Bi-amping: a remarkable improvement

Lovers of big installations, those who want to get the very most out of their system's performance and capabilities (especially the keenest music lovers), will at some point have heard of "bi-amping". At the very least, many of us have wondered at some point why our loudspeakers come with dual binding posts. The first step, sometimes unnecessary, is bi-wiring; but in fact the real goal of it all is bi-amping.

Broadly speaking, it involves improving performance and sound by using two (hence the prefix "bi") amplifiers per loudspeaker. It means moving the division of the audio signal according to the number of ways in a loudspeaker to before the amplification stage, doubling the number of power amplifiers and driving each way of the loudspeaker directly.

Below we will look at the advantages and drawbacks of this configuration method, which many people aspire to but which, and this does matter, is considerably more expensive. It also requires a certain amount of technical knowledge or the advice of an expert to prepare the whole configuration ahead of the amplification stage. It is not an easy setup for the great majority of enthusiasts, but it is a good "pastime" for anyone who wants to experiment with their system.

In its favor is the remarkable and noticeable improvement to the system, capable, for example, of making a pair of monitor-type loudspeakers sound as if they also had a subwoofer attached.

When we ask about the advantages of bi-amping, we are offered various arguments. But bi-amping is more than just using two amplifiers. In fact, there are variants that deliver one result or another.

The most common demand is the need to obtain twice the current delivery, which means using two identical amplifiers. This type of bi-amping is known as vertical (instead of "delivering" 50 W, each loudspeaker gets 100 W). Another, more complete method is horizontal bi-amping. This still uses double amplification, but with different power ratings: more power for the more demanding drivers (woofers or midrange drivers) and less for the tweeters. In this case, the aim is to match the demands of each driver rather than to seek the best synergy per loudspeaker.



VERTICAL BI-AMPING



In this case, both channels of a stereo amplifier are used for a single loudspeaker. It follows, therefore, that another identical amplifier will be needed for the other channel (one for the right channel and one for the left). Obviously, we are talking about a stereo configuration.

Each amplifier receives the analog signal for its corresponding channel, but each channel of the amplifier processes part of this signal. Before amplification, the signal is divided into "bass" and "treble", and each is amplified independently.

The aim of vertical bi-amping is to reduce the current demand on the amplifier. Bass and midrange drivers usually demand much more current, since they need to move more air. Tweeters or high-frequency drivers, on the other hand, are easier to drive.

In fact, using one amplifier channel for a bass-midrange driver (instead of having one channel handle both) makes it possible to get the most out of the amplifier's transformer. The stress on the driving electronics is reduced and the driver receives all the current it may demand.

This type of bi-amping should not be confused with mono amplification by means of "bridging". Bridged amplification is the result of adding together the power of the two channels to obtain an output with greater current delivery. A 100 W amplifier can deliver double the power or more when in bridged mode, although serious problems arise with regard to impedance. When bridged, the amplifier sees the load impedance as half its real value (in other words, if we connect an 8-ohm loudspeaker, it will "see" it as a 4-ohm one), which also means suffering the problems of a loudspeaker with the new impedance value. Since this is not a nominal value, with low-frequency sounds the impedance may drop very close to 0 ohms (a short circuit), or in other situations the distortion may be more than evident.

Vertical bi-amping often offers better results and greater safety than bridged amplification.



HORIZONTAL BI-AMPING



In this case, twice as many amplifiers are still used, but instead of dedicating one amplifier (with its two channels) to each loudspeaker, the two channels of one amplifier are used to drive the low frequencies of both loudspeakers, and the other amp handles the treble. In this case it is possible to use two different amplifiers. Another advantage is the possibility of matching the needs of each driver to the capabilities of the amplifiers. For example, a power amplifier with greater current delivery for the woofers and a less powerful one for the tweeters.

Passive crossover developed by B&W for its recent 800 Series. Its simplicity hides enviable performance.a>

CROSSOVERS



When we talk about bi-amping, we are also talking about two signals: high and low frequencies. A conventional loudspeaker is usually designed as a two-way system. This means that the signal received from the amplifier is split in two before reaching the drivers, which are divided into two independent circuits (each circuit is a way). Normally the tweeters represent one way (usually a single driver), while the other way, the bass, drives one or more mid-bass drivers. This division is necessary because there are very few drivers capable of reproducing the entire audible frequency range from 20 to 20,000 Hz.

