Constant voltage speaker system

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Constant voltage speaker systems refer to networks of loudspeakers which are connected to an audio amplifier utilizing step-up and step-down transformers to simplify impedance calculations and to minimize power loss over the speaker cables. Constant voltage speaker systems are also commonly referred to as 25-, 70-, 70.7-, or 100-volt speaker systems; Distributed speaker systems; or High impedance speaker systems. In the US, they are most commonly referred to as "70-volt speaker systems".

The term "constant voltage" has been seen as confusing—voltage is not normally constant in an audio signal.^[1]

Constant voltage speaker systems are analogous to electrical power transmission methods employed by power companies. Typically, a power company will step up the voltage of the power being transmitted, which correspondingly reduces the current and therefore also reduces the power loss during transmission. Similarly, in a constant voltage speaker system the amplifier uses a transformer to step up the voltage of the audio signal to reduce power loss over the speaker cable, allowing more power to be transmitted over a given wire diameter. Each speaker in the system has a step down transformer to convert the voltage back to a usable level.

Contents 1 Loudspeaker connection 2 Amplifier connection 2.1 External step-up transformer 2.2 Internal step-up transformer 2.3 High voltage transformerless 3 Higher power levels 3.1 High voltage 3.2 High current 4 Alternatives 5 Advantages 6 Disadvantages 7 References

[edit] Loudspeaker connection

Typically, each individual loudspeaker is connected to the constant voltage line by way of a step-down transformer. Such transformers can be set for a single power level or have multiple taps, one of which is selected to match the desired power level to be applied to the loudspeaker. Transformers with various taps allow the contractor to adjust the sound pressure level up or down at an individual loudspeaker. To minimize power losses, the wires connecting the loudspeaker to the step-down transformer are kept as short as possible. Purpose-built models are available that have the transformer contained within the loudspeaker enclosure, or, if intended for indoors, mounted just outside the enclosure.

Occasionally, two or more loudspeakers are connected to a single step-down transformer; one that is capable of handling their combined power needs. To minimize power losses, the speaker lines are kept as short as possible.

All the loudspeaker step-down transformer primaries are connected in parallel to the constant voltage line.^[2]

[edit] Amplifier connection

There are three different ways to connect constant voltage lines to an amplifier:

• External step-up transformer
• Internal step-up transformer
• High voltage, transformerless output

[edit] External step-up transformer

This topology allows the contractor to choose a general-purpose amplifier with typical low-impedance output by connecting its output to the primary of an external step-up transformer. It also allows the contractor to specify special-purpose transformers to tailor the system design to the project's target power levels. Multiple amplifiers can be hitched together via transformers to yield higher voltage and higher current capacity lines. For instance, three 70-volt amplifiers have been used to make a 210-volt line by connecting them to a special-purpose external output transformer hat has three primaries and a single secondary.^[3]

[edit] Internal step-up transformer

Amplifiers with built-in 70-volt output transformers are available with low impedance and high impedance output connections, the latter typically labeled "25v" and "70v." These are robust, purpose-built amplifiers with many application-specific design features such as overcurrent protection and aggressive high-pass filtering for flyback voltage protection. Some models can be configured so that one channel drives one or two low impedance 8-ohm speakers while the other drives a constant voltage string of speakers.^[4]

[edit] High voltage transformerless

In 1967, Crown International introduced the DC300 amplifier, the first amplifier capable of directly driving 70-volt lines without the typical narrowing of frequency response and loss of power associated with output transformers. In 1987, Crown introduced the Macrotech 2400, capable of driving 100-volt lines directly.^[5] Since that time, further developments in high-power amplifier technology have widened the choices; many manufacturers make amplifiers capable of direct connection to a high impedance constant voltage speaker line.

