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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.

Conservatively, let's only add 1 dB for the improvement in woofer output due to elimination of the woofer series inductor in the passive crossover. The tweeter is capable of an average output from 6-8 dB in excess of the woofer, and the phenomenon of masking will allow the tweeter to cover up the woofer clipping by at least half of that, or 3-4 dB of additional output before the output becomes noticeably distorted. Again, that is a conservative number. That's at least another 4 dB total increase on top of the nominal text book 6 dB for biamplification, or a total increased SPL over the single channel use of 10 dB.

As far as reliability is concerned, the tweeter is only exposed to 100 watts of power capability, not 400 watts, and the same goes for the woofer. Accidents or sudden overloads are much less likely to damage a driver, and the amplifier is likely to be less stressed than when in bridge mode operation, which does add some extra operating stress.

One thing that can be done with most amps used for biamplification is to double up on each band (channel), or load two biamp capable 8 ohm nominal rated speaker systems off of one amp. The load on the amp would then only be a four ohm load per channel. Two woofers can operate off of the low frequency amp channel, and two tweeters off of the high frequency channel. That extra speaker could provide an additional 3-6 dB more SPL, or more crowd coverage, all the while biamped and playing clean and loud. A bridged amp that is only rated down to four ohms per channel, eight ohms bridged, could only have one single 8 ohm speaker connected while in bridge mode.

Obviously, with biamplification, you can also mix and match amp capabilities to the drivers. If you have two power amps with different power ratings, use the bigger amp for the woofers, and a smaller amp for the tweeters, and as long as the tweeter amp was at least 1/3 to ½ the power rating of the woofer amp, you would still enjoy that increased output advantage over bridging. Remember, we still had about 3-4 dB of excess tweeter output, left over after the masking effect was overcome, so the tweeter amp does not have to have the same power capabilities as the woofer amp to keep up.

Biamplification does require an electronic crossover to operate properly, while most pro amps come already equipped for bridged operation. Some users may not want to deal with the additional complexity of an electronic crossover, most such stand-alone crossovers need to be adjusted, and the crossover settings are not always obvious or easy to make correctly for best sound. Peavey offers a pair of simple solutions to the electronic crossover "blues".

The new CS®-800S has provisions for different input modules to be inserted, and one of those that is available is the Peavey X1 crossover module. It has made biamplification a breeze for anyone. With it's crossover point cleverly set to a specific point, with jumper adjustment of polarity and mode, it can be used with a wide variety of Peavey sound reinforcement speaker systems. All EQ and crossover adjustments have already been made, and in most cases, equal gain for both amp channels will get you within a few clicks of nominal levels for the proper balance of highs to lows. The real plus is that the crossover output is defaulted to the two channels of the CS®-800S with the X1 module installed, so no extra patch cables are needed!

The speaker systems that work extremely well with the X1 are: The Peavey SP™-2 series, the SP™-4 series, the SP™-5 series, the new SP™-1G, the Impulse 500, the DTH™ 2 series, and the DTH™ 4 series. By following the set chart in the owner's manual, and using a simple system of movable jumpers on the crossover module, any of these speakers can be used with this crossover module, and biamplified precisely and correctly. Many other similar two-way sound reinforcement speakers can be used with the X1 module, where the jumper adjustments allow for fine tuning of another similar type of biamp capable speaker system. A similar type would be a 15" woofer in combination with a constant directivity horn driven by a 2" diaphragm compression driver tweeter.

The X-1 also has provisions for use as a subwoofer crossover, with the same easy hookup and set-up. And for that really all out stack, it has provisions for triamplification, with the subwoofer output going to another amp used in conjunction with the CS®-800S. Oh, and to make things even easier, the polarity of the subwoofer and the upper range output are both connected in the same (positive) polarity, and the blend between the sub and the top range cabinet is always correct! No frequency response suck-outs or bumps!

Now for the very best part: this X1 module for the CS®-800S only costs $74.99 retail!

The other electronic crossover option that is relatively painless is the tried and true Peavey PL™-cans. These $40 retail crossovers plug into the crossover island on the CS®-series, CS® X models and most prior models. For use with the mainstream Peavey speaker systems, the PL™-1200EQ is the recommended model, while the PL™-800EQ can also be used. For subwoofer biamping, there is a 150 Hz crossover version, the PL™-150, for use with subwoofers; or if you prefer a lower subwoofer crossover frequency, a PL™-100. In most cases, they can be hooked up all positive polarity, and levels adjusted, and you are up and running in bi-amp mode. Patch cables are required to get in and out of the crossover island jacks on the CS®-series amps.

So if you want to get the most from your system, and maximize the output SPL of your components, consider the minimal investment in a Peavey crossover module to put you into the biamp arena.

For those fans of bridged amp operation, there are a few tricks that may be of some help to you. There is one thing that a bridged amp that is capable of 2 ohm music operation can do that a biamped set-up can not do, and that is to drive a pair of speakers, one off of each channel, and then drive a subwoofer in bridged mode at the same time! All the speakers need to be an 8 ohm load. The sub should be an add-on type, one that has an inherent acoustic roll-off of the highs, or a built-in low-pass filter. A sub that dos not fit into either of these categories can be used by placing an inductor in series with the sub, to help roll-off the highs. This arrangement will have the subwoofer operating with 6 dB more gain than the two full range speakers, but this is usually not a problem, as most people run their subs at a level that is hotter than a flat match to the upper range speakers, and most subs are not as sensitive as the full range cabinets either, so the net result is a decent balance, and certainly not a problem for DJ, reggae or disco use if it is a little on the hot side.

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