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 systems, 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 (actually 70.7V) are most commonly used in commercial sound distribution systems in the U.S.

Because of the high power capability of the M•2600 amplifier, it can be used to directly drive 70V constant-voltage distribution systems without the use of a step-up transformer.

We recommend that you

do not use the M•2600 in

BRIDGED mode with a 70V

system, as it can produce

up to 1700 watts into 8 ohms, or 116V. This may

saturate the small 70V transformers. Use the STEREO or MONO mode instead, and you can run one or two separate 70V lines.

In STEREO, into a nominal 70V system, the M•2600 output can produce a maximum of 500 watts into 8 ohms (equal to 63V). Since this is less than 70.7V, 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, and P2 is the power delivered by 70.7V into 8 ohms (625W).

M•2600: K = 500W/625W = 0.8. Thus, a 2.5W tap becomes 2W, a 5W tap becomes 4W and a 10 W tap becomes 8W.

CAUTION: A characteristic of many tapped transformers is that they saturate at very low frequencies, 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•2600 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 am- plifier, as shown in the figure below.

M•2600

POWER AMPLIFIER

IN STEREO MODE RC NETWORK

+

+C1 C2+

CH 1

 

 

 

 

 

 

 

R1

 

 

 

 

–

 

 

 

 

 

 

 

 

 

 

 

 

RC NETWORK VALUES

 

 

 

 

 

C1 = C2 = 680∝F @ 250V

 

 

 

 

 

R1 = 4Ω @ 100W

 

 

 

 

 

Note: You can substitute

ALTERNATE RC NETWORK

 

 

 

 

 

a single capacitor for C1/C2.

 

 

C3

 

 

C3 = 330∝F @ 250VDC,

 

 

 

 

 

 

 

 

 

 

 

 

 

 

NON-POLARIZED.

 

 

 

 

 

 

 

R1

70V Constant Voltage

Distribution System

70V LINE

 

 

 

 

 

 

 

 

CONSTANT

 

 

 

 

 

 

 

 

VOLTAGE

 

 

 

 

 

 

 

 

TRANSFORMER

2.5W

5W

10W

2.5W

5W

10W

2.5W

5W

10W

 

 

 

 

 

 

 

 

POWER TAP

 

 

 

 

 

 

 

 

SWITCH

 

 

+

 

 

+

 

 

+

 

 

–

 

 

–

 

 

–

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