Benefits of B&W 704 bi-wire

[rado]

Could some explain me is it worth of bi-wire 704, or this is relate to Nautilus series?

Thanks,

[Luke Skywalker]

On bi-wire he follows a text of the Richard.

by: Richard Vandersteen

What is bi-wiring and what are the advantages?

Bi-wiring uses two separate sets of speaker cables to connect a single pair of loudspeakers to an amplifier. Coupled with a crossover designed specifically for bi-wiring, it offers many of the advantages of bi-amplifying the speakers with two separate amplifiers without the cost and complexity of two amplifiers.

We began experimenting with bi-wiring back in the early '80s, an era when horizontal bi-amplification was considered the ultimate way to drive quality loudspeakers. (Horizontal bi-amplification used one amplifier to drive the low-frequency section of a speaker and a second amplifier to drive the high-frequency section.) We noted that speakers sounded better when bi-amplified by two amplifiers than when driven by a single amplifier. Surprisingly, this superior performance was evident even when the speakers were bi-amplified by two identical amplifiers at a low volume level and the amplifiers were each driven full-range without an electronic crossover. We initially believed that the double power supplies and other components of two amplifiers were responsible for the improvement, however building amplifiers with twice the power supply and doubling-up on other critical components failed to provide the bi-amplification benefit.

So we looked at the speaker wires. With two amplifiers, bi-amplification used two sets of speaker cables so we experimented with doubling-up the speaker wires and with larger wire. Neither duplicated the bi-amplification improvements. Then we considered that in a bi-amplified system, one set of wires carries the low-frequencies and the other set of wires carries the high-frequencies. We modified a speaker's crossovers to accept two sets of cables and present different load characteristics to each set so that the low-frequencies would be carried by one set of wires and the high-frequencies by the other set of wires. Finally we heard the sonic improvements of bi-amplification with a single amplifier.

Additional experiments with a Hall Effect probe revealed that high-current bass frequencies created a measurable field around the wires that expanded and collapsed with the signal. We believe that this dynamic field modulates the smaller signals, especially the very low level treble frequencies. With the high-current signal (Bass) separated from the low-current signal (Treble) this small signal modulation was eliminated as long as the cables were separated by at least an inch or two. (To keep the treble cable out of the field surrounding the bass cable.)

The crossovers in Vandersteen bi-wirable speakers are engineered with completely separate high-pass and low-pass sections. The bass inputs pass low-frequencies to the woofers, but become more and more resistive at higher frequencies. The treble inputs pass high-frequencies to the midrange and tweeter, but become more and more resistive at low-frequencies. The output from the amplifier always takes the path of least resistance so deep bass frequencies go to the bass input (Low impedance at low-frequencies) rather than to the treble inputs (High impedance at low frequencies). For the same reason, treble frequencies go to the treble input (Low impedance at high-frequencies) rather than to the bass inputs (High impedance at high-frequencies). At the actual crossover frequency, the output from the amplifier would be divided equally between the two inputs as they would both have the same impedance at that frequency. Because of the different reflected impedances of the cables, the crossover between the woofer and midrange actually occurs at the wire ends where they connect to the amplifier.

The benefits of bi-wiring are most obvious in the midrange and treble. The low-current signal to the midrange and tweeter drivers does not have to travel on the same wire as the high-current woofer signal. The field fluctuations and signal regeneration of the high-current low-frequencies are prevented from distorting or masking the low-current high-frequencies. The back EMF (Electro-Mechanical Force) from the large woofer cannot affect the small-signal upper frequencies since they do not share the same wires.

The effects of bi-wiring are not subtle. The improvements are large enough that a bi-wire set of moderately priced cable will usually sound better than a single run of more expensive cable.

All the cables in a bi-wire set must be the same. There is often great temptation to use a wire known for good bass response on the woofer inputs and a different wire known for good treble response on the midrange/tweeter inputs. This will cause the different sonic characteristics of the two wires in the middle frequencies to interfere with the proper blending of the woofer and midrange driver through the crossover point. The consistency of the sound will be severely affected as the different sounding woofer and midrange drivers conflict with each other in the frequency range where our ears are most sensitive to sonic anomalies. The disappointing result is a vague image, a lack of transparency through the midrange and lower treble and a loss of detail and clarity.

Some of the benefits of bi-wiring are from the physical separation of the high-current bass and low-current midrange/tweeter wires. So-called bi-wire cables that combine the wires in one sheath do not offer the full advantages of true bi-wiring although they may be an excellent choice for mono-wiring the speakers.

