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Biamplification basics - an article

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Biamplification?

A combined effort by Audio Fan and John C.-Aussie with pirated comments from other sources. Note that the thoughts here are not claimed as original but are presented to assist the unwary and are not claimed to be the last word on the subject. (And the fact there is no last word is what makes this hobby so engrossing).

In general terms the purpose of biamplification is to improve sound by using two, (hence the bi-) instead of one stereo amplifier, to reproduce two channel sound. It may involve replacing the crossover components in speaker systems with other components. Basically it involves splitting the audio spectrum into two parts by means of a crossover and then use two separate amplifiers to process each part, so in a stereo system this involves two stereo amplifiers. Many advocate biamplification should eliminate passive crossovers in speaker systems by using active crossovers to improve the sound but another school of thought favors using passive crossovers.
Triamping is the use of three amplifier channels for each audio channel. Triamping follows the same concepts as biamping, so for simplicity only stereo biamping is discussed hereafter.


Arguments in favor of biamping are -

1. To double the current capability by using a second identical amp (vertical biamping).

2. To provide more power to the system by using a high-powered amp for the woofers and lower-powered amp for the highs (horizontal biamping).

3. To match the characteristics of the amplifiers to the drivers. For example, an amp with a smooth treble could be mated to an amp with a powerful bottom end. highs (horizontal biamping)

4. The use of an active crossover in biamping removes the limitations of passive components.
Vertical biamping

In vertical biamping, both channels of a stereo amp are used on one speaker. Vertical bi-amping thus needs two completely identical stereo amps, one for the right speaker, one for the left. Each amplifier has the same signal (left or right output) but one of the channels is fed the high range while the other is fed the low range.

The purpose of vertical biamping is to reduce the current demand on an amp. Because the main current draw is in the low frequencies, each power amp in a vertical setup only drives one woofer. The channel driving the woofer gets to hog most of the current provided by the amp's power supply, since the channel driving the high frequencies doesn't need as much current. This is particularly advantageous if the amp uses a common power supply for both channels. Vertical biamping in this manner requires identical amps for each channel to maintain balance integrity.


Horizontal biamping

In horizontal biamping, one stereo amp is connected to the low-frequency speaker posts on both the left and right channel, and a second stereo amp is connected to the high-frequency posts of the left and right speakers. This arrangement is used for biamping with dissimilar amps.

The purpose of this arrangement can be to maximize the virtues of different amplifiers. If amp X has superb bass and amp Y is a bit soft on bass but has a glorious mid and top then amp X is used for low end reproduction and amp Y for the upper end of the spectrum.


Biamping vs. bridging (monoblocking)

Bridging (also called monobridging or monoblocking) is the summing of two channels of an amp to give one higher-powered channel. An amp normally rated at 100W might deliver 300W to 400W when bridged. Because of the summing however, the load on the amp is seen as half of its normal value. In other words, an 8-ohm speaker becomes a 4-ohm speaker load, and a 4-ohm speaker becomes a 2-ohm speaker load. Speaker impedance ratings are nominal only. Actual impedance may dip to a much lower value through part of its range. When an amp's current load has been doubled due to bridging, it can often fail to provide the required amount of current into the load. Sonic effects include harshness in the midrange and highs, and thin bass. In almost all situations therefore, biamping with similar amps will result in better sound quality than bridging. Bridging is best left to professional sound-reinforcement applications, where sound quality is secondary.


Biamping with a subwoofer

A powered subwoofer can be used for biamping. A powered sub is a form of biamping because two power amps are connected to each stereo channel. There are two basic configurations to achieve this.

1. An active crossover is used to separate the frequencies that are fed to the main power amp and the sub amp. The main amp and speakers are fed a filtered signal and do not have to reproduce bass frequencies.

2. The subwoofer and main speakers are run in parallel. The sub is used to bolster the main speakers' bass response, but the main amp and speakers are run full range.

There are proponents of each of the above methods. One may benefit a certain system more than the other. Either configuration can be implemented using a line-level or high-level (speaker-level) signal, depending on the specific equipment used.


What is the Purpose of a Passive or Active Crossover in a Speaker System?

Crossovers are circuits used to split the audio spectrum into various sections. Few loudspeaker drivers are capable of reproducing the whole range of frequencies from 20 Hz to 20,000 Hz so crossovers are used to break up this frequency range to suit a particular speaker driver's characteristics.
e.g. a "2-way" speaker contains two drivers. One might handle all frequencies above 1400 Hz (the tweeter) and the other all frequencies below 1400 Hz (the woofer). Inside the speaker cabinet is the crossover, which does the job of splitting the audio and diverting the correct frequencies to the appropriate driver. A "3-way" system contains three drivers and thus requires two crossovers in each cabinet.

Most commonly, these crossovers are passive. A passive crossover involves resistors, inductors and capacitors with no amplifying or powered electronics.

A few loudspeaker systems have active crossovers (and sometimes amplifiers) inside the cabinet. An active crossover involves employing some sort of control of the filter or gain, whether it be through changing the slope of the filter, Q, frequency point, etc., or adding gain to the signal, especially if the filter is lossy. An active circuit contains electronics e.g. tubes or transistors, whereas a passive circuit contains only passive components i.e. components with no gain such as resistors, caps and inductors.

