November 07, 2013, by Jeff Kuells
I felt the need to write this article after a recent cross-country trip, and I’m pretty sure many of you can relate to my experience. At a newer, modern airport, I was waiting to board a connecting flight, and everything appeared to be going as planned until the departure time suddenly changed on the monitor - a one hour delay.
However, I never learned the reason for the delay, because the gate attendant making the announcement either had the mic down her throat - or, more likely - the paging system was defective. Speech intelligibility was poor to non-existent.
This can’t be, I thought. A modern airport certainly must have a paging system utilizing the latest technology, properly designed and installed to provide maximum intelligibility. We all need to “hear and understand” the announcements, right? After all, this is kind of the point, and unfortunately, it was a point completely missed by this system.
There can be several reasons for poor intelligibility with a paging system. In this case, the system was most plagued by bad loudspeakers combined with poor placement, and I’m sure “Mr. Budget” had negative impact as well, as it often does in systems of this nature. I wonder how many people miss their flights daily due to a poor paging system? One would think the amount of money wasted on compensating for missed flights would justify the cost of insuring every airport paging system sound do what it’s supposed to do: deliver intelligible audio to everyone in the facility.
Many audio professionals believe distributed audio cannot sound good. And in fact, it’s never going to sound as good as a finely tuned PA, but it also doesn’t need to sound like a badly tuned AM radio. If a distributed system is designed and budgeted properly, there is absolutely no reason for poor intelligibility or background music quality.
Distributed audio products have improved much over the past 15 or so years, because many manufacturers realized that distributed audio is big business. For example, there used to be only a few manufacturers offering ceiling loudspeakers, and now, just about everyone in the loudspeaker business has jumped on the bandwagon. Keep in mind that a major brand name on a loudspeaker doesn’t at all mean it provides the appropriate and necessary performance. Quality ceiling products take time to perfect.
THE WEAKEST LINK
While choosing proper loudspeakers for a distributed application is essential for good results, if you’re feeding these drivers limited audio signal and power, they still will not perform properly. The system will only be as good as its weakest link, therefore, every component in the chain - microphone(s), mixer, power amplifiers, step-up power transformers, wire gauge, step-down transformers and loudspeakers - must deliver good frequency bandwidth.
Another significant problem with distributed systems is the power amplifier side of the equation. Unfortunately, some installers lack understanding as to how to properly utilize power amplifiers for distributed applications. As with loudspeakers, most power amp manufacturers now offer products for distributed applications. I’m often asked about the differences between a standard two-channel power amp and a 70.7-volt or 100-volt power amp. Besides connectors and security features, there’s not much discrepancy.
The biggest difference is how high-output voltage is created. Some power amps require a step-up transformer to achieve the proper output voltage. Any professional power amp can be outfitted with a step-up transformer to make it a distributed amp. But first there must be proper understanding of why and how distributed audio is created.
Many distributed systems require only 100-watts at a 70.7-volt system operating voltage. (A 100-watt amplifier only has a voltage swing of 28 volts RMS at 8 ohms.) Thus a step-up transformer needs to be attached to achieve the required distributed voltage. There are two types of step-up transformers, isolation and autoformer, and both have pros and cons.
The autoformer is the easier to engineer, and the cheaper option. As a result, it’s the step-up transformer found in most power amps. Autoformers also supply good frequency response, but the downside is lack of protection between the loudspeaker and the power amp. Therefore, odds are pretty good that a system using an autoformer will have a short circuit on a line of 10 loudspeakers running on 300 feet of wiring. And if a short occurs, the power amp will shut down or even fail completely.
A step-up isolation transformer is a more solid approach in terms of power amp protection. With this approach, the power amp always sees a constant load impedance. If a short occurs anywhere in the chain, most likely the power amp will continue to be able to drive the system. However, frequency response suffers, limited to about 50 Hz to 8 kHz. A good bandwidth for an isolation step-up transformer would be 40 Hz to 18 kHz (+/- 1 dB).