There is therefore a circuit that splits the signal in two, and it is called a crossover. In this case, they are also of passive design (meaning they use resistors, capacitors and inductors, with no amplification or power elements).

There is another type of filter (English speakers use the dedicated word crossover): active ones. These do include electronic elements that "modify" the signal as well as splitting it. For example, amplifiers, filters with variable gain or Q, etc., so they include transistors, for example.

The advantage of passive crossovers is that, in addition to being relatively cheaper, they do not need to be connected to the mains; on the other hand, they have significant losses. In fact, a considerable part of the energy received from an amplifier is lost after passing through a passive crossover.

The advantage of active crossovers is that they are much more sophisticated, allowing the sound to be divided while taking into account various factors, such as impedance or the type of driver used. They are more complicated to adjust and use.

The vast majority of loudspeakers are fitted with a passive crossover. And when it comes to bi-amping (whether of horizontal or vertical design), either active or passive crossovers are therefore used.

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PASSIVE BI-AMPING



In this case, the loudspeaker's own passive crossover is used. The two amplifier channels that will drive a single loudspeaker receive and amplify the same signal (that is, the entire audible spectrum). The two amplified signals are connected to the respective binding posts of the loudspeaker (the loudspeaker must therefore have dual binding posts, and you must make sure you have removed the jumper -usually a metal strap- that links each pair of terminals). The loudspeaker's passive crossover is then responsible for filtering out the frequencies that each way or driver cannot reproduce.

Although this solution is cheaper than the next one, sometimes the results it delivers are not very different from what could be achieved with a more powerful amplifier in a traditional configuration. I recommend testing this type of configuration first to avoid wasting money. Of course, it can be useful as a step prior to an active configuration.



ACTIVE BI-AMPING



An active crossover is also known as an electronic crossover. It must be used before the amplification stage and after the preamplifier. Its purpose is to split the line-level signal before it is amplified, so that only the signal each way of the loudspeaker needs is amplified, bypassing the loudspeaker's passive crossover. We can use any loudspeaker that is prepared for bi-wiring or bi-amping (that is, with a double set of binding posts), while trying to take its built-in passive crossover out of the circuit.

By connecting the amplifier output directly to the drivers, energy efficiency is higher (we have already mentioned that the passive crossover "consumes" quite a lot of energy). In fact, one of the first impressions is that less power (or current delivery) is needed to obtain the same sound pressure level (an efficiency gain of 4 or 6 dB is usually achieved).

Other advantages of active bi-amping are that the filtering process does not interact with the loudspeaker's own characteristics (above all its impedance), that the amplifier's damping factor is maintained, and that the system's phase accuracy is more coherent.

But passive bi-amping also has its advantages. For example, there are very high-end loudspeakers whose passive crossover design can be described as excellent, and doing without it would perhaps be foolish; or there are those who advocate passive crossovers in order to avoid any modification to just one part of the signal (active crossovers can amplify either of the two ways independently, or use filter slopes that make certain frequencies predominate during playback); and so on.

In fact, an active crossover is so sophisticated that in inexperienced hands it can completely degrade the sound of a system.

Likewise, the quality of all the components involved in an active bi-amped installation will follow the golden rule of quality. If the design and construction of the active crossover are worse than any other component in the chain, the whole system will only inherit the sonic performance of the lowest-quality component, in this case the active crossover.



USING A PREAMPLIFIER



Clearly, bi-amping will require the use of a preamp-processor. Apart from processing analog and digital, stereo and multichannel signals, its main function will be to maintain the levels between channels, especially bearing in mind that we will now have to deal with more than 2 amplifiers (in a 2.0 configuration) or more than 6 in a 5.1 multichannel installation. In the case of an active crossover, it will be placed right after the preamplifier.



OTHER CONSIDERATIONS



To adjust the active crossover, you will need to know the characteristics of the loudspeakers and, in particular, of the drivers used. One of the most important values is the crossover frequency, which on paper determines up to which frequency one way operates and from which frequency the other takes over. We will also take into account the filter slope and the gain and/or sensitivity values of each driver.

Some manufacturers offer their users an "all-in-one" solution for converting their passive system into an active one. They use active crossovers on electronic cards which, fitted into their driving electronics, make the user's task easier. In fact, all that is needed is to insert the card and make the new connections. Obviously, these cases are only feasible when all the matching components are used, that is: loudspeakers, power amplifiers and crossovers.

Some specialist dealers have technicians on their staff with sufficient knowledge to configure a system of this kind.