Knowledge Base
ConstantVoltage Audio Distribution Systems
ConstantVoltage Audio Distribution Systems
Dennis Bohn, Rane Corporation
RaneNote 136 written 1997; last revised 3/07
 25, 70.7 & 100 Volts
 U.S. Standards
 Just What is "Constant" Anyway?
 Voltage Variations  Make Up Your Mind
 Calculating Losses  Chasing Your Tail
Background  Wellspring
Constantvoltage is the common name given to a general practice begun in the late 1920s and early 1930s (becoming a U.S. standard in 1949) governing the interface between power amplifiers and loudspeakers used in distributed sound systems. Installations employing ceilingmounted loudspeakers, such as offices, restaurants and schools are examples of distributed sound systems. Other examples include installations requiring long cable runs, such as stadiums, factories and convention centers. The need to do it differently than you would in your living room arose the first time someone needed to route audio to several places over long distances. It became an economic and physical necessity. Copper was too expensive and large cable too cumbersome to do things the home hifi way.
Stemming from this need to minimize cost, maximize efficiency, and simplify the design of complex audio systems, thus was born constantvoltage. The key to the solution came from understanding the electric company crosscountry power distribution practices. They elegantly solved the same distribution problems by understanding that what they were distributing was power, not voltage. Further they knew that power was voltage times current, and that power was conserved. This meant that you could change the mix of voltage and current so long as you maintained the same ratio: 100 watts was 100 watts  whether you received it by having 10 volts and 10 amps, or 100 volts and 1 amp. The idea bulb was lit. By steppingup the voltage, you steppeddown the current, and viceversa. Therefore to distribute 1 megawatt of power from the generator to the user, the power company steps the voltage up to 200,000 volts, runs just 5 amps through relatively small wire, and then steps it back down again at, say, 1000 different customer sites, giving each 1 kilowatt. In this manner large gauge cable is only necessary for the short direct run to each house. Very clever.
Applied to audio, this means using a transformer to stepup the power amplifier's output voltage (gaining the corresponding decrease in output current), use this higher voltage to drive the (now smaller gauge wire due to smaller current) long lines to the loudspeakers, and then using another transformer to stepdown the voltage at each loudspeaker. Nothing to it.
U.S. Standards  Who Says?
This scheme became known as the constantvoltage distribution method. Early mention is found in Radio Engineering, 3rd Ed. (McGrawHill, 1947), and it was standardized by the American Radio Manufacturer's Association as SE101A & SE106, issued in July 1949 [1]. Later it was adopted as a standard by the EIA (Electronic Industries Association), and today is covered also by the National Electric Code (NEC) [2].
Basics  Just What is "Constant" Anyway?
The term "constantvoltage" is quite misleading and causes much confusion until understood. In electronics, two terms exist to describe two very different power sources: "constantcurrent" and "constantvoltage." Constantcurrent is a power source that supplies a fixed amount of current regardless of the load; so the output voltage varies, but the current remains constant. Constantvoltage is just the opposite: the voltage stays constant regardless of the load; so the output current varies but not the voltage. Applied to distributed sound systems, the term is used to describe the action of the system at full power only. This is the key point in understanding. At full power the voltage on the system is constant and does not vary as a function of the number of loudspeakers driven, that is, you may add or remove (subject to the maximum power limits) any number of loudspeakers and the voltage will remain the same, i.e., constant.
The other thing that is "constant" is the amplifier's output voltage at rated power  and it is the same voltage for all power ratings. Several voltages are used, but the most common in the U.S. is 70.7 volts rms. The standard specifies that all power amplifiers put out 70.7 volts at their rated power. So, whether it is a 100 watt, or 500 watt or 10 watt power amplifier, the maximum output voltage of each must be the same (constant) value of 70.7 volts.
Figure 1 diagrams the alternative seriesparallel method, where, for example, nine loudspeakers are wired such that the net impedance seen by the amplifier is 8 ohms. The wiring must be selected sufficiently large to drive this lowimpedance value. Applying constantvoltage principles results in Figure 2. Here is seen an output transformer connected to the power amplifier which stepsup the fullpower output voltage to a value of 70.7 volts (or 100 volts for Europe), then each loudspeaker has integrally mounted stepdown transformers, converting the 70.7 volts to the correct lowvoltage (high current) level required by the actual 8 ohm speaker coil. It is common, although not universal, to find power (think loudness) taps at each speaker driver. These are used to allow different loudness levels in different coverage zones. With this scheme, the wire size is reduced considerably from that required in Figure 1 for the 70.7 volt connections.
Figure 1. LowInpedance SeriesParallel 8 ohm Direct Drive
Figure 2. 70.7V TransformerCoupled ConstantVoltage Distribution System
Becoming more popular are various directdrive 70.7 volt options as depicted in Figure 3. The output transformer shown in Figure 2 is either mounted directly onto (or inside of) the power amplifier, or it is mounted externally. In either case, its necessity adds cost, weight and bulk to the installation. An alternative is the directdrive approach, where the power amplifier is designed from the getgo (I always wanted to use that phrase, and I sincerely apologize to all nonAmerican readers from having done so) to put out 70.7 volts at full power. An amplifier designed in this manner does not have the current capacity to drive 8 ohm lowimpedance loads; instead it has the high voltage output necessary for constantvoltage use  same power; different priorities. Quite often directdrive designs use bridge techniques which is why two amplifier sections are shown, although singleended designs exist. The obvious advantage of directdrive is that the cost, weight and bulk of the output transformer are gone. The one disadvantage is that also gone is the isolation offered by a real transformer. Some installations require this isolation.
