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BACKGROUND: ANALOGUE RECORDING AND PLAYBACK (LP RECORDS) High quality analogue LPs have a frequency range extending to 30 kHz, a signal/noise ratio of 35-45 dB and a dynamic range of 60 to 65 dB. Most LPs begin their life as recordings on magnetic tape, which can introduce distortion and background hiss, the two sometimes being audible on playback. Direct-cutting of LPs eliminates both of these problems, but the inability to edit and the limited number of pressings available from a recording session can raise the cost of a direct-cut to five times that of a conventional record. Replay is apparently simple. The record moves past the cartridge at a constant rotational velocity, while the stylus sits in the groove and follows its side-to-side and up-and-down excursions. In order to recover the signal correctly, the stylus must move, relative to the pickup cartridge body, by exactly the right amount specified by the groove modulation. Meanwhile, the record must not move at all, except rotate. Any undue relative movement of cartridge or record will result in distortion. Incredibly, the smallest movement of the stylus which can be translated into an audible part of the signal approaches the dimensions of a living cell, about one ten-thousandth of a millimetre (0.1 mm). If the cartridge or record move relatively to each other by more than this amount, distortion of potentially audible magnitude will be generated. An unsupported record is quite incapable of resisting to sufficient accuracy the reaction force imposed on it by the stylus, so a solid support is essential. The cartridge body will always try to follow the stylus, and it must be restrained. The arm on its own cannot do this, and so some further restraint is desirable; and in order to maintain a constant distance between the record/platter assembly on the one hand, and the cartridge/arm assembly on the other, a rigid plinth is required. If the platter, arm and plinth were all perfectly rigid, there would be no further requirement of the deck. However, no real material is perfectly rigid; in practice, two undesirable effects occur – deflection and resonance – both of which contribute to distortion. To counteract these effects, the materials used in the construction of a deck must be not only highly stiff but also very well damped so that they do not resonate or ring when stressed. The objective therefore is to keep the cartridge body and the record surface in precisely the right position in relation to each other, and to control this relationship over a very wide frequency range, since any deflection or vibration in any part of the record deck itself will defeat this objective and render accurate replay of the record impossible. At around 10 Hz, the compliance of the cartridge cantilever mounting and the mass of the arm produce a mechanical resonance which will have disastrous effects on signal reproduction unless it can be controlled. The lower frequency limit of the "control region" is about 8 Hz, and the upper limit should ideally be the frequency of stylus/groove resonance, as high as 105 Hz for some cartridges. Over this wide range, many different sorts of resonance occur and manifest themselves in different ways. All must be controlled, ideally to the stringent criterion of less than 0.1 µm. The Rock V Where
to start describing the features of The Rock V? The appropriate
place must be the stylus tip, where it touches the record. Let
us assume that a high quality moving coil cartridge is fitted
and it is not mistracking. The stylus follows the groove, and
the cantilever follows the stylus. At the other end of the cantilever
are fixed the generator coils, which are forced against the rubber
suspension block, which in turn is forced against the cartridge
body. This force is supplied by the tie-back wire which attaches
to the end of the cantilever and the cartridge body after passing
through the compliant rubber suspension. The coils move relative
to the magnets in the cartridge body, generating the output signal.
The magnets are fixed in place in the body and the body is fixed
to the headshell. However, the movement of the cantilever causes
the suspension to move, which in turn causes the cartridge body
and headshell to move. Is the headshell fixed to anything suitably
rigid? Only in a Townshend Audio deck, for only in this design
is the headshell coupled directly to the plinth via a small paddle
suspended in the trough of viscous silicone oil, itself securely
fixed to the turntable plinth as shown in fig 1.
Fig 1 Headshell, paddle, trough and cartridge when playing. Without the trough, the headshell would have to rely for support on the arm tube, and hence on the bearing which is over 20 cm away. Even if the arm were damped at the bearing, a lot of vibration can happen over those 20 cm and if the arm is wholly undamped there is nothing to restrain it. What is more, the arm resonance at around 10 Hz will be at best only loosely controlled. This means that at the slightest provocation (such as any transient information in the groove, from warps to cymbals) the arm will want to wave to and fro at a frequency of 10 Hz in antiphase to the stylus. Worse still, the large stylus excursion that this entails will cause the generator assembly to approach the limits of its travel where linearity is least good, and the output will be unduly distorted and intermodulated with the 10 Hz signal. At the other end of the frequency scale, the arm ceases to be rigid at high frequencies (in fact, at frequencies as low as 3 kHz) and hence provides poor support for the cartridge. The slightest amount of slackness (play) in the arm bearing will compound these problems greatly. Against
all these potential disasters, the trough is a uniquely effective
and elegant safeguard. Its effect is to impart a 'viscous drag'
on the arm, which is proportional to its speed of movement. So
at low speeds, where the cartridge has to cope with tracking warps
and changes in groove pitch, the drag is negligible; but at higher
speeds the drag increases so that at frequencies of 20 Hz and
over the arm is effectively locked into position. In the critical
frequency band around 10 Hz where the arm tends to resonate, the
drag has been adjusted (by suitable choice of oil viscosity and
paddle dimensions) to damp the resonance "critically",
reducing its effects to a negligible level. At the other end of
the spectrum, the fact that the trough paddle is rigidly mounted
very close to the cartridge by way of the double outrigger space
frame assembly means that the cartridge is rigidly supported to
a much higher frequency than it would be by the arm alone.
