How To Design A High Impedance Distributed System - Toa A-503A Manual

TOA Electronics Amplifier Guide

How To Design A High Impedance Distributed System

In designing a high impedance speaker system, there is no need to calculate the total impedance
from the speaker impedance values, the way you would for a low impedance system. This is because
in high impedance systems (i.e. 70.7 volt and 25 volt line systems), the load impedance rating is
expressed in terms of the amount of power that would be delivered to it at the rated line voltage. The
rating is given in Watts, which can simply be added to the other speakers to get the total power drawn
by the load. Just add a little extra for headroom (see example below), and you know how much power
is needed. You don't even have to know Ohm's Law.
Here's the process in more detail: You should begin by choosing the type of speakers, how many, and
how much power each one will need in order to reach the desired volume in the listening area. Help
with this can be found in the TOA Speaker Guide. Once you know the type(s) of speaker(s) and how
much power each one will need, determine what is the lowest available transformer tap that will sup-
ply at least that much power to the speaker. For example, the SC-615T has 70.7 volt transformer taps at
15, 7.5 and 3.8 watts. To reach your desired level (maximum average level plus headroom for short-term
peaks), you decide you'll need at least 5 watts at the speaker. In this case, choose the 7.5 watt power tap.
When you have selected the proper power tap for each speaker, simply add them up and multiply the
total times 1.25. Your amplifier should have at least this much power into the selected line voltage.
For example, the job requires twelve SC-615T horns, each tapped at 7.5 watts, to cover the listening area.
Twelve times 7.5 watts = 90 watts, and 90 watts times 1.25 = 112.5 watts. Your amplifier should have at
least this much power.
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