Figure 3. 70.7V DirectDrive ConstantVoltage Distribution System
Voltage Variations  Make Up Your Mind
The particular number of 70.7 volts originally came about from the second way that constantvoltage distribution reduced costs: Back in the late '40s, UL safety code specified that all voltages above 100 volts peak ("max opencircuit value") created a "shock hazard," and subsequently must be placed in conduit  expensive  bad. Therefore working backward from a maximum of 100 volts peak (conduit not required), you get a maximum rms value of 70.7 volts (Vrms = 0.707 Vpeak). [It is common to see/hear/read "70.7 volts" shortened to just "70 volts"  it's sloppy; it's wrong; but it's common  accept it.] In Europe, and now in the U.S., 100 volts rms is popular. This allows use of even smaller wire. Some large U.S. installations have used as high as 210 volts rms, with wire runs of over one mile. Remember: the higher the voltage, the lower the current, the smaller the cable, the longer the line. [For the very astute reader: The wiregauge benefits of a reduction in current exceeds the power loss increases due to the higher impedance caused by the smaller wire, due to the currentsquared nature of power.] In some parts of the U.S. safety regulations regarding conduit use became stricter, forcing distributed systems to adopt a 25 volt rms standard. This saves conduit, but adds considerable copper cost (lower voltage = higher current = bigger wire), so its use is restricted to small installations.
Calculating Losses  Chasing Your Tail
As previously stated, modern constantvoltage amplifiers either integrate the stepup transformer into the same chassis, or employ a high voltage design to directdrive the line. Similarly, constantvoltage loudspeakers have the stepdown transformers builtin as diagrammed in Figures 2 and 3. The constantvoltage concept specifies that amplifiers and loudspeakers need only be rated in watts. For example, an amplifier is rated for so many watts output at 70.7 volts, and a loudspeaker is rated for so many watts input (producing a certain SPL). Designing a system becomes a relatively simple matter of selecting speakers that will achieve the target SPL (quieter zones use lower wattage speakers, or ones with taps, etc.), and then adding up the total to obtain the required amplifier power.
For example, say you need (10) 25 watt, (5) 50 watt and (15) 10 watt loudspeakers to create the coverage and loudness required. Adding this up says you need 650 watts of amplifier power  simple enough  but alas, life in audioland is never easy. Because of realworld losses, you will need about 1000 watts.
Figure 4 shows the losses associated with each transformer in the system (another vote for directdrive), plus the very real problem of linelosses. Insertion loss is the term used to describe the power dissipated or lost due to heat and voltagedrops across the internal transformer wiring. This lost power often is referred to as I^{2}R losses, since power (in watts) is currentsquared (abbreviated I^{2}) times the wire resistance, R. This same mechanism describes linelosses, since long lines add substantial total resistance and can be a significant source of power loss due to I^{2}R effects. These losses occur physically as heat along the length of the wire.
Figure 4. Transformer & Line Insertion Losses
You can go to a lot of trouble to calculate and/or measure each of these losses to determine exactly how much power is required [3], however there is a Catch22 involved: Direct calculation turns out to be extremely difficult and unreliable due to the lack of published insertion loss information, thus measurement is the only truly reliable source of data. The Catch22 is that in order to measure it, you must wait until you have built it, but in order to build it, you must have your amplifiers, which you cannot order until you measure it, after you have built it!
The alternative is to apply a very seasoned rule of thumb: Use 1.5 times the value found by summing all of the loudspeaker powers. Thus for our example, 1.5 times 650 watts tells us we need around 975 watts.
Wire Size  How Big Is Big Enough?
Since the whole point of using constantvoltage distribution techniques is to optimize installation costs, proper wire sizing becomes a major factor. Due to wire resistance (usually expressed as ohms per foot, or meter) there can be a great deal of engineering involved to calculate the correct wire size. The major factors considered are the maximum current flowing through the wire, the distance covered by the wire, and the resistance of the wire. The type of wire also must be selected. Generally, constantvoltage wiring consists of a twisted pair of solid or stranded conductors with or without a jacket.
For those who like to keep it simple, the job is relatively easy. For example, say the installation requires delivering 1000 watts to 100 loudspeakers. Calculating that 1000 watts at 70.7 volts is 14.14 amps, you then select a wire gauge that will carry 14.14 amps (plus some headroom for I^{2}R wire losses) and wire up all 100 loudspeakers. This works, but it may be unnecessarily expensive and wasteful.
Really meticulous calculators make the job of selecting wire size a lot more interesting. For the above example, looked at another way, the task is not to deliver 1000 watts to 100 loudspeakers, but rather to distribute 10 watts each to 100 loudspeakers. These are different things. Wire size now becomes a function of the geometry involved. For example, if all 100 loudspeakers are connected up daisychain fashion in a continuous line, then 14.14 amps flows to the first speaker where only 0.1414 amps are used to create the necessary 10 watts; from here 14.00 amps flows on to the next speaker where another 0.1414 amps are used; then 13.86 amps continues on to the next loudspeaker, and so on, until the final 0.1414 amps is delivered to the last speaker. Well, obviously the wire size necessary to connect the last speaker doesn't need to be rated for 14.14 amps. For this example, the fanatical installer would use a different wire size for each speaker, narrowing the gauge as he went. And the problem gets ever more complicated if the speakers are arranged in an array of, say, 10 x 10, for instance.
Luckily tables exist to make our lives easier. Some of the most useful appear in Giddings [3] as Tables 141 and Table 142 on pp. 332333. These provide cable lengths and gauges for 0.5 dB and 1.5 dB power loss, along with power, ohms, and current info. Great book. Table 1 below reproduces much of Gidding's Table 142 [4].
Wire Gauge > 
22 
20 
18 
16 
14 
12 
10 
8 

