25v/ 70v/ 100v in-line constant voltage
distribution transformers
(70v installation guidelines iii)
25v 70v 100v distributed audio systems
70V Transformer Systems are referred to as 'constant-voltage distributed audio systems'. The constant voltage system is the most economical way to install a multi-speaker sound reinforcement system.
The term‚ '70V system' relates to the maximum output voltage of the amplifier. 100V is the usual voltage in Europe, 70V in the United States. 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).
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)
| 1W | 5W | 10W | 30W | 60W | 100W |
| 5000 Ohm | 1000 Ohm | 500 Ohm | 167 Ohm | 83 Ohm | 50 Ohm |
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.
70v insertion loss
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.
| Insertion Loss | Power Consumption |
| 1.0 db | 1.259x |
| 1.5 db | 1.413x |
| 2.0 db | 1.580x |
| 2.5 db | 1.770x |
other reasons for 25v 70v 100v distributed audio systems
In addition to making calculations simpler, there are some other technical reasons for using distributed 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. 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 or because the customer wants some areas to be quieter than others. The multiple taps on most speaker transformers allow the installer to select how loud the speaker will be simply by attaching to a different power tap.
VU meter for 70V / 100V systems
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