SOUND SYSTEMS & PRO AUDIO
By Marty McCann
Copyright 1995
If you have read my previous article on the operation of power amplifiers in bridge mode, or if you already understand how bridge mode works, I will present an application to you that employs a CS-X amplifier in bridge mode to drive three loudspeakers in a single cluster or array.
In many rooms such as churches and auditoriums where a single source of sound is desired, a common approach is to employ an array of loudspeakers flown directly over the front edge of the stage or sanctuary platform. Some rooms have an aspect ratio where they are twice as long as they are wide, and perhaps a single loudspeaker system will not have adequate horizontal coverage to include all the seats to the near left and right in the pattern of the horn.
Modern loudspeaker systems employ constant directivity high frequency horns that exhibit uniform frequency response within their included angles of coverage. This constant directivity horns, called CD horns, are the best tools for accurate reproduction of high frequency information. However, these CD horns should not be placed side by side where they can overlap in their coverage angles. If two of these CD horns are allowed to overlap a great deal, they will have diminished output directly on axis between the two horns at some frequencies. Part of the rationale for trapezoid boxes is that they are supposed to minimize the overlapping of these horn patterns. However, it's a fact that many trapezoid boxes have trapezoidal angles that are smaller than the angles of coverage of their high frequency constant directivity horns.
Therefore with many boxes, if you pack the trapezoidal enclosures tightly, you still have severe overlapping of the high frequency output directly on-axis. Often the boxes must be splayed farther apart to minimize the overlap of the horns.
In many auditorium applications where you need coverage for both the near and far field seats, you can address the room with two sets of loudspeakers. The near seats are in the near field and the far seats, of course, are in the far field. You can solve the problem of excessive overlapping, and still address the near and far field requirements by employing an array of three loudspeakers. In my example I am going to use three HDH-244T enclosures. The three speakers would be flown in the center, directly above the front edge of the stage or sanctuary platform. The center HDH-244T would be flown right side up, with the horn on top, and it would have some downward angle to it. The two outside HDH-244T loudspeakers would then be flown upside down, with their horns on the bottom, and they would have more downward angle. With most of out Peavey enclosures, this downward angle would be twenty to twenty-five degrees greater than that of the center loudspeaker.
Now the loudspeakers would be "Arrayed" properly, but we must do something to allow the center speaker to truly take care of the Far Field seats. The inverse square law says that the sound coming from a loudspeaker is reduced in level directly proportional to the inverse of the square of the distance away from the source. The sound coming directly from the loudspeaker is called the direct field. The inverse square law can be simplified if we relate it to the decibel scale. The direct field emanating from a loudspeaker drops in level or diminishes, at a rate of -6 dB, every time you double the distance away from the loudspeaker. Therefore, in our application, in order for the Far Field loudspeaker to provide adequate coverage, the Near Field loudspeaker system must be turned down in level by -6 dB, so that the Far Field speakers can have a chance to provide the proper level to the farthest seats.
Also, if the Near Field speakers are not reduced in level, the gain of the entire system, before feedback occurs, will be limited by the Near Field speakers themselves. This is because the Near Field speakers will react with the open microphones, causing the system to feedback before the Far Field speakers can reach the level necessary to provide the desired SPL to the farthest seats.
In the past, if you wanted to use one power amplifier for this application, the two Near Field loudspeakers would be put on one channel of a power amplifier, and the gain or level control of that amp would be turned down -6 dB. The single Far Field speaker would be on its own channel and that channels level control would be set wide open. We would now have a situation where our gain structures would be set properly for the Near and Far Field components of the array. If we were to use a CS-800X in this situation, when the Far Field loudspeaker received 200 Watts from the power amp, each of the Near Field speakers would receive 50 Watts (-6 dB equals ? Power). I am basing this exercise on the condition that each individual loudspeaker has an impedance of eight ohms. Two hundred Watts may or may not be adequate power, depending on the room and the type of music to be reproduced. Also, I would worry that some individual, who did not understand the need for limiting the level of the Near Field coverage loudspeakers by -6 dB would come along and turn the amps' Near Field channel up all the way. This would completely change the performance capability of the system as a whole.