Max Current (A) > 
5 
7.5 
10 
13 
15 
20 
30 
45 

Max Power (W) > 
350 
530 
700 
920 
1060 
1400 
2100 
3100 

Load Power 
Load Ohms 
Maximum Distance in Feet 




0 
0 
0 
185 
295 
471 
725 


0 
93 
147 
236 
370 
589 
943 
1450 


0 
116 
184 
295 
462 
736 
1178 
1813 


117 
186 
295 
471 
739 
1178 
1885 
2900 


146 
232 
368 
589 
924 
1473 
2356 
3625 


194 
309 
490 
785 
1231 
1962 
3139 
4829 


292 
464 
736 
1178 
1848 
2945 
4713 
7250 


389 
618 
981 
1569 
2462 
3923 
6277 
9657 


486 
774 
1227 
1963 
3079 
4907 
7851 
12079 


584 
929 
1473 
2356 
3696 
5891 
9425 
14500 


729 
1161 
1841 
2945 
4620 
7363 
11781 
18125 


1167 
1857 
2945 
4713 
7392 
11781 
18850 
29000 
Rane Constant Voltage Transformers
Rane offers several models of constantvoltage transformers. The design of each is a true transformer with separate primary and secondary windings  not a singlewinding autotransformer as is sometimes encountered.
MA 6S Transformers
Though the MA 6S is discontinued, the TF 170 Rated 100 watts, 70.7 volt transformers are still available, sold individually from the Rane factory. Other transformers and the KTM panel are gone, though TF 170 specs are here in the KTM 6 Data Sheet (PDF).
MA3 Transformers
The MA3 had a design change in February 2007 affecting whether the transformers are mounted internally or externally. For MA3 amplifiers manufactured after February 2007, use the MT6 rack panel with up to six transformers installed. For MA3 amplifiers manufactured before February 2007, transformers can be mounted internally. If you aren't sure, the older MA3 has six transformer mounting holes above the input jacks. TF 407 and 410 transformers are sold individually for either rackmounting on the MT 6, or direct mounting inside the MA3 chassis:
 TF 407 Rated 40 watts, 70.7 volts (discontinued)
 TF 410 Rated 40 watts, 100 volts
See the TF 407 & TF 410 Installation Manual (PDF) for pre2007 MA3 Amplifiers.
See the MT 6 Data Sheet (PDF) for post2006 MA3 Amplifiers.
MT 4 Transformers
The MT 4 high performance toroidal transformers set a new standard for wideband frequency response and small size. The MT 4 toroidal transformers come assembled in a 1U rackmount open tray chassis or individually as follows:
 MT 4 Four channels (transformers with rack tray): 100 watts, 100 volts or 70.7 volts (tapped secondary).
 TF 4 (transformer only) Rated 100 watts, 100 volts or 70.7 volts (tapped secondary).
 KT 4 (tray only) Open 1U tray chassis with connectors, mounts (4) TF 4 transformers.
Use MT 4 transformers with any standard power amplifier and any combination of constant voltage loads up to 100 watts to improve frequency response and power handling.
MT 4 transformers use premium toroidal cores and windings to deliver excellent fullpower bass and a flat frequency response well above the audio range. Distributions systems will noticeably deliver better audio fidelity. MT 4 transformers are also smaller and lighter than other distribution transformers. See the MT 4 Multichannel Transformer for details.
References
 LangfordSmith, F., Ed. Radiotron Designer's Handbook, 4th Ed. (RCA, 1953), p. 21.2.
 Earley, Sheehan & Caloggero, Eds. National Electrical Code Handbook, 5th Ed. (NFPA, 1999).
 See: Giddings, Phillip Audio System Design and Installation (Sams, 1990) for an excellent treatment of constantvoltage system designs criteria; also Davis, D. & C. Sound System Engineering, 2nd Ed. (Sams, 1987) provides a through treatment of the potential interface problems.
 Reproduced by permission of the author and Howard W. Sams & Co.
"ConstantVoltage Audio Distribution Systems: 25, 70.7 & 100 Volts" This note in PDF.