From the FAQ section of the B&W website...
Bi-wiring and bi-amping
Most B&W speakers are provided with two pairs of speaker terminals; this allows you to either bi-wire or bi-amplify them. The aim of both these techniques is not to simply get the customer to spend more on cables and electronics (although no manufacturer objects to this spin off) but to improve the resolution of the speakers.
A multi-way speaker contains a crossover network that not only divides the incoming signal into different frequency ranges, appropriate to the working range of each drive unit, but also equalizes each driver’s response to be flat (raw driver responses are usually anything but flat).
There are two different basic types of crossover – series and parallel. Series crossovers have each filter section wired in series between the positive and negative input terminals. It is impossible to treat each filter section individually – each interacts with the others – and such crossovers are not suitable for bi-wiring or bi-amping. But by far the most common type is parallel. Here, each driver has its own filter wired between it and the input terminals. If there is only one pair of input terminals, the inputs to all the filters are connected in parallel to that one pair of terminals. If, however, you have more than one pair of terminals, you can completely separate the inputs to each filter. Why on earth would you want to do such a thing?
In the case of bi-wiring, the answer lies in the cable connecting the speaker to the amplifier and the fact that the amplifier is a voltage source but the speaker is a current driven device (force on voice coil = magnet flux density x length of conductor in the magnet gap x current).
Firstly, all cables are a compromise. Some types of construction work better at low frequencies and others at higher frequencies. Providing separate inputs to the speaker allows you to use different cable types, each optimised for the frequency range of use.
Secondly, consider that the cable has an impedance that causes a voltage drop along its length. Now consider the current flowing along the cable. Assume for the argument that the amplifier delivers a perfect voltage waveform to the cable and the cable itself adds no distortion. However, each driver has a non-linear impedance (for example, the inductance of the voice coil alters depending on its position in the magnet gap) that causes the current to be non-linear. This non-linear current through the impedance of the cable causes the voltage drop along the cable to be non-linear and thus the voltage across the speaker terminals is also non-linear, even though it is linear at the amplifier end.
If we were just concerned with one driver, things would not be too bad. But that non-linear voltage at the speaker terminals may contain harmonics within the frequency range of one of the other drivers and that driver will reproduce them, albeit at low level. If, however, you separate the inputs to each driver filter, each driver’s distortion is kept to itself and the total system distortion goes down. We are talking small changes here, but the resolution of some modern drivers is now so good that small improvements like this are readily detectable by keen listeners.
Many people ask us whether the load on the amplifier is different if you bi-wire. It is not. As far as the amplifier is concerned, it matters not one jot whether you parallel the inputs to the filter sections at the speaker end or the amplifier end of the speaker cable.
Bi-amping takes advantage of all this and adds some benefits of its own. Like with cable, you can choose different amplifiers that excel in different frequency bands. You may, for example, be keen on valve (tube) amplifiers. But even the most die-hard of aficionados would be hard pushed to claim that they are any good at keeping good control of the bass. Bi-amping enables you to combine a valve amplifier for mid and high frequencies with the control of a solid-state device at low frequencies.
Unlike bi-wiring, the load to each amplifier is different from that using a single amplifier full range. The voltage demands on each amplifier remain the same (each is still fed a full-range input and gives a full-range output), but the current demands are reduced. This of itself can improve the amplifier’s ability to deliver the signal to the speaker.
Be careful when bi-amping that the gain and polarity of each amplifier are the same, otherwise you will compromise the frequency response of the system.
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Bowers & Wilkins home audio and home cinema speakers 2003
My Bi-amp System Twist
I have B&W N803 mains. These have a bass crossover frequency of 350Hz, which separates the bi-wire inputs (and independent crossover boards for the (2) base drivers circuit, and the midrange/tweeter circuit). I have chosen to bi-amp these speakers for the reasons stated above, and because it allows me to incorporate a sound processor (parametric EQ) for the Low Frequency circuits (left main and right main) only.
The characteristics of my listening room make the use of a two channel parametric EQ for frequencies below 200 Hz very desirable. This interaction between the speaker and the room is not unusual, especially at low frequencies. While it is not severe, it is still significant enough to produce a very clear sonic unevenness. Bi-amping allows the use of signal processing for the LF signals only, and leaves the HF signals in a "bypass" mode, which I believe is most preferable in my circumstance.
The left main and right main pre-outs (Outlaw 950) are each split using a common Y splitter. The HF signal for each channel goes directly to the amplifier (Outlaw 770), without any signal processing. Yes this signal does include the total bandwidth of musical information, but only the HF information is ultimately demanded by the crossover/driver circuit, so the amplifier only pushes the HF signal through the speaker wire. I may someday use a tube amp for the HF signal path, but the Outlaw 770 is sufficient for now.
The LF signals first pass through a "Symetrix 552E" two channel parametric equalizer before going to the amplifier (Outlaw 770). Before I actually heard it, I was somewhat skeptical at the benefits this type of tweaking could have. I found that in my circumstance it has enormous benefits. Even my wife, who hates to admit anything I do regarding "that stuff" has any reasonable cost/benefit ratio, gave it her unconditional nod. As the volume goes up, the clarity and smoothness of the bass information is so balanced and lush it begs for more volume still. With the output gain/loss of the EQ carefully adjusted, and the phasing of the connections checked, this single enhancement has made very good speakers sound much closer to the B&W N800's ($17,000/pair) I will probably never own, but still covet.
I have two Velodyne HGS 15's, one for each main speaker to fill out the bottom end. The speaker configuration for the mains at the Outlaw 950 is set to large. The stereo signal leaving the Outlaw 950 pre-outs are actually split again before going to the parametric EQ so that it can be routed to each sub through an Outlaw ICBM (in stereo mode). The ICBM also allows LFE information to be adjusted (balanced) with stereo information from the main left and right channels. This is a very valuable feature of the ICBM. In this configuration, the ICBM outputs for the left and right mains are not used. The parametric EQ performs all necessary processing of the LF stereo signal for the front mains, and it is passed directly to the amplifier.
In my case the ICBM's setting for the front main channel crossover is 40Hz. While this works for me, a continuously variable crossover setting would be even better for most systems, as it could be dovetailed precisely with the speaker LF roll-off frequency.
For EQ, one could also use a 31 Band, 1/3 Octave Graphic EQ, such as the "ART 355", as this has unbalanced (RCA) in/outputs. Because the Symetrix EQ has balanced XLR in/output, I left out the fact that the signal is converted to a balanced signal, and then back again after the EQ by a "Rane BB 44X Balance Buddy." Believe me, if I told you that before now, even I would have gotten lost.
Rane also make very good EQ's, and the introduction of new digital EQ's to the studio environment is apparently growing in acceptance. I will defer to the true audio genius' of the Outlaw community provide any serious equipment recommendations. This remains a hobby, although a passionate one for me. There are professionals in here.
I look forward to any comments or suggestions... no matter how critical. This forum has been an incredible learning experience for me.
[This message has been edited by AGAssarsson (edited June 11, 2003).]
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