Resisting Change
Dec 1, 2001 12:00 PM,
By Alvin G. Sydnor
IT MAY SEEM LIKE A NEW WAY OF DOING THINGS, BUT THE constant-voltage distribution system has been around for a while. Many audio professionals have recently learned what the public utilities companies have known for years: Efficiently sending electric power over long distances requires minimizing loss due to the heating of power lines. This means that the impedance of the load over the transmission path must be very high so that the resistance of the transmission line is a small fraction of the resistance of the total circuit.
THE CONSTANT-IMPEDANCE METHOD
Many years ago, installers used the constant-impedance method for output impedance matching in commercial sound systems. This required the use of matching transformers. The constant-impedance method was originally adopted because it provided a convenient means for connecting loudspeakers. Various speakers within a system could be operated at different power levels, which was especially useful when installing sound systems in large industrial and commercial facilities that had both loud and quiet zones.
Speaker lines in the constant-impedance method were often refered to as high-impedance lines because they were usually connected to amplifiers providing 250- or 500-ohm output taps. The high-impedance line was connected to each loudspeaker voice coil through a line-matching transformer that had a secondary winding to match the loudspeaker voice coil’s impedance. The primary windings of line-matching transformers were connected to the proper primary impedance tap to control the amount of power that a particular loudspeaker would draw. Most line-matching transformers had four impedance taps, at 4, 8, 250 and 500 ohms.
The constant-impedance method worked well in its day, but it had a number of disadvantages. Among these were the complicated computations required to install a large sound system with many loudspeakers, as well as the need for large, high-power amplifiers that frequently delivered more than 200 volts. Obviously, the shock and fire hazards of these voltages were serious problems. Still another problem was that when a system had to be moved or expanded, it was necessary to readjust or replace every single transformer in the system. The constant-impedance method is no longer being installed; however, you can find older systems that still use the method today.
THE RISE OF CONSTANT-VOLTAGE
Following the decline of the constant-impedance method, a system of impedance matching emerged. The system, known as the constant-voltage method, is standard in almost every commercial sound system installation today.
The constant-voltage system was developed by the Committee on Sound Systems of the Electronic Industries Alliance (then known as the Radio and Electronic Television Manufacturers Association). In developing the new system, the committee complied with the recommendations of the Underwriters Laboratories regarding speaker-line voltage selection for the rated output of the amplifier. The constant-voltage system was accepted and adopted by the industry.
The system is based on a designated voltage rather than designated impedance. The European standard is 100 volts, and the American standard is 70.7 volts (usually referred to simply as 70 volts.) But let this point be clear: Constant voltage does not mean that the speaker lines carry an audio voltage that is always exactly 70 or 100 volts. During quiet signal spots, there is no voltage on the speaker line in the constant-voltage system. The voltage nomenclatures (12.5-volt, 70-volt, etc.) refer to the highest voltage an amplifier will develop while operating at its full-rated power. The actual voltage will be less when the amp is operating at lower output levels.
Today, commercial sound system amplifiers provide a variety of output terminals and can be used in numerous applications. Most amplifiers provide an output impedance of 4 or 8 ohms, as well as output taps at 12.5, 25, 70 and 100 volts. Most amplifiers provide 70.7-volt and 25-volt taps at the multiscrew output terminal strip.
What all this really means is that the available peak-signal voltage is regarded as a constant figure, at which value various currents, or wattages, may be drawn, for the determination of the relative power throughout the system. For example, let’s say ten 5-watt speakers are connected to a 50-watt amplifier. Correct matching will occur if the same constant-voltage design figure is used throughout the system. Thus, each speaker will receive one-tenth of the total output power.
STUDYING THE STANDARDS
Standardizing the output voltage of an amplifier simplifies the computation of the transformer taps needed for varying the sound levels of individual loudspeakers in a system. The line-matching transformers used in constant-voltage systems are available with the two standard line voltages, 70 and 25 volts. The size of the transformer will depend on the power it is designed to handle. As a general rule, constant-voltage transformers have taps for matching loudspeakers with 4-, 8- or 16-ohm voice coils; and they also have multiple taps marked in watts. This makes it convenient for the technician or installer to select the required transformer wattage needed to provide the required sound level for each speaker. The technician must ensure that the total power consumed by all loudspeakers in the system is equal to or less than the power output rating of the power amplifier.
In large systems that require long transmission lines, the 70-volt tap is ideal because it reduces the power loss caused by long lines. Smaller systems using shorter lines are generally installed using the 25-volt line tap. The trend is to use the 25-volt tap because many local building codes require that 70-volt transmission lines are run in conduit or armored BX-type cables. But take note: You must learn the local building codes before starting an installation or upgrading an existing system!
In some cases, constant-voltage transformers do not have taps marked in watts; however, transformers designed for use in a constant-impedance system with taps marked in impedance can be used. In order to use a constant-impedance transformer, you must first determine the wattage delivered by its marked impedance. For the 25-volt system, the formula to use is:
Z = E/P
whereZequals the desired impedance tap,Eis the voltage andPis the power in watts
For the 70-volt system, the formula is:
Z = 625/P
wherePis the desired power in watts
In constant-voltage sound systems, the amplifier can be seen as a constant-voltage generator, and the speaker’s feed lines as power lines. It’s no longer necessary to think in terms of impedance. Instead, think in terms of volts and watts.
