70-Volt Loud-Speaker Matching Systems

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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)
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
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).
     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).
  Primary         (Amplifier output voltage)2
Impedance.       Desired speaker power		      (3)  ohms
(use 1000 ohm transformer)
ohms
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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