So prior to purchasing a power amp with a step-up transformer, find out what type of transformer it is as well as its bandwidth. Be sure to check (or ask for) specifications of both frequency response and dB.
THE REQUIRED “SWING”
Increasingly, power amps don’t require a step-up transformer to deliver high-output voltage. A decade ago, Crown, for example, began offering a line without output transformers, instead outfitted with high enough internal DC rails to produce the required output “swing” for the load. These power amps also load protect against lower distributed impedances. It’s a very good solution for eliminating bandwidth limitations.
And in fact, many of the latest hybrid of high-power amps now on the market deliver enough voltage to drive a 70- volt system without need of step-up transformers. With higher power amps comes a larger voltage swing. For a 600-watt power amp running at eight ohms, voltage output is 69 volts. The impedance of a 600-watt 70.7-volt distributed system is 8.33 ohms. (This can be calculated using Ohm’s law.) What this means is that you don’t necessarily need to use all of the power provided by the amp. If you only need 300 watts, the 600-watt power amp used in my example will deliver the necessary power along with the required voltage swing.
Another question I’m frequently asked: can a power amp be bridged to get the voltage swing for a 70.7-volt system? The answer is yes, but you may not want to use a 600-watt power amp to drive a system requiring only 150 watts, and you may not want to use a step-up transformer.
On the other hand, a standard 150-watt power amp at eight ohms will not deliver the required 70.7 volts unless it is bridged or outfitted with a step-up transformer.
Here is an example of how to bridge a power amp for 70.7-volt operation. Let’s say we have a distributed system requirement of 150 watts at 70.7 volts. This will require a power amp that delivers 150 watts per channel at eight ohms, resulting in an output voltage of 34.6 volts per channel. For a 70.7-volt system requiring 150 watts, load impedance is calculated at 32 ohms.
When this power amp is bridged, it sees only half the impedance load applied to it. For a 32-ohm load, each channel sees 16 ohms. Most manufacturers do not publish 16-ohm specifications, so to determine this, divide the 8-ohm rating in half. So in this example, a 150-watt at 8 ohms is 75 watts at 16 ohms. The voltage swing for 75 watts at 16 ohms is 34.6 volts, and with the power amp bridged, power and voltage outputs double. Thus after bridging this power amp, output of 70.7 volts and 150 watts is attained, close enough to adequately drive the system.
CHECK THAT SATURATION
Overdriving is a common mistake with distributed systems. If more than 70.7 volts (or 100 volts) is applied, step-down transformers may saturate, causing them to go from high impedance to an almost dead short. Not only will sound quality suffer immensely, but the power amp may fail. And be extra careful that this does not happen if you are bridging a power amp. The channel outputs will be shorted together if saturation occurs, and this is not good!
Step-up transformers can also be saturated if too much power is applied to the primary. When selecting a step-up transformer, be sure to match the power output of the power amp to the primary impedance of the transformer. (Most primaries are either 4 ohms or 8 ohms.)
Another issue of which to be aware is the combined frequency response of a loudspeaker and step-down transformer. Most only go flat to 120 Hz. To conserve power from the power amp while also avoiding low-frequency saturation, place a high-pass filter on the input of the power amp, which will limit its low-frequency response. It is recommended to use high-pass filter rated at 80 Hz to 120 Hz, 12 dB to 24 dB.
Always remember that 70.7 volts or 100 volts is the maximum voltage to be applied to the step-down transformer. Further, a distributed system design should operate between 50 and 75 percent of its total capability, allowing for additional headroom when needed and preventing potential transformer saturation.
To attain a distributed system offering excellent intelligibility and quality background music reproduction, it’s essential to have a good understanding of all the pieces in the chain as well as the potential problems that may occur.
Jeff Kuells is an audio engineer and audio manufacturing consultant and was previously director of engineering for a major amplifier manufacturer.