Example: Assuming that the voltage across the line is 70 volts, it is only necessary to determine how much voltage is required at the voice coils of each speaker to consume a specified number of watts. If an 8-ohm speaker is to be driven at 12.5 watts, the voice coil signal would be 10 volts. The formula for this is:
whereEis the voltage (10),Zthe impedance (8) andPthe power in watts (12.5)
This hook-up requires the use of a 7:1 voltage step-down transformer.
Sometimes you may encounter a transformer that has no taps at all but has a 7:1 turns ratio. A transformer may have a tapped primary or secondary, or it may be a tapped autotransformer. The table on page 62 shows the relationship of speaker voltage and power consumption in cases like these.
Detailed mathematical calculations are eliminated in designing constant-voltage systems. And when it’s time to expand or add power to the system, higher power amplifiers can easily be substituted. There is no need to rewire the speakers. Another advantage is that after the system is installed and operational, it is easy to make individual loudspeakers louder or softer by simply changing the transformer tap on the loudspeaker to handle more or less power as needed.
In the constant-voltage system, it is also possible to switch loudspeakers in and out of the system with no noticeable change in volume of the remaining speakers. As mentioned earlier, this is not true in the constant-impedance system because any change in the number of loudspeakers will affect the impedance match of the entire system.
HORN-LOADED SYSTEMS
In horn-loaded systems, the low-frequency output of the horn is determined by its low-frequency cut-off characteristics. If frequencies below this theoretical cut-off point are fed to the driver, the unit will overload and the diaphragm may suffer damage. One way of preventing such low-frequency overload is to use a capacitor in series with the driving unit. This increases reactance to the lower frequencies and prevents them from being fed to the driver unit. Another method is to shunt the voice coil with a choke coil, which will bypass the low frequencies around the voice coil. In this scenario, the secondary of the transformer bypasses the low-frequency energy from the voice coil, and the diaphragm does not receive the acoustic load, thus avoiding the excessive burst of unused power.
70-VOLT CONSTANT-VOLTAGE SYSTEMS
Setting Up the System
There is often confusion as to how to set a constant-voltage amplifier when its full-rated power is not being used. It may seem proper to turn down the gain of a 100-watt amplifier when only 35 watts are being drawn from it. However, this is not true. The amplifier will not use power more economically, and the speakers will not be any less prone to overload. If, for example, you turned off all home appliances and lights, your wattmeter would stand still, but the line voltage would remain at 117 volts.
The same holds true in an audio 70.7-volt, constant-voltage system. If there is no load tied to the amplifier, then the amplifier will deliver no power and take only stand-by power from the speaker line. The power it will take from the line and the power it will deliver to the load will be completely dependent upon the actual load itself. In this type of system, overdriving speakers is really a non-issue. Excess voltage, not power, overdrives a fixed-impedance speaker; and since the voltage is kept constant by the amplifier irrespective of load, then even a single speaker (set at 2.5 watts) may be put across a wide-open 100-watt system with complete safety. Turning down the gain of a constant-voltage amplifier will naturally reduce the power input to the speakers, but that will upset the sound power distribution throughout the whole system.
If the system is set up after a legitimate sound survey has been made and the power ratings of the individual speakers within the system are set accordingly, there should be no reason to play with the gain controls of the amplifier. If variations are required in certain areas due to changes in ambient noise conditions, then these changes should be made by tapping at the transformer of the speaker located in that particular location. This will achieve the desired sound output there without upsetting the rest of the installation.
70.7-Volt System Advantages
- Impedance matching is completely eliminated in multispeaker systems. It is not necessary to arrange a network of speakers in fancy series-parallel combinations in order to obtain the proper impedance match to a transformer. In the constant-voltage system, the impedance is meaningful only to the extent that it determines how much power will get into the speaker, and this is set by the transformer manufacturer by fixing the wattage ratings on its terminals.
- The 70.7-volt line maintains a constant voltage irrespective of load. So, once an individual power adjustment on a speaker has been made, the speaker continually receives the same amount of power even when other speakers are added or removed from the system, resulting in more constant and uniform coverage.
- More efficient use of sound power is possible since one can adjust the speakers in each zone without affecting the others.
- Constant-voltage makes the best use of available audio power by eliminating volume controls, which burn up power. Transformers are essentially non-power-consuming devices.
- It’s relatively easy to determine one’s amplifier needs and to keep these needs to a minimum by simply counting up the actual speaker power requirements without having to allow for uncertain attenuation losses.
- More adequate overload protection is afforded to each individual speaker transformer on the constant-voltage system. A transformer failure in one location does not start a chain reaction.
With their increased versatility, ease of design, ease of expansion and safety, it’s no wonder that constant-voltage systems have all but replaced constant-impedance systems.
Alvin G. Sydnor is a former electronics engineer, retired from the audio products division of GE.
