Suresh:
The main reason for using a 70V output is more efficient transfer of power to the speakers. Using 100m of speaker cable in an 8-ohm system depending on the wire guage, more voltage is being dropped on the cable than the speakers. 70V systems raise the voltage, causing a proportionate drop in the current (Ohm's Law). The amount of power you can send through a wire is a function of the current not the voltage. So if the voltage is 10 times as great, the current is 1/10th as great. Therefore the efficiency would be 10 squared or 100 times as great!!! P=I^2R. Also speakers can be connected in parallel without having to worry about impedance. The only drawback is that frequency response below 70Hz is reduced.