I can show you how to get +6 dB more performance out of this system with no more financial investment, while having the system configured to prevent anyone from changing the calibration of the Near and Far Field gain structure. The secret to this application is to operate the CS-800X in bridge mode. Connect the Far Field HDH-244T across the two red binding posts of each channel, i.e., in normal bridge mode operation. Then you would connect one of the Near Field HDH-244T's, across the A channels' red and black binding posts. Next, the second Near Field HDH-244T would be connected across the B channels' red and black binding post, however (note) the second Near Field HDH-244T, connected to the B channel output, would have the loudspeaker leads reversed. This is very important, since the B channel is out of phase with the A channel, you don't want the second Near Field speaker to be out of phase, so reversing the loudspeaker leads will maintain the proper polarity. In this application I would recommend reversing the loudspeaker leads at the speaker itself.
This would prevent someone from noticing the different color wires on the A & B channels' red and black banana posts, and then they might be tempted to change them a round, thinking that they are "correcting" someone else's error.
In this application with CS-800X, the Far Field HDH-244T would automatically have +6 dB more gain that the Near Field HDH-244T loudspeakers. When the Far Field loudspeaker received 800 Watts from the power amplifier, each of the Near Field speakers would receive 200 Watts. I have shown you a way in this application to obtain +6 dB more performance in both the Near and Far Field, with no more investment in dollars. Typically, you would have had to quadruple the number of loudspeakers to obtain +6 dB more system performance.
The CS-800X can operate bridge mode into a four-ohm load. It can operate in stereo down to two ohms on each channel. In the above application we are connecting a single eight ohm loudspeaker across the two red binding posts of channels A & B, and then an additional eight ohm speaker is connected to channel A, and another to channel B. As far as the amplifier is concerned, it's as if we had a four ohm load hooked up in bridge mode.
Don't try to make this application anymore complicated than it already is. Many times when I have explained this approach to someone, they immediately try to figure out how to utilize even more loudspeakers in the same application. In some rooms, such as a pie shaped auditorium or church, where the farthest distance can be to the left and right rear corners, the same three loudspeakers can be employed in a different configuration. That is, the center loudspeaker is flown upside down with more of a downward angle, while the two outside speakers are flown right side up with less of a downward angle. The center speaker is now covering the Near Field, so it requires the -6 dB of attenuation, and the two outside loudspeakers are now playing to the Far Field, so they do not need the -6 dB reduction in gain. With this type of array, there is no way to employ the above outlined application.
Here is one more application that can work however with the CS-X amplifiers. Perhaps we are in a room that is long and narrow, and yet we still need both Near and Far Field coverage without increasing the horizontal coverage angle. We can fly the two loudspeakers one above the other and configure the top (Far Field) speaker in the above bridge mode application, and then the bottom (Near Field) speaker can be connected across the A channels' red and black banana posts. This will work the same way as the first three-speaker array, i.e., the Near Field speaker will have the proper gain reduction (-6 dB).
I believe we covered a lot of ground in this article. If you thoroughly understand the operation of power amplifier in bridge mode, you may want to undertake some of the outlined applications. If you are still a little cloudy on how bridge mode works, I would advise you not to try the above applications.
After I thought up this approach several months ago, I explained my proposed application to Jack Sondermeyer, Peavey Electronics' director of analog engineering. Since Jack is the engineer that actually designed the Peavey CS-X series of amplifiers, I knew that he would be able to spot any flaws in my thinking regarding this application. After I explained this method to Jack he laughed a little, and I said, "On no, don't tell me there is a fly in the ointment?" Jack laughed once again, and said that he was laughing because it was a great idea, he couldn't believe that he hadn't thought of it.
