Take the mystery out of 70.7v systems

Do you install sound systems in Houses of Worship? Perhaps you configure sound systems into sports bars and restaurants? Do you ever tackle large projects like stadium and arena sound systems? Maybe you put sound systems in banquet halls and event centers? Moreover, you might install sound into larger classrooms and lecture halls.  If you work in any of these kinds of venues, then it is imperative that you understand the principles of constant-voltage distributed sound systems.

Let me help you take the mystery out of designing and installing constant-voltage distributed sound systems, and help you understand the basic building blocks that make these systems efficient, cost-effective and believe it or not, great sounding!

Let’s first determine whether we should employ a distributed sound system rather than a traditional 8-ohm sound system? It all comes down to the number of loudspeakers that you need to cover a given space properly.  Now it is easy to understand when we assume a bunch of ceiling speakers, right? However, we typically don’t consider distributed sound for sound systems that might utilize delay or fill speakers for sound reinforcement, and we really should.

Some people tend to think that distributed sound systems do not sound very good compared to low-impedance sound systems, right?  Wrong!  It is a prevalent opinion.  To understand why, we must first dig into what a constant-voltage distributed sound system really is.

So let’s think of a power amplifier connected to a loudspeaker.  The amp has an 8-Ohm rated output, and we are using about 50’ of 16ga speaker cable to connect to an 8-Ohm rated speaker.  Let’s say that the amp outputs 60 watts into 8-Ohms. Now, let’s use this chart to find our transmission loss.  We have a 16ga cable running about 50’. So that gives us 11% or half a dB of loss. Now that is not too bad, right?  What it gives us is 53.4 watts delivered to that speaker.

If we run around 100’ or so to a second fill speaker, which as you know gives us a 4-Ohm Load.   Therefore, we lose 21% of our power, which equates to only 47.4 watts delivered to the main speaker.  Now you can see the 100’ cable loses probably somewhere around 38% of its power or can be expressed as delivering about 37.2 watts to the fill speaker.  Now some amps can push more output to a 4-ohm load, so that would help some, but you can see where I am going with this.

You may ask what happens when I need to add more speakers to a church?

You do not want to do that.

Seriously, you could wire the speakers in a series-parallel manner to get to 8-Ohms.  However, the loss over the long cable runs will start to add-up making the volume at each speaker uneven.  In a series circuit, if any one of the speakers goes bad, they all turn off, like lights on a Christmas tree.  Of course, you could buy more amplifiers.  If we look at the physics, you will see why constant-voltage distribution is a better solution.

Let’s begin with looking at where constant-voltage distribution comes from.  To derive the concept, we have to consider the Laws of Physics and more specifically, Ohm’s Law.  I guess that you already have a pretty good handle on Ohm’s Law.  What this tells you, is that Voltage is directly proportional to current and inversely proportional to impedance. 

Now, that means that that as voltage goes up, current goes up, and that when the impedance goes down, the current goes up.  Understanding this fundamental principle of physics, we can now look at what that means in the real world.

In AC power distribution, we discovered that between the power generating source and your home or business miles away, there was such a resistive power loss that varied with the length of cable, that there was little chance that a consistent voltage could be delivered to the end-user.  As we found earlier, we could have added more power generating stations or used much larger gauge wires to minimize resistive power loss.  Just like the undesirable choice of adding many more amplifiers to a sound system, that didn’t make practical, cost-effective sense.  So in 1949, it was decided that we would use Ohm’s Law to our advantage to reduce or lower current flow to reduce the effective resistive power loss due to I2R heating of the power cables. By adding step-up transformers at the source to raise the voltage and lower the current, we can use smaller gauge cable and achieve much greater distance. Then, we use a step-down transformer to give us the 240 volts we need, with increased current capacity that meets the need of our communities.

So using Ohm’s Law, in a standard 8-Ohm system like the series-parallel one that burned down that church, if my 8-ohm amplifier is outputting around 20 volts then each speaker will draw about 2.5 amps of current. In this system, the total current draw through the speaker cables would be approximately 22.5 amps. According to the Giddings Table, I would need to use about five hundred feet of 10 gauge speaker cable, which is expensive stuff!  That is for a 60-watt amplifier! Excellent for a thousand watt amp running subs maybe, but for all your speakers, not very practical!

So if we take the lead from the power companies, let’s put a 70.7V transformer at the amplifier, and one at each speaker.  So again using Ohm’s Law, we are going to transform the 8-Ohm output of the amplifier up, to a high impedance, about 50-Ohms.  This means that the current will proportionally decrease to about three amps. So we can now use cost-effective 22 gauge wire, or whatever cheap stuff we have laying around the shop.  We then use a step-up transformer at the speaker, and we have plenty of power at 8-Ohms to drive the volume we need.

To address the question about sounding poorly, transformer, especially low-cost transformers, have had a bad time saturating when driven with strong low-frequency signals.  The more iron in the core, the better for less saturation, but none the less it was still a problem.  So in order to eliminate that saturation of the transformer’s core, manufacturers would roll-off the bass.  This is where the old wives tale of bad sounding distributed sound comes from.  And not too many years ago, it was true. With the dawn of rare-earth toroidal magnets, such saturation is a thing of the past.  However, make no mistake about it, great transformers are not inexpensive.

So now you understand why you would choose a constant-voltage sound system.  Let me you show you how to design one properly.

Always choose a high-impedance amplifier that uses high-quality toroidal transformers, or a direct-drive high-impedance amplifier like Lab Gruppen.  This way you can throw out the idea that these systems sound bad.

Now, we will start with the speakers and work our way backward. This is not the time or place to discuss loudspeaker placement, but remember we are talking about full coverage, with a minimal overlap that would cause phase anomalies.

So once we determine the number of speakers and their placement to get us the coverage we need, what do we tackle next?

Chart A

Chart B

Chart C

In the example above, we have 36 ceiling speakers to give us the desired coverage. So according to chart A, we will need 512 feet of 16ga speaker cable, plus the length required to reach the equipment rack. It also shows us the proper amplification requirements.  Now as you know, this constant-voltage distributed sound is often called 70volt sound.  That is incorrect.  Because since audio is not a continuous constant amplitude signal, a 70.7-volt constant voltage sound system delivers up to 100-volt peaks at a 70.7-volt average.  So the rule of thumb is that you will choose an amplifier at least 1.5 times the needed output in watts. (Chart B)

So let’s recap.  I am tapping each speaker at 7.5 watts (Chart C), times 36 speakers, time 1.5 for peak headroom, and that gives me 405 watts. So you will specify an amplifier with at least 405 watts output into a 70.7-volt load.  Don’t forget to choose one with good toroidal transformers or one with a direct-drive 70.7-volt output.

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