D-fend SA300 User Manual - page 26
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D-fend™ SA300 User Manual
There are a couple of different 70V system architectures wherein you may consider using your SA300:
1. A 70V system utilizing a high-voltage amplifier directly driving a network of loudspeakers incorporating step-down
transformers;
2. A standard 70V system wherein a step-up transformer is used to develop a high voltage for driving a network of
loudspeakers incorporating step-down transformers.
The information below will help you configure your SA300 for these 70V systems.
High-voltage Amplifier Direct-Drive
In this configuration, a high-voltage amplifier directly drives a network of loudspeakers incorporating step-down
transformers. You must make sure the high-voltage amplifier does NOT use a step-up transformer to develop the 70V
(or higher) output voltage. In other words, you must ensure that the amplifier’s output impedance is considered a low-
impedance output drive, which is relatively common within modern distributed audio systems.
The SA300 should be connected between the amplifier output and the network of loudspeakers incorporating step-down
transformers. Threshold settings within the SA300 will be determined based upon the nominal RMS line voltage that
results in full power delivery to the loudspeakers. In many cases, the nominal line voltage is 70.7 Vrms with 100 Vpeak.
Configuring the SA300 for an RMS trip threshold of 70Vrms in this case will keep the output voltage of the amplifier at or
below 70V and thus ensure the loudspeakers are at or below their rated power handling when properly tapped.
Standard Step-Up Transformer Application
In this configuration, a normal amplifier drives the primary of a step-up transformer, and the secondary of said step-up
transformer in-turn drives a network of loudspeakers incorporating step-down transformers.
The SA300 should be connected between the amplifier output and the primary (or input) of the step-up transformer.
Threshold settings within the SA300 will be determined based upon the turns-ratio employed within the step-up
transformer required to achieve the nominal high-voltage line. For example, if your system is operating with a nominal
high-voltage line of 70 Vrms and the turns-ratio is 1.5, the SA300 threshold would be set for 47 Vrms (70.7 divided by
1.5). In this case, the SA300 will engage protection when the amplifier output voltage exceeds a level which results in line
voltages in excess of 70 Vrms.
Power Handling Considerations
When using a 70V system, one must be cognizant of the overall power delivered to the distributed network. This is
helpful when determining the size of the amplifier required for the system and ensuring the SA300 is operated within its
safe operating region (refer to the Voltage/Amperage/Watts table and calculations on page 9). In both of the previous
connection scenarios (Direct Drive and Step-Up Transformer), the SA300 is used to protect the entire distributed
network, and therefore will be required to handle the total system power. To ensure safe operation of the SA300 it is
recommended to calculate the total power and current required to drive the network.
For example, if you are using 50 loudspeakers, each with 5W tapped transformers, the equation would be as follows:
1. Calculate the impedance for one of the 5W tapped transformers as follows:
Z = (70.7Vrms)² / 5 = 1000 ohms.
2. Determine the net impedance seen by the entire system (assuming all 50 transformers are connected in parallel to
the 70V line):
Z = 1000 ohms / 50 = 20 ohms.
3. Calculate the total current delivered at 70.7 Vrms into the previously calculated net impedance:
I = 70.7Vrms / 20 ohms = 3.535 Arms
P = 3.535 Arms x 70.7 = 250 watts