High Impedance (70.7/25 Volt) Distributed Line Systems - Toa A-503A Manual

High Impedance (70.7 Volt / 25 Volt) Distributed Line Systems
In order to overcome the limitations of low impedance speaker systems, most medium-scale installed
sound systems in the United States use either 70.7 volt or 25 volt distributed line systems, also known
as high impedance or constant voltage systems. Often, they will be called simply "70 volt" or "25 volt"
systems.
These systems work by including transformers at the input to each speaker and directly after the
amplifier output (see fig. 8). The transformers are used to convert the impedance of each speaker to
a higher value, and to convert the amplifier output impedance to a correspondingly high value. In a
70 volt line system, speaker impedances (with transformers) may range from below 20 ohms to as
high as 10,000 ohms or more. But you won't need to calculate the load impedance in ohms, because
of how the high impedance approach works.
High impedance (70.7 volt and 25 volt line) systems have three major advantages over low-impedance
systems:
1) System impedance-matching is made much easier — it is simply a matter of adding up speaker
power taps and selecting an amplifier rated for at least that much power plus an allowance for
headroom.
2) Line loss is greatly reduced, especially over long cable runs, resulting in better performance and
reduced cost compared to long low impedance lines.
3) The amplifier output is electrically isolated from the speaker line by the output transformer, pro-
tecting the output stage against a grounded line and thus eliminating a potential source of sys-
tem failure.
Figure 8: High Impedance Distributed System
Step-up
Transformer
8 ohms
Amplifier
12
TOA Electronics Amplifier Guide
Step-down
Transformers
70 volt line
(high impedance) 8 ohms
8 ohms
8 ohms
8 ohms
Speakers