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Wilson Benesch Turntable and ACT Two Pickup Arm

by Keith Howard

Gramophone April 1996

When Compact Disc was launched in 1982/83 some pundits rashly predicted, even at that early juncture, that CD would wipe LP from the face of the earth within 10 years. Three years past the deadline, we now know this to have been a shining example of wishful thinking: LP has survived and even, in recent years, rallied to a small revival.

But sales of vinyl, particularly in the classical music sector, are today but a tiny fraction of what they were even 10 years ago, with obvious ramifications for a once thriving turntable industry. Thorens, Linn Products, Pink Triangle, to pick three obvious examples, all made their names as quality turntable manufacturers, and all have had to diversify into other product areas in order to survive.

In this parlous business context - a dwindling market still served by great names of the past - it might seem arrant folly to establish a new hi-fi company whose first product is a high-end record player. But that's precisely what Andrew Scholey and Craig Milnes did three years ago in forming Sheffield-based Wilson Benesch. A loudspeaker has since swelled the ranks, but the company's first products were the Wilson Benesch Turntable and its original matching pickup arm, the ACT One, since supplemented by the superior ACT Two reviewed here.

To the casual observer the WB Turntable - with such familiar features as belt drive, a heavy two-part platter, felt mat and sprung subchassis - appears much like any other high-end record deck. What sets it apart, and the arm too, is the application of carbon fibre composites to key structural elements - the subchassis of the turntable and the main tube of the pickup arm.

Why carbon fibre? If it were merely a matter of cashing in on the material's hi-tech image, born of its use in racing cars and high-performance aircraft, then it could be dismissed as a cynical marketing exercise. But there are two sound engineering justifications for using carbon fibre composite in structures that are subject to vibration (as a turntable is by its very nature), particularly where a requirement exists to minimize mass, as it does in a pickup arm. Weight for weight a carbon fibre laminate is significantly stiffer than steel or aluminium, which means that any given structure built from it will resonate at a higher frequency. As the material also has, because of the resin matrix which binds the fibres together, higher internal losses than metal alternatives, what resonances there are will additionally be better damped.

Quantifying carbon fibre's advantage requires some care. Standard-grade carbon fibre in its raw, fibrous form has a specific stiffness (tensile modulus divided by density) roughly five times that of steel - a figure commonly bandied about although in practice it means very little. To resist bending and torsional loads requires the fibre to be bonded into a rigid form by means of a resin, usually an epoxy, which adds mass while contributing very little to the tensile performance. Hence the specific stiffness of carbon fibre composite (fibre plus resin) is significantly less than suggested by figures for the raw fibre.

Further complication is added to the calculation if we compare materials on an equal mass basis- which is certainly valid for pickup arms if not so obviously for turntable subchassis. The fact that carbon fibre composite can be deployed, for a given weight, with greater material thicknesses than metals brings large benefits to bending and torsional performance. Side-stepping the physics involved, it calculates out that carbon fibre composite, evaluated on this basis, is about 40x superior to steel and around 5x superior to aluminium - improvements which are very well worth having. With an ultra-high-modulus grade of carbon fibre it is possible to do even better, although raw fibre prices of £300+ per kilogram mean these exotic materials have to be used sparingly if at all.

In the subchassis of the Wilson Benesch Turntable, the superior inherent performance of carbon fibre composite is further enhanced by a method of construction - carbon fibre skins laid over a Nomex honeycomb core - which will be recognised by anyone familiar with how the tubs of modern Formula One cars are fabricated. Sandwich construction of this type, although the obvious method to use where maximum bending stiffness is required, has only occasionally been used in turntables before, notably by Pink Triangle. To my knowledge the Wilson Benesch is the only turntable ever to have deployed a carbon fibre/Nomex sandwich in this role - Pink Triangle's subchassis, by contrast, is cut from aluminium-skinned Aerolam sheet.

Because carbon fibre composites are normally constructed using either unidirectional or woven cloths, they further differ from metals in being anisotropic in their physical properties. In the case of woven cloths, which are used for both the WB subchassis and pickup arm tube, tensile stiffness is greatest along the warp and weft, somewhat less at 45 degrees to the weave. This again can be advantageous, allowing the stiffness of a structure to be maximised in preferred directions.

