Quest Engineering Q-Tech QTA-4120P User Manual

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Quest Engineering Q-Tech QTA-4120P User Manual Manual pdf 15 page image

QTA-4060P/4120PUser ManualUser ManualQ-Tech Commercial SeriesQTA-4060P/4120P1315% discrepancy can mean that a 100watt system will only be able to power 7or 8 x10 watt speakers safely. Be aware ofthis booby trap. Always plan to have 20%more power than you think you will need.2. Incorrect output connection: If you ac-cidentally connect your terminal stripon the amplifier to the 8 ohm outputinstead of the 100/70V line, you will havedistortion and risk damaging the amplifier..3. Short circuits and no circuits: Check thatyour wiring has not been accidentally cut,miss-connected or generally damagedin the course of installation. This can becommon with building sites with multipletrades people installing equipment into thesame ceiling cavities as the audio system.How to calculate the correctnumber of speakers and whatwattage connection for a givenamplifier power.If you connect too many speakers to anamplifier you will have distortion, overheatingof the amplifier and generally poorperformance. The problem is not really “toomany speakers”, it is more a problem of “toomuch wattage draw exceeding the outputcapability of an amplifier. A similar problemwould be trying to draw 2,000 watts froma 1,000 watt generator. Sooner or later….Expect a system failure.Let us take a 100 watt amplifier. If you have20 speakers and you set them at the 5 watttaping, you will have a total 100 watt drawwhich is the maximum output of the amplifier.This is correct in theory but in practice youwill be safer connecting 18 speakers, not 20.The reason is that most ceiling speakers drawmore than their claimed wattage.The alternative would be to connect thespeakers to the 4 watt transformer terminalgiving you a theoretical total draw of 80 wattsand thus, plenty of “headroom”. One wattdifference is not very noticeable as far asoutput sound pressure level in concerned.A system that does not distort will givemuch clearer voice reproduction than alouder system that is distorting or loosingthe sibilant frequenciesThere is a formula for testing the potentialpower of the system by measuring theimpedance of each speaker and then addingall the figures to a total impedance, which isthen compared to the amplifier’s expectedimpedance/power figures. The quick way isto just add up all the wattages and then giveyourself 10-20% headroom by reducing thenumber of speakers per amplifier or loweringthe wattages slightly.For the benefit of those who want to calculatethe power of the system with a formula. Seebelow:Determining power by calculatingtotal impedanceThe alternative to counting the speaker tapingwatts is to calculate the total impedance ofthe line, which will also indicate the wattagenecessary to drive the system correctly.The computational formula is voltage squareddivided by impedance:Power = Voltage2 / ImpedanceWhen the speaker line reaches its top voltageof 70 Volts, the formula is:Power = 5000 / Impedance (because 70squared equals 5000).To measure impedance a dedicated meter isrequired. These use a 1k frequency to send analternating current through the transformer(s).The resulting figure indicated by the meter’sdial can be cross referenced with the meter’simpedance chart to see what value should beexpected and the power that will be required.