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The EIA 70.7 volt constant
voltage system of power distribution provides the engineer and technician with a simple means of matching a number of loudspeakers to an amplifier. To use this method: |
Since the voltage at rated amplifier
power is 70.7, this reduces to: _{(2)} |
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1. Determine the
power required at each
loudspeaker.
2. Add the power required for the individual speakers and select and amplifier with a rated power output equal to or greater than this total. 3. Select 70.7-volt transformer having primary wattage taps as determined in step 1.* 4. Wire the selected primaries in parallel across the 70.7-volt line |
From formula
(2) these relationships are: 1 watt requires 5000 ohm primary 2 watt requires 2500 ohm primary 5 watt requires 1000 ohm primary 10 watt requires 500 ohm primary Once the primary taps have been determined, continue on through step 4 and 5 as outlined above. When selecting transformer primary taps, use the next highest available value above the computed value. A mismatch of 25% is generally considered permissible. Example: Required |
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5. Connect each secondary to its speaker;
selecting the tap which matches the voice coil inpedance. |
One 6 watt speaker
with 4 ohm voice coil. Two 10 watt speakers with 8 ohm voice coils (use one transformer at this location). |
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For transformers rated in impedance, the following formulas may be used to determine the proper taps in step 3. |
(1-2) Total
power = 6 + 10 + 10 = 26 watts (use 30-watt amplifier or other amplifier capable of handling at least 26 watts). |
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Primary
(Amplifier output voltage)^{2}
Impedance. Desired speaker power |
(3)
ohms (use 1000 ohm transformer) ohms |
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or | (1) | ||
*These transformers have the
primary taps marked in watts and the secondaries marked in ohms. |
(4-5) See sketch below. | ||
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