Achieving the right balance of stiffness and internal damping depends on precisely controlling the relative amounts of fibre and resin, so Wilson Benesch employs what in composites jargon are called 'pre-pregs' - cloths impregnated with a precise amount of epoxy resin. No traditional wet lay-up using liquid resins is used, which eliminates the human factor and ensures good production consistency. Once the pre-pregs are cut to shape and correctly positioned (in the case of the arm tube, a 'sock' of carbon fibre is pulled over a mould), the resin is cured by application of pressure and heat in an autoclave.

Although the ACT One and Two cannot lay claim to being the first pickup arms to use carbon fibre (the pioneer, to my recollection, was the Infinity Black Widow - a design notorious for the fact that it threw away much of the material's performance potential through the 'anorexic' dimensions of its arm tube), but they are the first I know of to put it to something like optimal use. To ensure maximum rigidity in bending and torsion, and hence minimize structural resonance, the tube of a pickup arm must have as large an external diameter as possible consistent with traversing the record surface. In a manner reminiscent of the thin-wall cast magnesium tube of the SME Series V, this is achieved in the ACT designs by employing a tapered tube which is thinnest at its junction to the integral headshell and fattest where it is bonded to the bearing housing.

If you look carefully at the arm from the side you will also notice that the tube sides are slightly curved. It was originally intended that this curve be hyperbolic but, unable to find a lathe which could profile the mould to this shape, Wilson Benesch settled on a circular arc of large radius. The warp and weft of the carbon fibre cloth are aligned at 45 degrees to the tube's longitudinal axis, a configuration chosen through listening tests rather than any theoretical analysis though its effect is to maximise the stiffness in torsion and the damping in bending. A similar alignment is employed in the subchassis about the critical axis between the main bearing and arm base.

To my knowledge no one has ever published vibration analyses of turntable subchassis, so the achievements of the Turntable's composite sandwich item can only be guessed at. Not so with the arm. In late 1993 WB paid for the ACT One to be independently tested, the results (which I've seen) indicating an arm that is both exceptionally rigid and unusually well damped - just as you would anticipate. The first resonance of any severity occurs above 800Hz, and like all those appearing at still higher frequencies is sufficiently well damped to be of doubtful audible significance. This in an arm which boasts an effective mass half that of some competitors.

Other salient features of the arm (ACT One and Two) include a low-slung, two-piece stainless steel counterweight, mounted below the axis of the unipivot bearing to provide torsional stability and undecoupled to prevent any undesirable resonance of the counterweight mass. To describe the bearing as a unipivot is actually to malign it in that it eschews the familiar needle and cup arrangement, employing instead a so-called kinematic bearing comprising three 1mm diameter carbon-chrome balls, clamped together to form a horizontal triangle atop the arm pillar, on top of which rests a fourth ball, carried within the aluminium bearing housing, which is free to pivot about all axes. It's an arrangement which combines play-free (and therefore rattle-free) location with very low friction, independent of ambient temperature. It also requires no provision for adjustment, during manufacture or afterwards. Carbon-chrome balls are likewise used to clamp the hardened tool steel arm pillar to the arm base, and to locate the counterweight. Light foam placed within the hollow of the arm tube dissipates any resonances within the enclosed volume of air. Bias ('anti-skating') is applied by John Crabbe's ubiquitous thread and falling weight mechanism, the weight, moment arm and pivot in this case being gold-plated - the only example of needless extravagance throughout the entire arm.
All these features the ACT One and ACT Two share, the principal difference between them being the use of a higher-modulus grade of carbon fibre in the latter, which allows the weight of the arm tube to be reduced still further. The £249 costlier ACT Two also has a parking device. Cardas cable is used in both arms, but the ACT Two additionally offers the option of balanced wiring (£151 extra).

As I remarked earlier, the turntable looks a good deal less remarkable than the arms because here the carbon fibre is hidden from view. You might almost call the result dour. A featureless black aluminium top plate is relieved only by a traditional wooden plinth, offered in standard finishes of red cherry, black ash or oak, with piano black and piano white lacquer available to special order at a £255 premium.

There are no controls whatsoever on the turntable itself, speed change (33 1/3 or 45rpm) being effected by a push switch on the outboard power supply box, adjacent LEDs indicating which speed is selected (green for 33 1/3 rpm, red for 45rpm). The supply generates 18 volts quartz-referenced AC for the turntable's Papst hysteresis motor, and +35 volts DC for the optional RIAA-equalised preamplifier, recently introduced at £995, which mounts out of sight within the plinth. During manufacture the frequency of the motor supply, which is internally adjustable in 0.1 per cent steps, is preset to ensure accurate platter speeds regardless of machining tolerances.
Changing speed is not achieved simply by switching motor frequencies. Instead the drive voltage is first ramped down, the supply frequency switched, and the voltage then ramped back up again in order to eliminate ungainly torque-induced convulsions of the subchassis. During the course of the switch-over the LED for the newly selected speed flashes until full motor voltage is restored.

