70 Volt Distributed Audio Systems

Most of us are familiar with impedance matching for distributed audio systems. Resistive and autoformer impedance matching volume controls, unrealistic wattage ratings and more can conspire to make your life difficult if you’ve got lots of speakers to install for a distributed audio system. Feat not. There’s another more effective and efficient way to solve this problem. Originally used exclusively in commercial applications, 70 volt audio distribution systems are becoming more common in today’s larger residential installs, not to mention their use in light commercial applications. So here’s a brief explanation of how they work and where you might consider using them.

And don’t forget that you can order Terra AC Series speakers with high quality built-in 70 volt transformers. Plus we offer the Crown CDi 1000 70V/low impedance DSP power amplifier as well, independently and in select speaker/amplifier packages.

What’s a 70 volt distributed audio system?

Amplifiers designed to drive typical, low impedance speakers (4, 6 or 8 Ohms) are constant current devices. The current delivered depends upon the impedance of the speaker load, but in order to make the system work properly with lots of speakers connected, you have to use series or series/parallel wiring arrangements and devices that maintain at least a minimum impedance load. If you don’t and the impedance drops too low, the amplifier will eventually overheat and (hopefully) protect itself, or worse.

However, if you choose an amplifier designed to drive a 70 volt line (or put a transformer on the output of a low impedance amplifier) and attach a matching transformer to each speaker, you create a constant voltage system that’s isolated from the combined speaker load impedance. In other words, you’ve eliminated impedance matching concerns. All you’ve have to do is ensure the amp can deliver the wattage needed to drive the connected speakers.

So with a 70 volt system I don’t have to worry about impedance matching any more?

Correct. But you still have other issues you need to address in order to make sure that the system works properly.

  1. Amplifiers If you’re not already using an amplifier that has 70 volt outputs (like our Crown CDi 1000), you’re going to have to either get one or use a standard low impedance amplifier and purchase and install a transformer at the amplifier’s speaker outputs. These types of transformers have wattage ratings, so be sure you get one that can handle the total wattage you’re intending to distribute to all the speakers. However, we strongly recommend that you simply use a 70 volt amp for ease of system design and installation.
  2. Watts, Watts, Watts As stated above, you no longer have to be concerned about impedance matching, but you do have to calculate the system’s wattage requirements. Each speaker in the system will typically have its own 70 volt to low impedance step down transformer. And each step down transformer will have several “taps” for connecting to the main 70 volt trunk line from the amplifier. The taps are marked in watts and which one you choose determines how loudly that particular speaker will play (and how many watts it’s fed), relative to the others in the system.

In order to determine the system’s total wattage you’ll need to add up all the watts, based upon the taps you’ve chosen for each speaker and the watts it will draw. Then you have to be sure that both the amplifier and the step up transformer you’ve installed at the amp (if you’ve used one) can deliver that amount of power.

The minimum required amplifier power is the number of speakers multiplied by the tap value. For example, using 6 speakers set to a tap value of 16W would require about 100W of power (6 x 16 = 96). However, it’s recommended that you use an amplifier with 20% more power than the minimum required; in our above example, about 120W. The formula for this equation would look something like this:

(tap value) x (number of speakers) + (20% – headroom) = total power required

So how do I decide which tap to use?

Let’s assume that you have step down transformers with connection taps rated at 4, 8, 16, and 32 watts. (‘Cause that’s how ours are rated.) Note too that these are multiples of each other.) Each higher number tap will deliver higher power to that speaker and in turn that speaker will play louder. So, if you’ve got a room that needs more acoustic power, simply choose a higher rated tap and bask in the glow of more. Or vice versa. If necessary, you can simply connect a speaker to the different taps in order to experiment with how much difference each tap appears to make using a particular model speaker. But remember if it’s an in-wall or in-ceiling speaker and it’s not installed in a baffle of some kind you’re not going to get any bass response. It’ll sound significantly louder overall once it’s installed in the wall or ceiling too.