• Constant voltage systems (70V, 100V lines)

    Constant voltage, high impedance or line transformer systems are often a somewhat confusing concept for professionals used to rock and roll. They are actually very simple to design, among other things because they are intended to be installed by electricians and the like, with little experience in sound systems. In this article we will try to clarify the simple concepts needed to understand this type of system.

    There are doubts about the origin of the term, although it probably derives from the fact that the different amplifiers used have the same electrical output voltage, which is usually 100V in Europe and 70.7V (which is usually written simply as 70V) in the US. They are also sometimes referred to as "high-impedance systems" because the speakers used can reach impedances of even several thousand ohms, compared to low-impedance systems where it is rare to see a load greater than 16 ohms per speaker. We also speak of "distributed line systems" as well as "100V line systems" (or 70V), giving rise to the abbreviation "line systems".

    Systems in which the voltage at the output of the amplifier is raised and reduced before the speaker began to be used in the US in the late 1920s, with the term "constant voltage" appearing as early as a 1931 publication (Radio Physics Course 2nd Ed., Radio Technical Publishing co., N.Y.).

    The illustration shows an example with three speakers. Typically, such a system uses many speakers (as indicated by the suspension points in the illustration), but for simplicity only three are used.


    There are several reasons for using constant voltage systems:
    1. Minimize cable losses or reduce the cost of cabling
    There are many applications where the cable runs are extremely long (think of a hotel or an airport) and therefore the power loss in the cabling is a critical factor. Cable losses are a function of the square of the current and therefore if we can increase the voltage (thus significantly reducing the current intensity) that travels through the speaker line and reduce it when it reaches the speaker, we will have managed to reduce the power loss produced by the cable, or we will be able to afford to use a smaller calibre cable. It is a process similar to the transport of electricity by the electric companies, which carry it at several tens of thousands of volts and then lower it to 230V or 115V (which raises the current) when it reaches our neighborhood. As an example, we can carry 250W with a 100V line at a distance of 1200 meters with a loss of only 3 dB of power for a cable of only 2mm2 (AWG14). The maximum loudspeaker cable lengths table lista wiring values for high impedance installations.

    2. Allow many low-power speakers to be connected
    With impedances such as 8 or 16 ohms it is difficult to connect a large number of boxes in parallel (and serial-parallel connections are normally avoided as the failure of one unit affects all others in series with it). However, in constant voltage systems, hundreds of speakers can be connected in parallel.

    3. Simple installation calculation
    Calculating the required amplifier could not be simpler: just add up the input power of each of the boxes connected to the same line and choose an amplifier with the same or higher output power capacity (optionally adding an oversizing factor). In this sense it is the same as the electrical installation of a home; you only have to make sure that the available power of the line is sufficient for the sum of the devices connected to it.

    4. Flexibility
    Input transformers for constant voltage line speakers usually provide a range of input power taps. This makes it easy to select different levels for different zones. We can utilize the input power taps of a system's speakers to conveniently give more volume to some zones and less volume to others. For example, the teenage section of a store could be assigned more power than the areas intended for adult consumers who may not want to listen to music at such a high volume. And those settings could easily be changed if the volume requirements in each zone changed (such as if the zones were re-located).
    5. Connection of speakers of different power and type on the same line
    Whereas in a low impedance line it is almost impossible to mix speakers of different input power, in a constant voltage line we can combine speakers of any type and power, as each speaker naturally receives the required power from the amplifier.

    High voltage lines. Step-up and step-down transformers

    100 or 70 volts are high voltages considering that the speakers are usually of low power. To achieve these high voltages with amplifiers that do not generate them with their power electronics, a step-up transformer is used at the output of each channel of the amplifier. Those amplifiers that are specifically designed for constant voltage lines have it built-in, although external transformers coupled to conventional amplifiers can also be used. These step-up transformers usually have several windings that allow for both 70V and 100V output, and sometimes also 25V, which is a low voltage used in the US since it allows for less electrical safety for legal purposes. Even less common is the use of 50V lines. Often amplifiers also have a direct output, without going through the transformer, for low impedance loads. The different outputs can often be used simultaneously; for example, we could use the same channel to amplify an 8 ohm speaker and a group of auxiliary boxes with an input transformer.

