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25v 70v 100v constant voltage distributed audio systems
Why 70v? These systems are often referred to as 70v distribution systems, in-line distributed audio systems or constant voltage systems and are often spoken of as “fooling” the amplifier into operating a battery of loudspeaker systems as “one”. This is of course not true and modern 25v, 70v, 100v systems offer a variety of very useful applications. In a typical commercial setting, telephony, paging or PA and or announcement systems are operated through a number of loudspeakers through a single amplifier. Something the final Ohms load presented to the amplifier in a normal parallel speaker system would prohibit or at least not be recommended.
The term‚ "70V system" relates to the maximum output voltage of the amplifier. 100V is the more common voltage used in Europe, 70V commonly in the United States. Although 25v, it should be mentioned, does have its uses and produces full output at 25 volts RMS and therefore falls below UBC or other regulatory installation requirements. This makes it particularly useful in school systems, houses of worship and other area's where running new conduit might be problematic. A higher voltage up to 200V can be used too, for very long cable runs and higher power requirements. To generate this high voltage, the amplifier is equipped with a step-up transformer, which transforms the regular output voltage, in the 15 to 30 Volts range, up to the necessary 70V (or 100V respectively).
70v installation using TIC SP70vTA transformers
A battery or series of loudspeakers presented to a single amplifier must operate and be powered at different levels and the work up for this is quite laborious therefore, 70v systems and their like offer real world and convenient solutions for these installations. 70v systems were developed to make all these calculations or difficulties simple & straightforward. In these systems the output of the amplifier is full at an output of 70v RMS making the series calculations a mere matter of adding watts. For parallel impedances the full work up calculation for those mathematical masochists amongst us, is;
Simply put; in installation, a series of loudspeakers are run from the amplifier using distribution transformers at each unit which match the load impedance of each speaker so they will only draw a set amount of power across the load.
More advantages of 70v distributed audio systems
In addition to making the base calculations simpler, there are some other practical reasons for using 70v distribution systems.
Paralleling Impedances. If you were using 8-ohm speakers, you would not be able to parallel very many speakers before loading the amplifier with too low of an impedance. Series-parallel configurations are often not the right way to go. In distributed systems, the impedance of each speaker is transformed upward to allow the connection of many speakers in parallel.
Wire Gauge & Economy. When running into the higher impedance of the transformer, you can typically use smaller gauge cable. The amount of voltage (and therefore total power) eaten up in the cable is a function of cable impedance divided by the load impedance. If the load is 8 ohms, then the cable impedance better be pretty small, which requires large gauge cable. On the other hand, if the total load on the amp is, perhaps, 100 ohms (where the sum of taps is 50 watts), then the cable impedance can be higher without eating up very much voltage. The smaller gauge cable can save the installer and customer a substantial amount of money. It would get very costly if you had to wire every speaker with 12-gauge cable or larger!
Tap Selection. In many applications, speakers need to be set at different volumes, either because of different ceiling heights, different densities of speakers, outdoor ambient acoustics including landscape architecture or because the customer wants some areas to be quieter than others. The multiple taps on most 70v speaker transformers allow the installer to select how loud the speaker will be simply by selection of a different power tap.
A simple speaker/ 70v transformer diagram
Many TIC products are supplied with a factory installed, wire tapped, 25v / 70v In-Line Distribution Transformer for PA, telephony or commercial use. The supplied 70v transformer may also operate for many 25v and 100v operations. TIC always recommends the services of a trained professional Audio contractor/ Systems Integrator to complete such installations, particularly a CEDIA or NSCA trained professional. Careful note should be taken of the special 25v/70v/100v power Amplifier used in such installations as the deciding factor for determining correct installation procedures and, voltages/ wattages used. While no guarantees are expressed or implied, please note our “general” installation guidelines below.
