Power Amplifiers in Bridge Mode

First, some background information.

The maximum positive- and negative-voltage capability of a power amp is limited by the power supply's limits, i.e., its voltage rails. The amplifier can produce so many volts positive and negative at its output. The audio signal essentially swings between the positive and negative supplies within the power amplifier. These voltage rails are also referred to as the DC (Direct Current) supply. The DC supply in a CS®400X is +/- 52 Vdc; in a CS-800X, +/- 74 Vdc; in a CS-1000X, +/- 82 Vdc; and in a CS-1200X, +/- 86 Vdc. Each of these CS-X amplifiers can produce a continuous sustained sinusoidal signal level that is somewhere in the range of 55-58% below the rated value of its DC voltage rails. Power (in watts) is equal to the voltage produced by the amplifier times itself and divided by the rated loudspeaker load.

W = V x V/R

Now that the current Peavey CS-X series can deliver full rated power into a two-ohm load in stereo, each of these models can produce even more power in bridge mode into a four-ohm load.

When operating the CS-X series in mono bridge mode, you can think of the A channel as being the amplifier's positive voltage rail and the B channel as the negative voltage rail. Although this is not actually the case, it's convenient to simplify it this way, since the positive speaker terminal is connected to the A channel's red binding post, and the negative speaker lead is connected to the B channel's red binding post. In bridge-mono mode, no connection is made to the black binding posts on channel A or channel B.

As an example, let's look at how we can improve a typical sound system's performance by employing power amplifiers in bridge mode. Suppose your sound system included two Peavey SPTM 2XT loudspeaker enclosures powered by a single CS-800X amplifier. Each of the SP-2XTs are eight-ohm enclosures, and they would each receive 260 watts of power. If you are doing sound in mono, you could put the CS-800X in bridge-mono mode to power both SP-2XTs, connected in parallel across the amplifier's two red binding-post terminals. The CS-800X would now produce 1200 watts of total power or 600 watts to each of the SP-2XTs. You will now have an increase in SPL (Sound Pressure Level) in the order of +3.6 dB. Typically, you would have to double the number of enclosures to obtain a +3 dB increase in SPL. I just showed you how to obtain more than a +3 dB increase in performance for absolutely no increase in cost.

As another example, let's say you have either of these two systems: (a) two SP-4XTs and one CS-800X, or (b) two DTHTM 4s and one CS-1000X. If you added one more power amplifier to either of these two systems and operated both amps in bridge mode, you would basically have increased your financial investment by 33%, but your total available system power would increase by 285%, and your maximum SPL increase would be better than +4.5 dB.

If this interests you so far, let me explain further how bridge mode works so that you may understand an application, involving a three-loudspeaker array powered by a single CS-X. I stated above that you could think of the A channel as the positive voltage rail and the B channel as the negative voltage rail (Figure 1). Well, it's a little more complicated than that. To illustrate exactly what's going on, I am going to break each channel of a typical stereo amplifier down into two basic component stages. The first stage we will call the very first stage of amplification, and the second stage will be all the rest of the channels' components.

When the very first input stage amplifies the input signal, you have a certain amount of signal gain, as well as a 180-degree reversal in phase (polarity). When the amplifier is switched into bridge-mode operation, the signal at the output of the first stage of amplification of channel A is attenuated, (reduced in gain) so that it is at the exact level that was first inputted into the A channel, and it then becomes the B channel's input. If the first stage of channel A has a gain factor of 10, then the out-of-phase output of this first stage is attenuated to 1/10 its value. The bridge-mode switch then sends this out-of-phase signal into the input of the B channel. Now each channel is identical in level, yet opposite in polarity. If we hooked up a loudspeaker to channel A's output and another speaker to channel B's output, the two speakers would be moving in opposite directions, since the two channels are out of phase with each other.

In bridge-mode operation, however, we actually hook up the load between the two red terminals of both the A and B channels. While the A channel moves so many volts positive, the B channel moves the same number of volts negative. When the A channel swings negative, the B channel swings positive by the same amount of voltage. The loudspeaker doesn't know that it is hooked up between the two red binding posts, so it reacts to the difference in electrical potential (voltage) between its own red and black input terminals. When channel A moves +10 volts, channel B simultaneously moves -10 volts, and the loudspeaker sees a 20-volt difference in potential between its two terminals. In other words, the positive loudspeaker (red) terminal is 20 volts more positive than its negative (black) terminal.

Some people who don't understand bridge-mode operation put the amplifier in bridge mode and then hook up two loudspeakers, one on each channel. The two loudspeakers are now operating out of phase, and the bass response is considerably reduced, due to the two speakers opposing each other. Years ago, some power amplifiers were equipped with a switch that bridged (paralleled) the amplifiers' inputs. This allowed you to drive the two-channel amp monaurally with the same input signal, without patching the two channels' inputs together. However, this is not the case when operating a stereo amplifier in bridge-mono mode.

Now that you understand bridge-mode operation, I will discuss an application that employs a CS-X amplifier in bridge mode to drive three loudspeakers in a single array.

In rooms such as churches or 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. For rooms that are twice as long as they are wide, a single loudspeaker system may 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. These constant-directivity (CD) horns are the best tools for the 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. Trapezoidal boxes are supposed to minimize the overlapping of these horn patterns. However, many of these boxes have trapezoidal angles that are smaller than the angles of coverage of their high-frequency constant-directivity horns. Therefore, if you pack the trapezoidal enclosures tightly, you still have severe overlapping of the high-frequency horns, resulting in diminished high-frequency output. Often the boxes must be splayed farther apart to minimize the overlap.

In auditorium applications where you need coverage for both the near and far 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 excess 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 HDHTM 244T enclosures. The three speakers are flown in the center, right above the front edge of the stage or sanctuary platform. The center HDH-244T is flown right side up, with the horn on the top, and has some downward angle to it. The two outside HDH-244T loudspeakers are then flown upside down, with their horns on the bottom, and have more downward angle. With most Peavey enclosures, this downward angle is twenty to twenty-five degrees greater than the center loudspeaker.

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. The constant directivity, or CD horns, is the best tool for the 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 that the angles of coverage of their high frequency constant directivity horns.

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