The cables should all be the same length. This is not due to the time that the signal takes to travel through a cable, but rather that two different lengths of the same cable will sound different. If the cables connecting one speaker are a different length than the cables connecting the other speaker, the resulting difference in sound between the two speakers will compromise the imaging and coherence of the system. If different lengths of cable are used for the bass and midrange/tweeter inputs of the speakers, the effects will be similar to those experienced when using two different cables as described above.

Since short runs of speaker cable sound better than long runs, consider placing your electronics between the speakers rather than off to one side. If for convenience or aesthetic considerations, the electronics must be located a considerable distance from the speakers, it is usually preferable to place the amplifier between the speakers and use long interconnect cables and short speaker wire.
:T

[Kobus]

I've done it and might do it again BUT, do not be to unhappy if you can not hear the difference.

Kobus

[miner]

I have bi-wired my N804s and I did hear a sonic improvement. Even more when I added the RB-1092 (vs RB-1070).

[Russ L]

Could some explain me is it worth of bi-wire 704, or this is relate to Nautilus series?

Thanks,

An alternative to Biwiring is to run the cable to the upper set of inputs and run a short bit of cable down to the lower inputs instead of using the metal bridge. Seems to give a better connection and sound...clearer and a lower noise floor to my ears.
Heres what B&W have to say about biwiring their speakers...

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 equalises 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.

Best regards, Russ

[pbarach]

OK, so (1) Vandersteen says you should use the same type and length of cable for each of the 4 connections, (2) B&W says you could benefit from choosing different cables for bass and treble frequencies, and (3) a lot of people have said that bi-wiring helps/doesn't help. There are various detailed and sometimes even mathematical justifications offered for both points of view.

You can't get a reliably noticeable improvement by using bi-wiring; if you could, then nearly everyone would agree. So I've concluded that either the apparent difference is an illusion, or it is audible only on certain system configurations, or it is such a subtle change that it's not particularly important compared with other factors that can make a bigger difference (e.g., using a good source and amp, speaker placement, room treatments).

[Alaric]

Would a lower powered amp benefit more from bi-wiring? That would seem to allow the woofer (and mid in my case) to draw a heavier current load as required-or am I all wet so far as the crossover's function is concerned?
My integrated is fairly low power , and my speakers are pretty efficient. My mid bass and bass drivers are on the bottom post and the top speaker posts only feed my tweets. I got a noticeable improvement in bass -contrary to some of the opinions I've seen. :scratchhead:

[Kal Rubinson]

Would a lower powered amp benefit more from bi-wiring?

Not likely. Of course, it won't benefit less. :

[Russ L]

You can't get a reliably noticeable improvement by using bi-wiring; if you could, then nearly everyone would agree.

Its not reliable across everyone because like most products and tweaks in in Home Audio when people hear them they interpret them in a highly subjective way. As always whats important is what you hear and like. Personally I can hear a small difference between biwiring and not. I can hear a reduction in noise when polysorbothane isolation feet are used under my CD player. I like the sound of B&W speakers. Others don't agree with me. Regards, Russ

[baztroy]

Could some explain me is it worth of bi-wire 704, or this is relate to Nautilus series?

Thanks,

Hello - I have bi-wired my 704's in my two channel set up. They are connected to a Rotel RC 1090 / RB 1090 pre and power amp with both Rotel RCD 1072 and Roksan Xerxes Turntable input sources. I play everything from classical music to heavy metal and so hopefully my comments will cover your listening tastes.

As noted in one of the other postings, the major difference was in the midrange, and in my set up was quite noticeable. The general handling of the midrange improved, solo female voices / violins became sweeter and better resolved, acoustic guitar strings sounded more precise and tonally pure, and the overall soundstage and stereo imaging widened and improved. I didn't notice much change in the bass, other than the general improvement in soundstage.

I tried the bi-wiring after only a few hours of use and then did a "back to back" roughly 4 months later. The differences were still present.

In the early 1990's I had a Musical Fidelity based system and chose to Bi-amp my speakers (Pro-Ac's - all good British stuff). This had a similar magnitude of effect but at greater cost.

I believe the rules of this forum prevent me from mentioning cable brands etc, but suffice to say that I purchased them from my local dealer (i.e I have no personal reason to promote extra cable sales).

I hope that this helps. Graham

[Alaric]

I've noticed most people seem to find the benefits of bi-wiring in the upper-mids and highs. I found my Paradigms opened up , and sharpened , the low end. When I tried unplugging the bottom posts , all I had left was the tweeters! Seems to be the opposite of most folks' experience. Anyone else had a similar experience? Or have I royally screwed up by mentioning the "P" word in the B&W forum?

[bigburner]

I didn't notice any difference from bi-wiring but seeing I went to the trouble of putting it in I have left it there. Perhaps the extra length of copper needs a few hundred hours to break in, or perhaps not.

Nigel.