The 'low cogging' (smooth torque delivery) characteristics of the motor and the use of belt drive do much to prevent internally generated vibrations reaching the platter, but isolation is further enhanced by mounting the motor on a three-point sprung suspension. Similar mounting of the entire subchassis acts to isolate external vibrations, though like most turntables the Wilson Benesch will still repay careful selection of the shelf or table on which it is operated. Hidden beneath the felt mat, a polymer ring separates the inner and outer sections of the platter and acts to damp yet another set of structural resonances. A glass lid and vinyl instrument cover are provided to protect the turntable from dust.

Performance
For the purposes of this review Wilson Benesch provided two items in addition to the Turntable and ACT Two - a prototype carbon fibre-bodied low-output moving-coil cartridge (now available) based on stylus, cantilever and generator components made by Benz in Switzerland, and a prototype discrete transistor version of the aforementioned IC-based RIAA preamplifier. The entire combination I treated as a package, making no attempt to substitute different cartridges or preamplification. The line-level output from the preamp fed my own passive preamplifier (a pair of DNM's conductive plastic volume control potentiometers mounted in a cheap and cheerless ABS box), and thence an Audio Innovations First Audio Amplifier. Initially the loudspeakers were Mordaunt-Short Performance 860s, substituted latterly by Acoustic Energy's promising AE100s (review pending).

I have to be honest at this juncture and confess that I've done very little listening to vinyl in recent years, although my four-figure LP record collection still clutters the hall. In a period of freelance penury I sold my Townshend Rock five years ago, since when CD has met the bulk of my listening needs. Having the Wilson Benesch combination at home for an extended period provided the motivation for re-discovering some half-forgotten gems, and reminded me just how fine LP reproduction can be. However, I am in no position to deliver chapter and verse on how WB's finest stacks up against its obvious competitors. I've contented myself instead with a non-comparative assessment of its prowess, content in the fact that anyone with this sort of money to spend on a turntable will surely take time and care to make their own comparisons of the prime contenders.

Undoubtedly that's a category within which the Wilson Benesch combination deserves to be numbered. You'd expect a turntable and arm developed with resonance suppression foremost in mind to provide an unusually clean, coloration-free sound from LP, and that is precisely what the Turntable/ACT Two partnership delivers. Not the slugged, compressed output that too many 'low coloration' loudspeakers are wont to serve up, but rather a sound devoid of any obvious emphasis or clouding anywhere in the audible spectrum. It's a characteristic I particularly appreciated on choral music, Simon Preston and the Christ Church, Oxford choir's matchless Purcell Funeral Sentences (Archive 2723 076, 5/89) being reproduced as well as I've ever heard from LP.

One of the most important attributes of any high-quality turntable, and an important indicator of its ability to cope with transient signals, is low surface noise. It used once to be thought that ticks, pops and the sound of frying bacon were the inevitable consequence of using a record player capable of extracting the utmost detail from a record groove. Today we understand the reverse to be true: that record players which are intolerant of surface defects are guilty of making mountains out of molehills, while those which better control the vibrational energy generated by tracking a record groove are able to keep surface noise in correct proportion. The Wilson Benesch combination conveys reassuringly low levels of surface noise, so dynamic range is maximised while the annoyance value of pressing imperfections is minimized. With performance like this, and a well looked after record collection, who needs click suppression?

Pitch stability too is of a high order, as it should be in a turntable which combines a synthesized AC motor supply with a heavy, high inertia platter. LP reproduction will never match the unimpeachable performance of digital in this regard, but only the most sensitive will find piano reproduction unacceptably compromised.

In summary it strikes me that these two are remarkable products. Not just because they offer top-fight performance, but as much for the fact that they demonstrate remarkable confidence and design flair in the context of a small new company's first attack on what, one way and another, is a most difficult market to conquer. The peculiar advantages of carbon fibre composite are well enough known by now; there exists a specialist moulding industry well versed in making consistent, well-finished products in the material. Yet it has taken a newcomer to the specialist hi-fi industry to grasp the nettle and apply carbon fibre in circumstances where it has obvious potential to better the performance offered by traditional materials. Good for them, I say.

Reprinted with kind permission from Gramophone


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