In our posts Distributed Speaker Systems 101 and How To Wire 70-volt Distributed Sound Systems, we gave you some great info on the basics of 70-Volt sound systems and how to effectively wire them together. In this article, we’re going to cover the separate units that make up a complete, well-designed 70-volt distributed sound system.
What Parts Make up the Basic 70 Volt Sound System Design?
Brief Overview of 70-Volt
- 100 volts peak = 70.7 volts
- 70 Volt is the highest “safe” voltage authorities permit.
- [Voltage X Current = Power] Power remains the same as long as the voltage/power ratio remains the same
- 100 watts of power = 10 amps X 10 volts OR 100 watts of power = 100 volts X 1amp
- Raise the voltage = Lower the current
- Example – Power companies send 200,000 volts through “small” wire. It drops to 120 volts in your home
Why Use 70 Volts?
- Lots of speakers powered by one amplifier
- No need to home-run each speaker
- Higher voltage allows use of smaller wire
- Impedance-matching speaker selectors restrict the number of speakers possible
- Speakers can easily be added and removed anywhere in the system
- No need to calculate impedance, just total wattage
- EASY to design!
What Parts Make up the Basic 70 Volt Design?
- Source components
- Step-down Transformers
- Attenuators (Volume Controls)
- Transformer Substitutions
Types of Source Components
- CD players
- Juke boxes
- VCR audio
- Digital message announcers
- Cassette decks
- Satellite receivers
- Telephone systems
- iPhone, iPod, iPad
- MP3 Players
- Any device with an Audio Out!
- Transforms 70 volt signal down to 8 ohms, or high voltage and low current to low voltage and high current
- Attached the speaker in most cases
- Usually have one common lead and multiple primary leads on the primary side (input)
- Usually two leads on the secondary (output), but may have several, i.e., 4 ohms, 8 ohms, and 16 ohms
- Step-down transformers are what the amp sees, not the speaker
- Frequency response varies depending on materials used to build it and size of core and windings
- Mix sources, i.e., Microphone and music
- Has pre-amp & power amp circuitry
- Provides level control for each input
- Provides master level control for output
- Usually provides both mic and line level inputs and several outputs
- Usually provides several outputs
- Can be a combined mixer-amp, or a mixer and 70-volt amplifier, separately.
- For the safety of your speakers and amplifier, it is a good idea to use no more than about 80% of the amp’s maximum wattage
- Requires a mixer to control signals into the amplifer
- Connects line and mic inputs to the amplifier
- Provides individual control for each input
- Provides master level control for output
- Outputs Mono signal
- Most commercial systems require 18 gauge 2 conductor WITHOUT a shield
- Stranded wire is necessary, solid will not work
- Wire gauge depends on power load, as voltage increases, the resistance of the wire becomes less significant
- Higher voltage allows smaller gauge wire
- Wire connections are all parallel
- Pots – rheostats used to adjust levels for just one speaker; low power handling and poor durability; show variable resistance to amp
- L-Pads – two pots in one switch that show a constant load to the amp while changing resistance to speaker; same drawbacks as pots
- Autoformers – clean, durable way to adjust level; changes impedance seen by the amp. Highest power handling
- Can be cone drivers, horns, or enclosed assemblies
- If not 70-volt, 8 ohms is most common; other impedance may be 4 or 16 ohms
- Add total wattage used by all speakers, must be less than maximum amp wattage
- Any mixture needed is ok with the amp
- Available as In-Ceiling or Surface-Mount
- Horns, Two-Way Speakers, or Subwoofers
- Same qualities as above, except:
- Rated for environmental temperatures and weather
- Ruggedly built
- Available as Surface-Mount, In-Ground, or as a Rock aesthetic
Can be used with drop-tile ceilings
- Low-voltage, making it easy to run many in a single system
- Wide range of prices and aesthetics
- Easy to install and connect
Autoformers (not transformers): one winding with multiple connections to various points in the winding
- Available in 10, 35, 100 watt versions
- Wattage rating refers the total load they can handle
- Available in standard or Decora
- Colors include white, ivory, almond, & stainless steel
- Require large boxes or plaster rings
- Able to control any load at or below it’s rated value, i.e., a 100-watt attenuator will work with just a one-watt load, or with 100 speakers tapped one watt each.
70 Volt Transformer Substitutions
- Use a 25 volt transformer on a 70 volt system and you will deliver 8 times the power to the secondary
- Use a 70 volt transformer on a 25 volt system and you will deliver 1/8 the power to the secondary
- Substitution is acceptable as long as secondary voltage does not exceed the transformers’ rating
- A 4 ohm load (speaker) on an 8 ohm secondary tap will draw twice the power
- An 8 ohm load on a 4 ohm secondary tap will only draw half the tap’s power
Article Via Atlas Sound’s 70 Volt Rules
Recap of Distributed Speaker Systems 101
In a typical paging, background music or noise-masking system, several loudspeakers are placed across a single amplifier in parallel. They must often be powered at different levels, and the calculations involved in determining the actual load impedance at the amplifier’s output are quite tedious. A solution for this problem comes in the form of the 70-volt distribution system, which was developed to make all calculations simple and straightforward. In this method of distribution, amplifiers are designed so their full power output exists at 70 volts RMS. The load impedance that corresponds to several output power ratings is shown below:
|Power||1 W||5 W||10 W|
|Power||30 W||60 W||100 W|
In application, many loudspeakers are placed across the output using distribution transformers, which match the load impedance of each loudspeaker so that it will draw a specified amount of power from the line when the amp hits its maximum output of 70 volts.