Passive circuits are always lossy, active circuits have gain. Active circuits are capable of more sophisticated and tunable frequency responses, partly because filtering components are inserted into the feedback network of an amplification stage, and partly because active circuits also provide buffering and hence remove or reduce impedance matching problems between multiple filtering stages. It is also easier to eliminate phasing problems in an active circuit. Most experts agree that active operation is better, given proper implementation and high quality parts, but is more expensive to implement.
Further interesting reading on crossovers may be found at -
http://www.linkwitzlab.com/crossovers.htm


Passive biamping

In passive biamping, two amps feed one channel of a pair of stereo loudspeakers. The speakers used must have separate terminals for the high and low frequencies. Any speaker system that allows biwiring can be biamplified. In passive biamping, the speaker's internal crossover is used to route the highs to the tweeter and the lows to the woofer, just as they are when one amp is used.
Caution! When contemplating passive biamping, one should always consider whether a single higher quality amp would result in better sound than two amps. Note also that not all amp combinations will work together to provide coherent sound. Experimentation is necessary to avoid costly mistakes.


Active biamping using electronic crossovers

Active biamping requires the use of a separate line-level crossover between the preamp and the power amps. The active crossover (sometimes called an electronic crossover) divides the signal into high and low frequencies and routes each to a separate power amp. The speaker's passive crossover must be bypassed. The amps in the system are thus wired directly to the woofer and tweeter terminals.


Active or Passive biamping?

An excellent article on this topic can be found at http://www.bryston.ca/

Another detailed argument in favor of active crossover biamping is given by Rod Elliott at http://www.sound.au.com/bi-amp.htm

Direct connection between the amp and the speaker element provides important sonic benefits. Resistors radically decrease an amp's damping factor. Capacitors and inductors slow the signal and cause phase changes. Once freed of these intervening reactive components, the amp's grip on the speaker improves greatly, and it sees a benign resistive load instead of a reactive impedance that varies with frequency. Also, since each element of a passive crossover wastes power, a net gain of 4 to 6 dB of efficiency can be achieved in active operation.


Summary of arguments in favor of active crossovers -

1. An active filter is easier to tune to a given crossover point since it does not interact with the speaker element's impedance.

2. High impedance, lower voltage elements are smaller and have reduced sonic effects compared to higher impedance higher voltage elements.

3. The amplifier's damping factor is preserved.

4. An active crossover maintains the system's phase accuracy.

5. The amp sees a primarily resistive load for superior power delivery.

Active crossovers provide better control of the speaker element. Drivers sound more dynamic when driven directly by an amplifier.


Summary of arguments in favor of passive crossovers -

1. Quality speakers are designed with a crossover network matched to the particular characteristics of the drivers. Substituting an active crossover defeats the speaker designer's efforts to truly match the drivers, cabinet, and crossover together.

2. Anything that is added to the signal path alters it in some way. Passive components do too (a cable is a passive component, for example), but they may be less likely to add coloration than active components.

3. An active crossover performs a sophisticated function, roughly similar in complexity to a phono stage. An active crossover has the potential to degrade the sound of a system if its quality does not match the rest of the components.

Ultimately arguments for or against either configuration are academic - the proof is in the listening.

As with all links in the chain, the quality of the active crossover influences the sonic result. A poorly designed active crossover might not yield better results than the internal high quality passive crossovers in a well designed loudspeaker with components carefully matching the drivers.


Misconceptions

(i)Speakers which allow for biwiring give better sound if two amplifiers are used - not necessarily so, it depends on many factors such as cabling, passive component quality etc.

(ii)I have a spare amp so am thinking of bi-amping - this may reflect a misunderstanding of the processes involved. i.e. if the amps are a poor match, the net result could be negative.

(iii)Bi-amping is a simple way of improving sound.
Quite the opposite. For the most satisfying sound, biamplification may require -

* a high-quality active crossover to achieve any significant improvement

* twice as many expensive interconnects and speaker cables

* two expensive and compatible amplifiers

* excellent drivers & a good cabinet to achieve best results - biamplification, by itself, does not guarantee that every speaker system will give dramatically improved bass

* if biamplifying to achieve a better bottom end with a sub, that sub needs careful matching the with the rest of the system - not always easy.


The bottom line: Bi-amping is neither simple nor inexpensive IF the potential benefits are to be realized, and there is no one simple answer to every situation.


A number of active crossovers are available. The Bryston 10B has a high reputation and a review of this can be found at http://www.cybertheater.com/HW_Reviews/Bryston_10B/bryston_10b.html

Active crossovers are also manufactured by FM Acoustics, Krell, Linn, and Pass Labs

Marchand also make a variety of units. See http://www.marchandelec.com/

Some manufacturers (Linn, Naim, Vandersteen, PMC) provide an upgrade path that replaces a passive crossover with active.

Some manufacturers (e.g. Wavelength) sell speakers designed around an active crossover, which require the use of more than one power amp.
Some manufacturers (eg Paradigm, PMC) sell powered speakers that include active crossovers and amplifiers. They only need a line level signal to the speaker.

The Dahlquist DQ-LP1 was designed for sub woofer application and used an electronic filter for the bass, tunable from 40 to 400 Hz but a simple passive circuit for the upper frequencies.

External sub crossovers are also supplied by Energy, Mirage and Paradigm, among others.



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Topic - Biamplification basics - an article - John C.-Aussie 15:01:25 04/30/00 (6)


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