600 Series Technical paper


This new Series replaces the 600 Series 4, model for model. However, the individual model names carry the suffix S2 (eg 686 S2), as this is only the second time we have used those particular model numbers. The new Series incorporates both technology enhancements and a new visual treatment; this paper will describe both. Some changes are common across several models and these will be discussed separately from those that are particular to individual models.

Bass/midrange drivers (2-way models): •  New dust cap design Bass drivers (3-way models): •  New colour-coded aluminium cone construction •  New dust cap design Technology carried over from the previous Series Tweeters: •  Neodymium-Iron-Boron (NdFeB) magnets •  Nautilus™ tube loading

Improvements common across the Series Tweeters (all models): •  Damped dual-layer (double) dome •  Mechanical decoupling from the cabinet •  Protective grille mesh •  New colour-coded trim •  Easier servicing

Bass/Midrange drivers (2-way models) •  Woven Kevlar® cones Midrange drivers (3-way models) •  Woven Kevlar cones with FST™ technology

Crossovers •  Minimalist topology •  Bi-wiring Cabinets •  Flowport™ venting

Tweeter response

The tweeter is common to all models in the Series and features the same dual-layer or double dome as the CM10. Instead of the normal single layer of aluminium found in previous 600 Series loudspeakers, this diaphragm has a sandwich construction. At the back of the visible dome, which is thinner than the standard aluminium dome, is attached a second aluminium layer that replicates the main dome’s profile with the centre cut out. This unique approach allows the second layer to form a circular girder section that raises the first break-up frequency from 30kHz to 38kHz, making the diaphragm’s behaviour in the all-important audible band below 20kHz purer and more piston-like. This double dome construction may be compared to the carbon-braced design used on the PM1 in that both offer a greater level of performance over the HF unit on axis No crossover 685 685 S2

Figure 1 - Comparative frequency response, old (red) and new (green) tweeter, showing the raising in frequency of the first breakup resonance

Figure 2 – Exploded view of tweeter assembly

previous single-layer aluminium dome construction, but without the significantly higher expense of our diamond dome. They may be regarded as an in-between design. The dual-layer approach, however, does not suffer the increase in moving mass afforded by the carbon bracing and gives appropriately higher unit sensitivity.

Tweeter decoupling

The performance is further improved by the addition of mechanical decoupling from the cabinet. Decoupling has featured in several of our more expensive models and the benefits of reducing vibration transmission between drive units and cabinets has been discussed in previous publications. It has long been featured in our ‘tweeter-on-top’ designs, but this is the first time we have applied it to a baffle-mounted tweeter. In common with previous designs, it is a soft gel material in the mounting arrangement of the unit that affords the decoupling. In this case, the gel encapsulates the magnet assembly rather like a tyre and allows the unit to float in its mounting. This mounting in turn is screwed rigidly to the cabinet, allowing the unit to be readily removed for servicing; a far easier 685 685 S2

Figure 3 – Accelerometer measurement of baffle adjacent to tweeter, showing reduced excitation for new (red) compared to old (blue)

Figure 4 – Exploded view of tweeter assembly

process than having to remove the driver immediately below the tweeter and release the tweeter from behind, as was required in the previous Series. The screw fixing is hidden by a decorative front plate that is a simple friction push fit onto the mounting plate. It is fronted by a brushed aluminium layer that is anodised a different shade according to the cabinet finish and carries a diamond cut Bowers & Wilkins logo. This front plate surrounds a protective mesh that precludes accidental or malicious instore damage to the radiating dome and has minimal effect on the tweeter output. It is similar to the mesh used on the CM10, for which the hole pattern was optimised.

Bass/midrange driver

This type of driver is found in all the 2-way systems and the general construction is the same whether it’s a 5” or 6½” unit. Carried over from the previous Series is the familiar yellow colour of our woven Kevlar cones but, at the centre, the dust cap is quite different. Out goes the bullet-shaped plastic dust cap and in comes what is more accurately described as a dust plug. It was first introduced in the PM1 and its cross-section shows it to be shaped rather like a mushroom. The cap of the mushroom is the visible part, but the stalk extends into the voice coil former and is a tight fit. Each part has a different purpose, but both rely on the material, which is a lossy, closed-cell foam polymer. The target is to achieve a smoother response at the upper frequencies of the driver’s response. This is particularly important when using, as we do, minimalist crossover networks with low roll-off rates. The response of each driver must remain smooth well above the nominal crossover frequency; otherwise the remnants of resonance can mask fine detail. Any dust cap forms part of the total radiating diaphragm. If the coil former were to be left completely open, the sensitivity

Figure 5 – expanded impulse responses old (blue) and new (red)

of the driver would fall by approximately 1dB, so the contribution is not insignificant. Traditionally, dust caps have comprised a thin membrane, which may be formed from paper, plastic, metal or whatever. The use of stiffer materials has the advantage of reducing any likelihood of the dust cap inverting under the effects of the changes in pressure inside the cabinet as the cone moves back and forth. Their disadvantage is that such dust caps tend to be fairly ‘live’ and exhibit resonance. Sometimes, highly damped (lossy) plastics have been used for the membrane. They tend to give a smoother response, but are usually so floppy that there is always the danger of inversion, especially if the speaker is driven hard. The foam dust plug avoids any compromise in this respect. The cap portion is lossy and radiates smoothly, but the solid form eliminates any tendency to invert. This is all well and good, but still the majority of the radiation comes from the cone itself and, at upper frequencies, there is always cone break-up to deal with. Woven Kevlar cones, when impregnated with the correct amount of resin and with a coating of highly damped polymer, already

have a well-controlled break-up region. Nevertheless, there are still low levels of resonance in the cone that induce the voice coil former to go out of round. This is where the stalk portion of the plug comes in. By being a tight fit in the former, the plug reinforces its circular shape, which in turn further reduces the levels of resonance in the cone. The result of this fairly simple modification is a smoother driver response and less low-level coloration just above the crossover to the tweeter. This improvement is most clearly shown by examining the impulse responses of the old and new drivers. The previous model shows a distinct ringing in the tail of the response, indicative of a high frequency resonance. The rapid fluctuations associated with the resonance are completely absent from the new driver.

