Lowell MA250 - Manual (Page 12)

A
SI E
LO I PE ANCE
SPEA E LINE
OHMS OHMS OHMS W W W W W W W W W
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WIRE LENGTHS IN FEET
V SPEA E LINE
O V LINE IVI E ALL V LEN T S
NO INAL PO E IN LOA
I E COPPE CA LE LEN T S O SPEA E LINES AT – LOSS IN SPL PO E LOSS IN ATTS
Instruction Sheet
IS-MA60/125/250
Issued: 5-1-14
Lowell Manufacturing Company 100 Integram Drive Pacific, Missouri 63069 U.S.A.
Call: 800-325-9660 Fax: 636-257-6606 Click: www.lowellmfg.com
12
Cable distance should be kept as short as possible to minimize power loss. The chart below can be used as a
general guide showing various wire sizes and the maximum distance related to cable loading that speakers can be
placed from the unit for an approximate 0.5dB loss (-12.5% power).
Wiring Size and Length:
70V and 25V Amplifier Load Calculations and Amplifier Selection:
In fact, it is good design practice to limit the speaker load to 85% of the amplifiers power rating. That designed-in
headroom allows for future expansion (adding a few speakers) and also the reduced load will increase the lifetime of
the amplifier by allowing it to run cooler. Continuing with the example from page 11, the designer knows that the
expected speaker load is 100 watts. What would be an appropriately sized amplifier for that system? If the
designer considers a 125W amplifier, 125 X .85 = 106.25 watts. It would be good design practice to limit the
speaker load to no more than 106.25 watts, so since the speaker load in the example is 100W, the 125W watt
amplifier would be a good choice. Sometimes the amplifier has already been purchased and the number of
speakers required is known, but the question becomes what transformer tap setting to use. Desired speaker SPL
calculations to determine the speaker transformer tap required are beyond the scope of this paper, but there are
some practical considerations when choosing the power tap. If a 125 watt amplifier has already been purchased,
using the 85% loading rule of thumb, 125W X .85 = 106.25 watts. It would be good design practice to limit the
speaker load to no more than 106.25W. Since we know from our example that 100 speakers would be required, it is
obvious that tapping the speakers at 1W is the best choice.
Some system designers feel it is best to tap the speaker transformers as high as possible. Consider an example
where a sound system that has a 125W 70V amplifier, only has 10 speakers. If the transformers are tapped at
0.25W each, the total speaker load will be 0.25W X 10 speakers = 2.5W. If the amplifier is turned all the way up so
its output is 70 volts, those speakers will still only draw a maximum of 0.25W each and it would not be possible to
get the speakers any louder.
That would result in tons of headroom but you wouldn’t have much volume out of the
speakers. Some designers would suggest that the speakers in this example should be tapped at 5W each so the
speaker load would be 5W X 10 speakers = 50W. Again, plenty of headroom for the amplifier but more than 12dB
more sound pressure level out of the speakers. Other designers would prefer that the speakers be tapped at 1W
each (if they are sure that the 1W tap would provide sufficient SPL) because the 1W tap provides 2 higher powered
taps that are available if this particular speaker needs to be louder, and 2 lower-powered taps if this speaker is too
loud. The designer needs to determine the appropriate tap setting after considering the requirements of the system
design. The question might be asked, what happens when the designer has a 125 watt amplifier, and needs 25
speakers tapped at 5 watts each? That results in a speaker load of 125 watts with no headroom. Sometimes a
designer is confident that adding speakers in the future will not be necessary, and competitive pressure in a bid
situation will not make it possible to use a larger amplifier to provide the desired headroom. Although headroom is
always desirable, be assured that the Lowell amplifiers will survive and operate safely when fully loaded.
Never use a speaker load (the sum of all speaker transformer taps) that exceeds the amplifier power rating.
Overloading the amplifier can cause distortion and could damage the amplifier and void the warranty.
WIRE LENGTHS IN FEET WIRE LENGTHS IN FEET