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Biamplification vs. Bridging Power Amplifiers

By Jon M. Risch, Senior Project Engineer, Peavey Electronics Corp.

When seeking more output from a sound system, it is common to bridge the power amplifier to increase the power available and generate more output from the system. In some cases, this is the best thing to do, especially if it is a subwoofer being used with a medium to low output power amp.Bridging the amp will give the usual 6 dB more power output, or 4 times the single channel power output into the same load impedance. However, the bridging must be into a load impedance that is twice the minimum allowable load for the single channel. Subwoofers often need some headroom to maintain punch and impact, so use of a bridged amp is not that unusual.

However, if the load is a full range speaker system, the bridged amp now has enough power to overdrive the tweeter very readily if an accident should occur, such as microphone to monitor feedback, or an electronics related feedback loop. Before you can even react, the tweeter may be damaged, or a protection circuit may trip or blow. In this situation, the increased power may affect reliability.

An alternative to power amp bridging is biamplification. When using full range speakers that have a biamplification option, biamplification can often provide more actual sound output capability than bridging an amp can, with better reliability. What exactly is biamplification? Biamplification is the splitting of the full range sound signal into high frequency components, and low frequency components, similar to what the passive internal crossover is doing inside the speaker, but only it is done at line level via an electronic crossover circuit, and then fed to two separate power amp channels.

In order to more clearly explain biamplification, let's look at some diagrams. We'll examine some diagrams that are analogous to an oscilliscope "snapshot" of a particular signal. In Fig. 1, a low frequency sound is being reproduced by a single channel of a power amp into a speaker load, and it is not clipping.

In Fig. 2, a high frequency is being reproduced through that same amp channel, and is also being reproduced cleanly.

But add the two frequencies together, and the power output required to reproduce the combination exceeds the available power output of the single channel. This combined signal is shown in Fig. 3, and the clipping of the high frequency component is readily noticed.

If the high and low frequencies are separated at line level, and sent to two different amp channels, then they can be reproduced cleanly, and the summing will occur acoustically. This is shown in Fig. 4. The net result is that actual output SPL has been increased.

The use of a line level crossover to biamplify has several advantages. First, the passive crossover has internal losses, losses in the series inductor for the woofer, and deliberate losses in the padding for the tweeter to match level to that of the woofer. The losses through the inductor for the woofer can be from 1 to 2 dB, so you can see that some output level is lost from using the passive internal crossover. A direct connection to the power amp without any intervening series inductors will usually increase woofer damping, and tighten up the bass character.

Second, most pro sound speaker systems that are biamp capable use compression driver tweeters mounted on a high frequency horn. These compression drivers are as much as 10-12 dB more sensitive than the woofer at the peak of their output, and an average of 6-8 dB more efficient over the entire range of the tweeter, requiring the tweeter to be padded down to match the woofer output. This loss of the inherent sensitivity of the tweeter has important implications for biamplification.

Third, there is a hearing phenomenon known as masking, where one frequency can dominate and overshadow another. In the case of biamplification, the clipping of the woofer will be masked by clean output from the tweeter, so that even when the woofer section of the biamplified full range system is clipping, it is not as easily heard, due to the clean tweeter output. This effect will work until the woofer amp is generating enough clipping harmonics to overcome the clean tweeter output levels. This is in contrast to the single channel of amplification, which clipped the high frequencies first, making the clipping much more obvious. Fig. 5 shows the woofer signal clipping by 3 dB.

See Fig. 3 again, and compare to Fig. 6, which shows biamplified woofer clipping of approximately 3 dB.

The undistorted combined waveforms are shown in Fig. 4. The sound of the clipped waveform in Fig. 3 will be much more audible than the clipped waveform in Fig. 6, both show equal levels of clipping.

So how much extra output does biamplification give you compared to bridging the same amp? First let's start with the assumption that the power amp is not a very small one, but has a nominally adequate amount of power for the woofer of a full range system, say around 100 watts per channel into 8 ohms. If you bridge such a power amp, and it is capable of 4 ohm per channel operation and 8 ohms bridged, then the power output will be 400 watts in bridged mode. This is a power increase of 6 dB, and, if the speaker system can handle the full amount of this increased amount of power, then the SPL should increase by about 6 dB also. As a benchmark, it helps to realize that the common criteria to double the apparent loudness of a sound is to increase the SPL by 10 dB, or ten times the power.

Biamplification is classically said to increase acoustic output by 6 dB also. However, this assumes equal power for the highs and lows, and also assumes equal speaker sensitivities. In addition, it must also involve a signal with a spread of frequencies across the audio band, in order to take advantage of the division of labor that occurs. This describes most modern music as played through a full range speaker system. So a 6 dB increase in sound output for full range music through a biamplified system is a minimum amount, not taking into consideration those three factors we wrote about earlier. Let's add those in to the equation now.

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