Constant Voltage (70-Volt) Audio Systems For Beginners
High-Impedance Loudspeaker Distribution
Audio distribution systems, especially those commonly used in department stores, warehouses, around race tracks, etc., require that many loudspeakers be driven from a single audio amplifier over long runs of speaker wire. It is certainly possible to connect 20 or 30 speakers in series/parallel combinations to achieve a reasonable impedance speaker load for the amplifier. But this type system can be difficult to install and troubleshoot, and can be unreliable and inflexible. A "constant-voltage" or "high-impedance" audio distribution system is commonly used to overcome these and a number of other possible problems. These systems are also referred to by their nominal voltage, with "70-Volt" and "25-Volt" systems being the most common references.
The heart of these systems is the impedance and voltage transformation that allows the single amplifier to drive many speakers. Amplifiers that are designed for these systems are usually monaural (single channel) as opposed to stereo. They differ from "standard" audio power amplifiers in that they employ a large transformer at the output of the amplifier. The amplifier's "load" is the input or "primary" side of this transformer. The output or "secondary" side of the transformer is connected to all of the speakers in parallel. This means that electrically, the positive output of the secondary is connected to the positive side of each speaker, the negative to the negative side of each speaker. The wire can be run from speaker to speaker, or can easily be split off to speakers in another area.
It's time for a little theory here. Power is most commonly calculated by multiplying the voltage in a circuit times the current flowing through it. So, a 6W circuit can have 2 amps of current from 3 volts, or 3 amps of current from 2 volts. One amp at 6 volts would also be 6W. The transformer in the 70V amplifier is designed such that the voltage from the amplifier is greatly increased in the secondary, usually 70.7 volts at maximum, but the current is greatly reduced. So the power output of the amplifier is the same, just transformed from a lower voltage/higher current to higher voltage/lower current. It is excessive current flow that causes amplifiers to overheat, so it should stand to reason that being able to lower the current and increase voltage in our speaker system would allow one amp to drive many speakers. By the way, 2.83 volts applied to an 8-ohm speaker equals 1W of power.
Attached to each speaker in a 70V system is another transformer that changes the power back to "lower voltage/higher current" so that it can drive the 8-ohm speaker. These transformers usually have two output taps, one for 8-ohm speakers, and one for 4-ohm speakers. There are generally four input taps to choose from... more on that in a moment.
Wattage is Easier
We can design and analyze a 70V audio system from the aspect of impedance (in ohms, for the experts), but using power (in watts, for the rest of us) to arrive at the same end result is definitely more straight forward. The wattage of any 70V power amplifier is always given by the manufacturer. Most are from 20W up to about 100W. The transformer on each of the speakers in the system will generally have four different wattage taps on the secondary, often ranging from ½ watt to as high at 30W or more. If each of 10 speakers were connected to the amplifier using a 1 watt tap, then at full power they will draw 10 watts from the amplifier. If 10 speakers were on a 2 watt tap, 20 watts would be drawn. Each individual speaker can be set to any available wattage tap. The higher the wattage tap, the more loudly it will play for a given setting on the amplifier's volume control. Some speakers may need to be set at a lower or higher wattage level based upon location, ambient noise, and distance from people.
To ensure stability and good service life, the sum of the wattage taps on all connected speakers should not exceed about 80% of the amplifier's rated power. So 40 speakers set on their 2 watt tap would equal 80W, and could be safely powered by a 100 watt amplifier.
Advantages of 70V (High-Impedance) Audio Distribution Systems
- All loudspeaker transformers are wired in parallel across the amplifier output, making wiring simple.
- If one or more speakers fail, the remaining speakers continue to operate just as before.
- The loudspeaker transformers generally have assorted primary winding taps allowing different power levels (and therefore overall volume) to be selected for each speaker. The individual loudspeaker power level can be easily adjusted after the system is installed. Using switch-selected transformers provides volume controls at the individual loudspeakers. There are also wall-plate type attenuators so that the volume of a speaker, or even several speakers, can be controlled locally.
- Adding or removing speakers from the system does not change the signal/volume level of the other speakers as the voltage output of the amplifier is constant. This makes adding or removing speakers of no consequence to the performance of the rest of the system, so long as added speakers do not overload the amplifier.
- High-impedance, constant voltage systems easily overcome the nominal resistance of long runs of speaker wire. As current flow is very low, a light gauge speaker wire can be used without a performance tradeoff. 18 to 14 AWG speaker wire is the most commonly used.
Alternate Output Voltage Taps
100V, and even 140V outputs can be found on some of these distribution amplifiers. A 25V output could be used where fewer speakers are to be used, and the runs of wire are not extremely long, such as in a doctor's office. 100V and 140V systems are often used around sports arenas, golf courses, amusement parks, etc., where the speakers are very widespread, higher volume levels are needed, and speaker wire runs can easily exceed 100 yards.
Sources of Audio
There are of course a number of common features in these systems which affect how they work and what they can do. They can be as simple as having a single microphone for paging, to background music from a variety of sources including CD players, iPods, satellite feed, etc., and telephone system interface. The amplifiers vary greatly in their features, but the most simple will have basic volume and tone controls, a microphone input, and standard RCA jack(s) for audio input. For basic background music systems, a simple home audio receiver can be used to provide AM/FM radio reception and inputs for a CD or cassette player. Such receivers will have an auxiliary or tape line output that can be routed to the distribution amplifier with a standard RCA patch cable.
More sophisticated systems have precedence circuits that automatically reduce or mute music while paging. They can also interface with a telephone system so that the hold music is the same as the background music, and paging can be done from any telephone. Sophisticated multiple zone systems can allow different paging, music, and control to be available in each zone and controlled separately.
The most common speaker found in business offices, classrooms, etc., is a standard 8" ceiling speaker. They often come with a baffle (grill), and the transformer attached, and are designed to be mounted into a drop ceiling. Most drop ceiling tiles are not sufficiently robust to support the weight of a speaker, so a drop ceiling "bridge" is used to allow the ceiling frame to support the speaker and baffle. There are also back covers for the speakers, called "top hats". These are not necessary for the speaker's best performance, though may be required to keep debris out of the back of the speaker's cone, or possibly required by local electrical or fire code. Due to some code requirements, the bridges and top hats are available in UL-approved metal construction, and plastic. These speakers vary in price, and of course in tonal quality.
For the best possible tonal quality, 70V systems can be used with any high-fidelity speaker, including in-wall or ceiling speakers. These speakers are intended for mounting directly into drywall, which is easily robust enough to support them. The transformers can be purchased separately and added to any such speaker if they do not come with it. Nonetheless, 70V systems are generally not used in home audio distribution systems as there are a number of other options that can provide better sound quality and features.
For outdoor public address applications, horn-type speakers are used to direct and focus the sound over the area where it is wanted. This improves efficiency as sound energy is not wasted in going where it is not wanted or needed. Specifications on these speakers will usually include the vertical and horizontal dispersion angles or the horn lens. This info is used to gauge how far away the speaker is placed and its optimal position for the best coverage. These speakers are best for the spoken word (dialogue or announcing). They generally do not have the low frequency response to make music sound natural. They are built from materials making them suitable to continual outdoor use in all weather.