Fig 2 Frequency
Response and Rumble The trough also provides a considerable reduction in bearing rumble. By rigidly coupling the cartridge at signal frequencies to the plinth, and therefore to the platter, the trough ensures that any rumble vibration is common to both record and cartridge; so there is no relative motion of the two and therefore no rumble signal generated by the cartridge. Fig.2 shows how effective the trough is at reducing rumble. The left hand graph shows the rumble level without use of the trough – most other decks have a similar or higher rumble level. On the right is a graph of rumble level with the trough in use, to the same scale, showing that rumble has been reduced by around 20 dB, from low to very low indeed. Note that the frequency response of The Rock is far more accurate than that of an undamped turntable, being -3dB at 10Hz and 0dB at 20Hz, compared with +3dB to+5dB with an undamped turntable. Further, the response at 2Hz, the rise time of a fast warp ripple, is the same with the undamped and damped case. Thus it is clear that the front end damping imposes no additional load on the cantilever and suspension when playing warps, handling a “swinger” (0.5Hz) or simply playing the record (0.0001Hz) In fact, the load on the cartridge suspension is much less in the damped case due to the precise control of the 8-10Hz arm/resonance. The control is so effective that it is possible to tap the record quite hard during playing without upsetting the tracking. The turntable may even be lifted up and tilted to an angle of 30 degrees without jumping. Heavy foot-fall, including dancing on a sprung wood floor, has no effect on ability of The Rock to play perfectly. Changing records is simple: the pickup is lifted from the record and the paddle is placed at the right hand end of the trough as shown in Fig 3.
Fig 3 Paddle in right hand end of trough. Much thought has gone into the design of the trough to make it as easy as possible to use and to prevent spillage of the trough fluid. Two conveniently positioned opening handles come naturally to hand. Silicone oil is basically harmless; it won't damage furnishings or clothing – it slowly seeps away. If it is spilled on polished wood, cleaning up is easy – remember that silicone oil is the basis of most modern furniture polishes! Full instructions for cleaning silicone oil from records are provided in the operating instructions. When records are being changed, the arm is parked so that the paddle rests in the right-hand end of the trough. When the arm is locked in the armrest, inevitable drips of fluid are caught in a small funnel fitted directly underneath the paddle as shown in Fig 4.
Fig 4 Clamp released, trough open, arm in armrest. Having thus proceeded from stylus to cartridge body to headshell, we come to the trough, which is fixed, by means of two very stiff pillars, to a heavy, rigid plinth. This plinth is made from a composite material (steel, acoustic damping and Herculite Plaster-of-Paris) chosen for strength, rigidity (stiffness) and damping; it has the lowest possible tendency to stretch, bend or resonate, as can be appreciated by tapping it. The plinth links three major components: the trough, the arm and the main bearing. The main bearing must support the platter as it rotates, without introducing any rumble or other vibration into the platter or plinth. To achieve this, The Rock V has a precision bearing using a hard steel ball against a polished case-hardened shaft. The bearing support, like all other parts of the deck, is set into the plinth for ultra-high rigidity.
The platter is manufactured from high density white polyethylene
plastic mated to a disk of D Flex acoustic damping sheet which
in turn rests upon a solid glass disk. The unique polyethylene
was chosen because it has an acoustic impedance very close to
that of the vinyl, from which records are made and is naturally
homogeneous. Matching the impedance ensures that the sound wave
that radiates outwards from the stylus is fully absorbed, without
reflection, into the platter. Where there are reflections, due
to the platter having a very hard surface (e.g. acrylic) or a
very soft surface (e.g. felt) the sound becomes full, slightly
reverberant and blurred. This phenomenon is known as “vinyl
ring”. It is an aberration not tolerated in The Rock V.