[edit] Higher power levels

[edit] High voltage

High voltage constant voltage systems can be designed to use 140-, 200- and 210-volt lines, depending on the transformers selected and the amplifier connection topology. Such high voltage systems have been used in locations where small diameter wire is already in place, where long distance wire runs are involved and at especially loud installations such as Daytona International Speedway^[6] and the Indianapolis Motor Speedway prior to its redesign in 2003.^[7] Safety considerations involved with such high voltages require speaker line installation within conduit in most of the world.

[edit] High current

600 watt transformers are widely available for contractors needing high power loudspeakers in constant voltage installations.^[8][9] Special purpose transformers capable of handling 1250 watts down as low as 50 Hz are available.^[10] One problem with high power, high current transformers is that fewer can be used on a single constant voltage line. Larger diameter speaker wire is recommended. Larger transformers needed for high power handling have reduced high frequency response.

[edit] Alternatives

The traditional alternative to constant voltage speaker systems are low impedance speaker systems (commonly referred to as "8-ohm speaker systems" in spite of the fact that their impedance may not be 8 ohms), in which the amplifier and speaker are directly coupled without the use of transformers. The disadvantages relative to constant voltage systems are that speaker cables need to be shorter or larger diameter and that more amplifiers are needed if different listening levels are desired at different locations.

Recently, self-powered loudspeakers have become widely available. One advantage of self-powered loudspeakers is the possibility that they have been carefully engineered for maximum performance without component damage. The main disadvantage is that both signal and power cables must be run to each loudspeaker.

[edit] Advantages

The main advantages of using a constant voltage speaker system over a conventional low impedance speaker system are:

• Multiple loudspeakers: Many loudspeakers can be driven by a single amplifier without complex series/parallel connection schemes.^[11]
• Multiple power levels: Different sound pressure level targets can be achieved at different listening areas while still using a single amplifier.^[12]
• Less expensive: Since the voltage of the signal has been stepped up and the current is relatively low, lighter, less expensive cable can be used without incurring additional power loss. Where a typical 8-ohm speaker system might require 12 gauge cable, a 70-volt system could use 18 gauge or smaller cable.^[13]

[edit] Disadvantages

• Frequency Response: Inexpensive transformers may have poor reproduction of low and high frequencies.^[14]
• Distortion: Overdriven transformers can add ringing distortion to the audio signal. Low cost transformers are prone to distortion at higher power levels, especially with regard to low frequency response.^[15] Low level signals can fail to energize a poorly designed transformer core enough to prevent higher than normal amounts of harmonic distortion.^[16]
• Variation: Unit-to-unit variation can be observed in poorly made transformers.^[17]
• Delay: More distant speakers on the same constant voltage line can not be delayed to match the speed of sound in air so that the impulses from a string of loudspeakers arrive at the same time from the point of view of a distant listener.
• Insertion loss: The tranformers themselves commonly reduce total power applied to the loudspeakers, requiring the amplifier to be some ten to twenty percent more powerful than the total power that is intended to be applied to the loudspeakers. Typical transformer insertion loss measurements are taken at 1,000 Hz in order to make the transformer's specifications appear as good as possible.^[18] Using this method, typical insertion losses are about 1 dB, a 20% power loss. Unfortunately, most of the power in voice-application audio systems is below 400 Hz, meaning that insertion loss at lower frequencies would be greater. The best transformers reduce mid-band frequencies by 0.5 dB (approximately 10% power loss) or less, resulting in a ten watt loudspeaker drawing 11.1 watts from the amplifier.^[19]
• Capacitance: To achieve higher power levels, transformers must be physically larger. Large transformers (above 200 watts) begin to suffer from high frequency attenuation due to self-capacitance.^[20]
• More expensive: If high-power loudspeakers are used with an emphasis on low frequency response, the required transformers will be much larger and will add significant cost to the project. Additionally, in some areas, local building and electrical code requires 70-volt cabling to be carried within conduit, increasing the overall project cost.^[21]
• More sensitive: Since constant voltage systems operate at relatively high impedances, they are more sensitive to small amounts of leakage current and partial short circuits. Running 70-volt speaker lines in conduit that may collect water can result in crackling sounds heard in the system.^[22]

[edit] References