70V DISTRIBUTION SYSTEMS

A distributed sound system uses a constant- voltage, high-impedance network that feeds a number of tapped transformers which, in turn, deliver power to individual speakers. Each tap is rated in watts, so you can select the amount of power delivered to the speaker. Developed for distributed paging and public address sys- tems, one benefit of such a system is that it eliminates complicated impedance calculations when setting up a multi-speaker system. You just add up the total wattages of all the speakers in the system and make sure that it doesn’t exceed the total power rating of the amplifier (allowing at least 10% for insertion losses). Another benefit is that by using high voltage and low current, losses in the speaker distribution wiring are kept to a minimum. Standard voltage levels include 25V, 70V, and 100V, but 70V systems are most commonly used in commercial sound distribution systems.

FR SERIES

 

POWER AMPLIFIER

 

IN BRIDGE MODE

RC NETWORK

Because of the high power capability of the M•1400/M•1400i amplifier, it can be used to directly drive 70V constant-voltage distribution systems without the use of a step-up trans- former. When the M•1400/M•1400i amplifier is operated in BRIDGE mode, it can produce 850 watts into 8 ohms, or 82.5V. This is slightly higher than the standard 70.7V for which the system was designed. You can recalculate the actual power delivered to each tap by multiplying the tap’s rated wattage by a correction factor (K). The correction factor is P1/P2, where P1 is the power delivered by the amplifier into 8 ohms (BRIDGE mode), and P2 is the power delivered by 70.7V into 8 ohms (625W).

M•1400i: K = 850W/625W = 1.36. Thus, a 2.5W tap becomes 3.4W, a 5W tap becomes 6.8W and a 10 W tap becomes 13.6W.

CAUTION: A characteristic of many tapped transformers is that they saturate at very low fre- quencies, which causes their impedance to decrease, approaching the DC resistance of the copper wire. This can result in overloading the amplifier if the signal contains lots of low frequen- cies. When using an M•1400/M•1400i amplifier in a 70V distribution system, set the LOW CUT FILTER to 100Hz or higher. In addition, install an RC network at the SPEAKER OUTPUT

of the amplifier, as shown in the figure below.

+

+C1 C2+

CH 1

 

–

R1

–

 

CH 2

 

+

 

RC NETWORK VALUES

C1 = C2 = 680∝F @ 250VDC

R1 = 4Ω @ 100W

ALTERNATE RC NETWORK

Note: You can substitute

 

 

C3

 

 

a single capacitor for C1/C2.

 

 

 

 

C3 = 330∝F @ 250VDC,

 

 

 

 

 

 

 

 

 

 

 

 

 

 

NON-POLARIZED.

 

 

 

 

 

 

 

 

 

 

R1

 

 

70V LINE

 

 

 

 

 

 

 

 

CONSTANT

 

 

 

 

 

 

 

 

VOLTAGE

 

 

 

 

 

 

 

 

TRANSFORMER

2.5W

5W

10W

2.5W

5W

10W

2.5W

5W

10W

 

 

 

 

 

 

 

 

POWER TAP

 

 

 

 

 

 

 

 

SWITCH

 

 

+

 

 

+

 

 

+

 

 

–

 

 

–

 

 

–

70V CONSTANT VOLTAGE DISTRIBUTION SYSTEM

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