The record is secured to the platter by means of a screw-down
clamp with an adjustable raised centre section surrounding the
centre spindle of the platter. This causes the record to be very
slightly “belled”, giving close contact between platter
and record and ensuring that any vibration of the record itself
is properly conducted away into the bulk of the platter. The height
of the washer may be adjusted to take account of records of differing
thickness and degree of warp. With this arrangement, it is possible
to flatten all but the most severely warped records. In the case
of 180/200 gram pressings, the washer may be screwed right down
below the record surface so as to not contact these (usually)
very flat records. The record is restrained by the platter to
within about O.1µm, and while we can't think of a way of
measuring it, the results speak for themselves. A further feature
of the centre washer is that it may be raised sufficiently so
that a large-hole or doughnut 45 disk may be easily centred on
the periphery of the adjusting ring as shown Fig 5.
Fig 5 Centre for large-hole record. To prevent scratching the mirror stainless steel case, two clamp holders are provided, one on the left-hand side of the platter (for left handers) and one on the right hand side of the platter, to provide a convenient place to put the record clamp when changing records. Although the trough does the lion's share of the work in keeping the cartridge where it ought to be, the arm still has an important part to play. The ideal arm for use with The Rock V is the new Townshend Audio Excalibur, which includes a rigid space-frame outrigger and paddle to dip into the trough; however other arms may be used provided the double outrigger and paddle are fitted. Outwardly more conventional in appearance than the deck, The Excalibur nevertheless represents the same degree of thought and attention to detail – a full description is given later in this paper. Fig. 3 shows how the arm base pillar is attached to a thick, non-resonant polycarbonate arm plate, itself screwed (with adjustment for setting overhang) to four mounting points set into the plinth. This completes the loop of firm supports for the cartridge, as shown in the diagram. Compare this with the conventional deck, where the cartridge is about as far from a firm support as possible. Drive to the platter is supplied via a circular section belt by a synchronous motor. Voltage selection (220-240 or 110-120) is by way of a wire link in the motor compartment. The Rock V can be supplied with a drive pulley suitable for 50 Hz or 60 Hz working. The motor is switched on and off by an attractive push-button switch that lights blue with the Townshend Audio logo visible. The light is dimmed when the power is connected and the motor is off.
Finally, just as no structure can be perfectly rigid, it cannot
be completely immune to external vibration either. To reduce external
structure-borne vibration from entering the enclosed sub-chassis
by up to 40 dB, the Rock V incorporates a unique three-point suspension
similar to that found in aerospace instrument mounts as illustrated
in Fig 6.
Fig 6 Spring Bellows suspension. The spring bellows-damped suspension is located as close as possible to the plane of the centre of gravity of the deck to provide optimum stability. Each suspension unit comprises a “soft” coil spring which is enclosed in a rubber concertina bellows sealed but for a very small (1mm dia) hole in one end. The spring rate is chosen to form a very high Q resonant system with a fundamental resonance of about 2Hz as shown in fig? as 0.05. where the resonant peak is high (amplification at 2Hz) but the isolation above a transmissibility of 1 (equivalent to 2Hz) is very high.
Fig 7 Influence of Damping in Vibration Isolation. If the springs were used alone, the isolation would be very effective, but the bounce resulting from a large external disturbance would be intolerable as the oscillation could last for minutes. The bellows/small hole arrangement however, acts as a viscous brake on the oscillation as the trapped air in the bellows must pass rapidly through the hole as the assembly moves. The friction loss as the air passes through the hole rapidly dissipates the spring/mass energy (C/Cc=2 above) and the suspended structure quickly settles down. When at rest, however, minute vibrations in the base structure will cause only a very small amount of air to pass through the hole which it will do without restriction, so there will be no damping. Thus we have the best of both worlds – maximum isolation with minimum oscillation. The three spring/bellows assemblies have threaded fittings that mate with threaded shafts to allow for fine level adjustment. When the turntable is sitting normally, this suspension is so effective that the sound from The Rock standing on a normal table or bookcase surface is only slightly affected when the surface is struck hard. When mounted on the Townshend Audio Seismic Sink Stand, the isolation and resulting in purity of reproduced sound is further enhanced. The underside of the suspension is attached to the turntable base which has four non-slip rubber feet at the periphery. For safety during transport, locking catches are provided, accessible when the platter is removed, to lock the sub-chassis securely down onto the thick steel base. The motor is rigidly attached to the turntable base so that any motor vibration is eliminated by massive mass damping. An extremely attractive, soundproof glass lid, with counterbalanced opening top and mechanically linked opening front, is available as an optional extra. The lid fixing holes for the lid are visible in fig 3. Now you know why we have gone to such lengths to get the Rock's engineering just right – and naturally, we have also given such a lovingly crafted product looks to match its performance – but how successful have we been? To say "very" would be both facile and pointless. Instead, because the design of the deck has been a rational process, based as far as possible on satisfying known physical principles, we are able to point to specific areas of improvement over other decks and also – who else does this? – give reasons. The overall sound has unprecedented