Let’s assume that we want to drive a particular loudspeaker at 5 watts. The connection is made and the impedance of the loudspeaker as seen from the primary side is calculated. The 8-ohm loudspeaker is transformed across to the primary as a 1,000-ohm load to the amplifier. The 70-volt primary has, in process, been transformed down to 6.3 volts RMS at the speaker’s voice coil.
Ignoring for a moment the insertion loss of the transformer, we can do the calculations at either the primary or the secondary of the transformer. The formula is voltage squared divided by impedance:
Primary Power = Secondary Power
(70)2/1000 = (6.3)2/8
Loudspeakers are placed across the line and tapped as needed, and all the designer has to do is count watts — the sum of all the taps. When the total wattage drawn by the line equals the power output rating of the amplifier, then the maximum number of speakers have been attached to the amplifier’s output. In practice, it is best to keep the total sum of the taps below 80% of the power amplifier rating.
The simplicity of this method means that the user never needs to calculate load impedance in parallel combinations.
In reality, not all the power that goes into the transformer’s primary gets transformed to its output. There is some insertion loss. These days, transformers in distributed systems should probably exhibit no greater than 1 dB of insertion loss.
70 VOLTS VS. 100 VOLTS
The standard in North America is 70 volts for a distributed system. Most other areas of the world use 100 volts, in which case figuring out the total load on the amp is still simply a matter of adding up the sums of the speaker taps, using the 100-volt rating of the tap instead of a 70-volt rating. Any impedance calculations, if necessary, are done by dividing the tap rating into 10,000 (100 volts squared) instead of into 5,000 (70.7 volts squared).
Today, there are speakers which use the same taps for both 70- and 100-volt systems. For example, a tap with 500 ohm impedance is a 10-watt tap when used on a 70-volt system and a 20-watt tap on a 100-volt system. The manufacturer must ensure that the transformer is capable of handling the higher voltage (which almost all are) and that the transformer is capable of transforming the full power at 100 volts without saturating (which many 70-volt transformers are not capable of doing).
OTHER REASONS FOR DISTRIBUTED 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.
WHY IS IT CALLED “CONSTANT VOLTAGE?”
You may have heard a distributed speaker system referred to as a “70-volt constant voltage system.” Does this mean that there is a constant AC or DC voltage of 70 volts always going through the speaker line? No, it doesn’t.
I’ve heard that the term goes back to early telephone systems. Audio engineers of that time were concerned with how the voltage arriving at the receiving device varied from the voltage sent out by the sending device, and how the voltage transfer would vary in conjunction with changes in the impedance of the receiving device. In distributed speaker systems, where the impedance of the receiving device (in this case, the transformer) is very high relative to the impedance of the sending device (in this case, the power amplifier), then the receiving device receives the same voltage regardless of the impedance of the receiving device (within reason). For example, if the amplifier is putting out a sine wave of 70 volts RMS, then the full 70 volts goes across the primary of the transformer whether you’ve connected to a 5-watt tap (which is 1,000 ohms) or to a 50-watt tap (which is 100 ohms). So there is “constant voltage” transfer regardless of the impedance.
Does this mean that multiple low-impedance speakers (8 ohms) driven by an amplifier is not a “constant voltage” system? Driving a low-impedance speaker system (16 ohms, 8 ohms, 4 ohms) with a power amplifier is also a “constant voltage” system until you have too low of an impedance for the amplifier to drive. For example, a 10-volt sine wave from the amplifier driving a 16-ohm speaker will continue to be a 10-volt sine wave if you connect an 8-ohm speaker instead. The 8-ohm speaker will simply draw more current from the 10-volt signal, resulting in more power draw. The voltage stays the same but the current draw varies, which results in different power taps.
In my opinion, the term “constant voltage system” is not very useful or meaningful in describing 70-volt distributed speaker systems, but it has somehow stuck with us through the years.
Understanding the concepts in designing a distributed system is the first step in successfully completing the project. Using these simple math examples can help you install large-scale systems in theme parks, stadiums and more. Don’t be confused by the term “constant voltage system.” It is a hold-over from an earlier time and won’t help you understand the distributed systems of today. As with everything, experience is the best teacher. Try multiple configurations and listen to what sounds best.
Rick Kamlet is market director for commercial sound at JBL Professional. This text was adapted from the JBL Technical Note, Vol. I, No. 2.