOUTTHETOTALIMPEDANCESeries“Series” means that the positive amp outputconnects to the first speaker’s positive terminal, thefirst speaker’s negative terminal connects to the secondspeaker’s positive terminal, the second speaker’snegative terminal goes to the amp’s negative output.Series connections are not normally used in PAapplications because it ruins the amplifier’s ability toDAMPCONTROL THESPEAKERS4HEOTHERSNAGIFONEspeaker goes out, they all do. The audience may notappreciate this, and things could get ugly fast.Doing load calculations with series configurationsis easy — just add the loads. For instance, two 4-ohmspeakers in series, equals 8 ohms (4 + 4 = 8).Parallel“Parallel” means that the positive amp output con-nects to the positive terminals of all the speakers,and the negative amp output connects to the negativeterminals of all the speakers. If one speaker opens in aparallel configuration, the others will still work, but theload will change. That lets you breathe a little easier(the show will go on), except that you may have a deadspeaker and not even know it.Calculating parallel loads is also easy, as long as eachspeaker has the same value — just divide the value bythe number of speakers.For example, four 8-ohm speakers, connected inparallel, will equal 2 ohms (8 / 4 = 2).If the parallel loads aren’t all the same, things gets alittle more complicated, but nothing that you can’t dowith a simple calculator. The total impedance (Z T) isgiven by the following formula, where Z 1, Z2 and Z3 arethe impedances of your speakers.There are other, more complicated configurations,like series-parallel (using a combination of series andparallel links to arrive at a desired load) and parallelconfigurations of unmatched loads (usually notrecommended). But rather than get too deep intoTHIS LETSJUSTSUMMARIZETHEBASICS ASTHEYAPPLY TOYOUANDYOURAMPs 4HELOWERTHESPEAKERIMPEDANCE THEMOREpower can be put out by the amplifier.s $RIVINGLOWERIMPEDANCESPEAKERSMAKESTHEamplifier work harder and heat up quicker.s $ONOTCONNECTATOTALIMPEDANCEOFUNDER2 ohms per channel in stereo and mono modes.s $ONOTCONNECTATOTALIMPEDANCEOFUNDER4 ohms in bridged mono mode.s #ONNECTINGSPEAKERSINSERIESORPARALLELCANdrastically alter their frequency response.s #ONSIDERUSINGMULTIPLEAMPLIlERSRATHERTHANoverloading one.s 2EDUCETHELOW FREQUENCYOUTPUTBYENGAGINGthe low cut filter.s .EVERPLUGTHEAMPLIlEROUTPUTSINTOANYTHINGexcept speakers (unless you have an outboardbox designed to accept speaker-level levels).s "ECAREFULINBRIDGEDMONOMODE ASBOTHspeaker wires are live. In this mode, do notconnect the speaker wires to any externaldevice which is grounded.ZT = 11Z1+ 1Z2+ 1Z3+ . . .