    In other cases, the amplifiers generate either 100V or 70V directly without the aid of a transformer. Typically, this type of transformer-less line amplifier only outputs one of these two voltages, so we often find that American manufacturers make models with 70V output, while European manufacturers use 100V. The more nodern amplifiers might do both.

    If we can choose between 70 or 100V we will choose to use 100V lines, since the higher voltage will allow us to reduce the cable losses or use thinner cable (and therefore lower cost). For exceptional long cable runs, most of the amplifiers for constant voltage applications that have two or more channels allow us to join two channels in bridge mode, obtaining 140V or 200V output, in which case the power of the speakers will be four times more than what their position indicates for the unbridged voltage (for example, the position of 15W for 100V would correspond to 60W for 200V), but here one should prevent the possibility that someone could unknowingly and dangerously select a tap that could mean too much power would be provided to the speaker.

    Another possibility is to use conventional power amplifiers. For example, a 1250W per channel 4-ohm amplifier produces a nominal output voltage of 70V (using Joule's Law, one of the basic formulas in electricity). And an amplifier of 2500W per channel at 4 ohms gets a nominal output voltage of 100V. If this means excessively high power amplifiers for the number of speakers that the line will carry, smaller amplifiers can be used in bridge mode. For example, an amplifier that does 625W at 8 ohms in bridge mode (for 70V) or one that does 1250W at 8 ohms in bridge mode (for 100V).

    The speakers also require a transformer, a step-transformer down that reduces the voltage so that they receive the right amount of power. This transformer is usually built into the speaker in products that are specifically designed for constant voltage lines, although external transformers can also be used in conjunction with conventional loudspeakers, particularly when they are high power cabinets that lack a transformer version provided by the manufacturer, though the popularity of high SPL powered speakers has made this an even rarer occurrence. These step-down transformers often have several windings that allow you to select the power level you each speaker receives from the line; in these cases there is usually an input power tap selector to select between a range of input power levels in steps that are usually spaced 3 dB (power levels are doubled) and are usually marked with their corresponding 70V and 100V power levels. The positions of the transformer selector are shared for 70V and 100V, with the position for 70V of a given power corresponding to twice the power with 100V (for example, the position for 15W and 70V corresponds to 30W and 100V and, similarly, the position for 30W and 100V corresponds to 15W at 70V). Sometimes there is an additional position that bypasses the transformer and allows the loudspeaker to be used as if it were not equipped with a transformer; this can be useful as the same model can be used for both high and low impedance applications, but the installer should be careful not to accidentally select this transformer-less position within a 70 or 100V line.

    The quality of the transformers, both step-up (those that go with the amplifier) and step-dpon (those that go with the loudspeakers), is critical to sound quality. Transformers commonly used in power supplies are not usable for this application because they do not provide uniform impedance with frequency, and therefore affect the frequency response of the system. Within the transformers used specifically for loudspeakers in this type of system, only the highest quality transformers will allow a correct response in the upper part of the high frequency spectrum. Another problem with transformers is core saturation. At high power levels, transformers become saturated, which lowers the line load impedance at low frequencies. This is a problem because the amplifier delivers more power (or attempts to do so, possibly by activating the amplifier's over-current protection) and the speakers receive more power, which can jeopardize the safety of the installation in addition to affecting the sound quality.

    We can distinguish between two types of transformers for constant voltage applications. The first is the isolation transformer, which is one that has two windings on a closed iron core. The term "isolation" is used because there is no electrical connection between the input and the output (there is what is called galvanic isolation), the current being conducted electro-magnetically. The second type of transformer is the auto-transformer (or autoformer), which consists of a single winding with an iron or steel core inside. This type of transformer is cheaper and lighter, but it is not considered suitable for quality sound installations and is less safe than insulation, since a winding failure will result in the delivery of maximum voltage at the output. Like power transformers, those used for constant voltage lines can also be toroidal, somewhat lighter and more efficient. On the other hand, isolation transformers can avoid potential ground loop problems.

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