For the purpose of longevity I have reproduced the TIC article here....

whitepaper tech sheet:

TIC Corp, TIC Corporation Pros choice


25v / 70v / 100v in-line constant voltage
distributed audio transformers
(70v installation guidelines i)

connecting speaker systems in a 25v / 70v / 100v
constant voltage distribution systems

25v 70v 100v systems are typically used for paging, telephony, background music and other multiple inputs, where high sound volume or even, sometimes, the quality of sound is not the main consideration.

TIC Corporation ed. note: the following is meant as a general guide to assist our customers world-wide in their basic understanding of the wiring and installation of TIC Corporations range of audio products and exterior speakers in general. Due to the many diverse conditions and requirements for individual installations and site conditions no guarantees are expressed or implied within this document by TIC Corporation.

TIC Corporation ed. note; this printed guide comprises a simplified explanation of general 25v/ 70v/ 100v transformer use and is not meant to be or imply a theoretical or experts dissertation of 70-volt in-line distribution systems transformers.

how to install 70v distributed audio systems

The section previously, TIC Audio Installation Basics II, explained how to connect speaker systems in a typical sound reinforcement system. Public address (PA) systems typically use a different mechanism for connecting speaker systems. Each speaker system is connected to a special transformer, called a line transformer, which is connected to the 70-volt distribution line from the power amplifier. Many 70v power amplifiers designed for PA systems provide both standard 4/8/16-Ohm speaker connections and the 70v in-line distribution connection.

Fig. 3

70v Installation basic speaker wiring

Fig. 3 shows a typical PA system using a 70v constant voltage distribution system. Note that a separate line transformer is used for each speaker system. Each line transformer has multiple taps so that you can control the amount of power reaching the speaker system. Typical taps are 2.5, 5.0, 10, 15 and 20 watts. To install a constant voltage PA system, you must solder the wires from the primary of the line transformer to the distribution line and the secondary of the line transformer to the speaker system. Plug and Jack connectors usually are not used because PA systems are typically semi-permanent.

Constant voltage distribution, as used in public address systems, uses tapped transformers, such as the TIC Corporation model number SP 70T unit, to provide the signal to each speaker system on the line. The transformer taps allow you to select a different wattage for each speaker to control the volume level for different areas of the building.

One benefit of the constant voltage distribution system is that you don’t need to worry about the aggregate impedance of speaker systems you attach to the 70v power amplifier. You can add or remove speaker systems anytime and it will not make any difference in the performance of the amplifier or the speaker systems, as long as the maximum number allowed is not exceeded. This total depends on the wattage taps used and the design of the power amplifier. Note; Consult the operator’s manual that came with the 70v power amplifier you are using to determine the practical maximum number of speaker systems you can attach to the constant voltage line.

Another benefit is that wire length is not a major consideration as it is with standard sound system speaker system connections. The 70v distribution line can be hundreds of feet long (because it feeds high-impedance line transformer primaries) and it will have little effect on the power delivered to each speaker system. Each line transformer is usually mounted very near its speaker system so the wires connecting the low-impedance, high-current secondary to the speaker system are short.

Many TIC products are supplied with 30 or 50 Ft. of double insulated burial cable, model dependent, therefore the unit must be opened to make use of the factory-installed (SP 70T) 25/70v line transformer supplied with these models. As each installation is potentially different, the 70v line transformer is not pre-tapped. Selection must be installation specific.

Note: many of the professional - commercial grade TIC outdoor speaker units have factory installed 25v / 70v transformers, which are easily switch-able at load.

Example; a typical installation requiring a 5 watt supply at 8-Ohms would require accessing the Driver of the speaker system. Disconnecting the standard, direct wired, 8-Ohm connections. Isolating, then soldering the 8-Ohm and common taps from one side of the line transformer to the positive and negative spades of the driver. Isolating, then soldering the 5 watt and 70v common taps to the incoming power amplifier line. Most of TIC Corporations “Pro” series i.e. GS3, GS7L, TFS6 thru TFS25 come fitted with a factory installed “on the fly” switch-able 8 ohm standard to 70v distribution system.

The standard 25v / 70v in-line distribution transformer supplied with TIC Corporations professional/ commercial models is perfectly capable of providing adequate usage throughout North America, and usually in South American 50v, US 70v and EEC 100v applications. A rough example of how this breaks down is, when using the TIC Corporation standard 70v transformer line: 50v x 5W, 70v x 10W & 100v x 15W are equivalent output taps.

a note on testing 70v line transformers

25v / 70v / 100v line transformers are notoriously “power hungry”. It is entirely common for the line transformer to absorb 30% – 40% of the incoming line current. Example: Using the same 5W installation mentioned above, if you were to test the system using a Taos® testing device, set at 1000hz input, you may only read approx. 3W output from the line transformer. In a real world example the reading will rarely be precise being dependent upon too many other factors. It should be mentioned however, that a defective transformer would rarely read low, but most commonly provide no reading at all.

vu meter for 25v / 70V / 100V systems

70v installation VU meter readings

Note: TIC Corporation always recommends the services of a trained professional when completing an exterior audio installation. This is particularly true when dealing with some of the complexities that may arise during a 25v 70v or 100v in-line distribution, constant voltage, installation.