70v installation transformer wiring
Notes: Polarity at either or both ends may be adjusted to the site installers preferred methodology at his own risk. Each and every installation is different in outdoor speaker quantity, equipment used, local conditions or ambient background acoustics. No guarantees are expressed or implied by the enclosed general guidelines. The enclosed guidelines references open 70v in-line distribution transformers and does not refer to the many TIC models with switch-able "on the fly" 25v / 70v switch/ crossover assemblies.
25v 70v 100v step down transformers
The main difference to a regular low-impedance system (4 or 8 Ohms) is the way, individual loudspeakers are connected to the loudspeaker line. A large number of single loudspeakers, each equipped with a step-down transformer, can be connected to one single output line. Each speaker's step-down transformer has relative high impedance at the primary side to connect to the 70V line. The secondary side of the transformer matches to the speaker itself (mostly 8 Ohms). The ratio between the amplifier's output impedance and the individual speaker's transformer impedance is usually between 1:100 and 1:1000. Depending on the maximum power of an amplifier, each 70V amplifier matches to a certain minimum impedance than can be connected to this output. It does not matter how this impedance is achieved. A large number of smaller speakers (with higher impedance at their step-down transformers) or a small number of larger speakers (with lower impedance at their step-down transformers) can be connected to the 100V line or any combination of them. The only rule is that the total impedance of the 70V line should not fall below the minimum impedance of the amplifier's 70V output. This solution was borrowed from the electrical power line distribution system.
The regular signal (voltage) during the transmission of music or speech is mostly far less that the maximum 100V. The voltage in the 70V line changes proportionally in the same way as the voltage at the audio input of the amplifier, because the output voltage of an amplifier is determined by the input signal only (the term '100V' defines the maximum voltage in the system in the same way as the term '+6 dB' defines the maximum level for a line signal, even if the actual level is much smaller).
Typical power rates and matching impedances (70v systems)
Also a much smaller wire diameter (AWG) can be used as in a low-impedance system (Increasing voltage and decreasing current minimizes the amount of current flowing in the wire).
A distribution transformer is required to step up the output voltage of the amplifier so that the current flow is kept as low as possible. 25V, 70V, 100V and sometimes even more than 200V are used. Many loudspeakers can be placed across the output by using in-line distribution transformers. These second transformers are required at the loudspeakers to bring the voltage down to the low-voltage/high current signal needed to drive the loudspeakers. The distribution transformer usually has several taps to choose the proper power that will be drawn from the distribution line by the transformer (and finally by the loudspeaker).
These taps normally look as follows:
The input taps of the distribution transformer lets you choose the power drawn from the line-in and the output taps you choose those connected to the loudspeaker (4 Ohms, 8 Ohms, 16 Ohms). The downside of the use of transformers is that they always degrade the sound quality in a certain way (especially the low end).
The simplest way to wire a constant-voltage system is to parallel all the speakers on only one long run of wire. But the amount of power lost in the wire may not allow the required amount of power to get to the farthest loudspeaker. And if there should be a short on the wire run, it would take down the entire system.
The better approach is to wire separate rows of loudspeakers. These separate wire runs can be disconnected for troubleshooting and could be powered by different power amplifiers. They also can be switched differently. It must be calculated with a certain power loss in the transformers (step-up and step-down transformer). Usually it will be 10 to 15 % for both transformers together.
How to install 70v distributed audio systems
The section previously, TIC Audio Installation Basics, 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.
This 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.
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.
Insertion loss in 70v systems
Insertion loss is the loss of power through the transformer. If the loss is 3 dB, half of the amplifier's power is lost. You should not have more than 1 dB loss in any audio transformer. The insertion loss of the transformers is measured at 1000 Hz. One problem is here that most of the power in the sound spectrum is below 400 Hz and the insertion loss at 1000 Hz could give a false information. All transformers should be tested at 60 Hz too.
Cable wire runs for 70v Systems
A simpler "real world" 70v systems contractor use chart below
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
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, audio system installation.