Midrange driver

In both the 3-way models, 683 S2 and HTM61 S2, the bass and midrange functions are split between different drivers. For midrange, as before, we use an FST driver, well proven to offer the highest levels of clarity in that most critical region and first featured in our high-end 800 Series. The 683 S2 carries the same 6” size driver as the outgoing model, but the centre speaker features a smaller unit that will be discussed later in this paper when we look at each individual model in turn.

Bass driver

The dedicated bass drivers in the 3-way models still feature an aluminium cone, but there are two distinct differences compared to the outgoing models. From the front, one will only discern the different dust cap. It is smaller than before and concave rather than convex in profile. This pushes the first break-up of the dust cap to higher frequencies, well above the bass to midrange crossover frequency. Not immediately obvious, and of greater importance, is what goes on at the back of the cone. Aluminium is a stiff material and stiffness is favoured for drivers that

Figure 4 – comparative responses 683 (red) and 683 S2 (cyan) bass drivers, showing the raising in frequency of the first break-up resonance

are dedicated to bass frequencies only. Stiff materials, however, tend to have quite severe resonances. Yes, these resonances start at higher frequencies than with less stiff materials, but they tend to be more severe (technically described as having a higher Q). These resonances are controlled by adding some damping and, in previous models, the damping was provided by a second layer of paper/Kevlar fibre behind the aluminium and the glue that bound them together. With the new cones, a supplementary ring of aluminium is glued to the periphery

at the rear in a similar manner to the dual-layer tweeter construction. Once more, the second layer provides girder-like strengthening to the cone, significantly raising the frequency of the first resonance, and the intermediate glue layer provides the necessary damping.


In common with all recent designs, these speakers use a minimalist approach to filter topology. The guiding principle is that no electronic component, however good, can inherently improve the quality of the signal passing through it. Yes, the overall sound may improve if the filters serve to give an overall flatter response, but increasing complexity tends to remove some of the life in the sound. Indeed, with speakers, it has been found that a system with fewer crossover components and a slightly undulating response can sound better than one that uses a more complex crossover to

achieve a near ruler-flat response. Of course, fewer components mean lower roll-off rates either side of the crossover frequency, so it is important to use drivers that have a smooth and controlled response well outside their nominal working range; hence the driver attributes described above.

Model by model - 686

This 5” 2-way is the smallest speaker in the range and, apart from the inclusion of the new driver designs, the most obvious difference is the migration of Flowport™ from the rear to the front of the speaker, with a significant change in dimensions. Having the port on the front has forced an increase in height, which in turn has enabled a reduction in cabinet depth. This is important if the speaker is to be hung on a wall surface and the familiar hanging bracket is retained at the rear. A small decrease in the cabinet width further enhances the visual aspect.


686 S2


This, the larger of the two bookshelf systems, features a 6½” bass/midrange driver, which gives improved bass extension and higher sensitivity than the smaller 686. It is 5mm narrower and 5mm taller than the old model. Small changes, maybe, but the overall effect is to give an improved aspect.


685 S2


The new 684 features 5” drivers compared to the 6½” drivers of the outgoing model. This means a significantly narrower and more attractive speaker. The smaller drivers, with their closer centres, have allowed the new system to operate well as a 2-way, compared to the 2½-way of the previous design. There, the larger drivers would beam unacceptably in vertical plane if one of them were not rolled off at a relatively low frequency.


684 S2


This model continues as a full 3-way design, based on twin 6½” bass drivers. Like the 685 (which uses the same driver size) it has been possible to reduce the cabinet width slightly. Of course, the bass drivers incorporate the new aluminium cone design and the FST midrange driver is well-proven to deliver exceptional resolution and detail.


683 S2


This, the smaller of the two centre speakers, carries over the twin 5” bass/ midrange with central tweeter concept of the outgoing model. The cabinet height has been reduced in the same way that the 686 S2 width has decreased, but in order to keep the same internal volume without increasing depth, the cabinet width has increased a little. However, both centre speakers retain rear-facing ports in order to minimise cabinet width. The speaker may be considered as a 684 S2 on its side, with the tweeter moved to a position between the bass/midrange drivers to guarantee a symmetrical horizontal dispersion. Typically, it would be included into any 600 Series home theatre system that did not feature the 683 S2 at front left and right.


HTM62 S2


This model has probably seen greater change than any other in the range. The outgoing model, in an effort to minimise baffle area while still offering a full 3-way system, had only a single bass driver and an asymmetrical horizontal dispersion pattern around the upper crossover frequency. The new model takes a similar design philosophy to the CM Centre 2 and has grown considerably as a result. Placing the tweeter directly above the 4” FST midrange driver guarantees a symmetrical dispersion and the extra bass driver matches the power output of the 683 S2, with which this centre speaker should be used in a home theatre installation.


HTM61 S2


The 600 Series from Bowers & Wilkins, in all its iterations, has always set the benchmark for affordable true-hi-fi speakers. Each successive Series has raised the standard for both industrial design and sonic performance. This new 600 Series, with its new driver designs and new visual treatment, yet again offers true improvement in both these categories.


Kevlar is a registered trademark of Dupont. Nautilus, Flowport and FST are trademarks of B&W Group Ltd. Copyright © B&W Group Ltd.