stability and solidity. This is partly a result of the superior speed stability of the deck; the ear is sensitive to speed variations as small as 0.01 %, and while it is extremely difficult to measure accurately to this level (even test records cannot guarantee such accuracy), we can certainly achieve better than 0.025% from a deck merely placed down on the factory bench with no special setting up, thanks to the very close machining tolerances employed, the high mass and excellent balance of the platter and the low and uniform friction of the bearing. The trough also contributes largely to the unique sound. By reducing the effects of arm resonances to virtually zero, as shown in Fig. 2, it ensures that the signal from the cartridge truly represents the signal put on the record, neither more nor less, not modulated by low frequency signals and not full of high frequency rattles. It even aids the tracking of warps, contrary perhaps to instinct, because undamped arms tracking warps will invariably resonate badly, with severe ill effects. The maximum stylus excursion when tracking warps is actually less with damping than without. Not only does the trough improve the deck's sound, but the amplifier's too; since the amplifier no longer has to deal with low frequency garbage (either by filtering it out or by amplifying, depending on the subsonic characteristics of the particular amplifier), it can concentrate its efforts on amplifying the music signal. Amplifiers, which pass subsonic frequencies, can cause undamped decks to damage loudspeakers by their low frequency output – remember that few loudspeakers and especially valve (tube) amplifiers can handle much power at 10 Hz. Rumble is particularly low on The Rock V, even without the trough, due to the materials used and the very high quality bearing and (as mentioned above) is still further reduced with the trough in use. Test this by tapping the trough or the record while playing -you will have to tap hard to hear any effect at all in the loudspeakers. It is a remarkable fact of audio reproduction that the ear interprets pitch instability, under certain conditions, as a lack of dynamic range. This is certainly true of "dynamic wow" - slowing under load -in a record deck. The heavy platter and high quality drive on The Rock reduce this effect markedly compared with conventional designs and the result is a considerable increase in perceived dynamics. Observation of the output power meter on an amplifier fed by a Rock reveals that comfortable listening levels are actually lower than normal; the absence of subjective compression of transients causes this. Listening to a Rock V, one comes to realise just how full of resonance-induced foibles is the output from other decks. Long-term listening is far less fatiguing than with other players (or, of course, with CD), a sure sign of reduced levels of spurious output. The ear generally does not detect low-level spurious signals directly – the brain filters them out automatically – but it lets let us know that it is doing so by becoming tired after a few minutes and starting to lose focus and concentration on the music. This effect does not happen at a live concert (of acoustic instruments, anyway) and it does not happen when listening to a good analogue recording via a Rock. The sound of a Rock, equipped with Excalibur and a good cartridge and played through high quality amplifiers – for example the Townshend amplification and Townshend Isolda cable into Townshend Glastonbury loudspeakers – is possibly the most natural, lifelike experience this side of a first-generation master tape. Even clicks and scratches are less audible on a Rock V. It has long been appreciated that most ticks and small scratches on records are equivalent to a single cycle of 10-15 kHz, in itself barely audible; and yet these same scratches throw most record playing systems into resonances which can last up to 50 times as long as the original tick, and are certainly very audible. In the Rock V, resonances right across the band are so well suppressed that for once we can approach closely the ideal of the inaudible tick. The effects are simply over so fast that the ear does not notice, and mildly 'clicky' records can become enjoyable once again.
Townshend Audio is the only turntable manufacturer to advocate
direct comparison of LP sound with that from open-reel tapes copied
from the original master (as we regularly demonstrate at hi-fi
shows). This test shows up in an instant the serious limitations
in performance of conventional turntables – The Rock V stands
close to master tape sound! If a turntable assembly cannot reproduce
the master tape sound, it is inaccurate.
Fig 8 Excalibur II arm with Townshend Audio “The Grail” cartridge. Compared with the obviously unique aspects of the Townshend Audio turntables, the Excalibur arm similarly appears very unusual. Look at the peculiar features: the small paddle at the front, the way the headshell is attached to the arm tube, the length. In fact, Excalibur reflects just the same design principles as The Rock V: Kinematic design and resonance control. Resonance control is an obvious requirement of any mechanical means of reproduction, since resonance implies added, spurious information in the output signal. Kinematic design demands the minimum number of mechanical restraints (e.g. three mounting feet rather than four), and that all the forces to pass through the centre-of-gravity. This principle, well established in precision machinery, reduces turning moments on parts of the system and improves stability, which is clearly beneficial in as precise a machine as a record player. So we find in the Excalibur that everything possible is damped and that forces are arranged to act in or near to a plane co-incident with that of the record surface as it rests on the platter. The headshell is made from magnesium, a material with good self-damping properties and high specific stiffness (stiffness per density). The magnesium piece used is angled for high rigidity and includes a small paddle for use with the damping trough on Townshend Audio decks. For the utmost rigidity, space frame assembly is arranged to grip the paddle to couple the paddle as securely as possible to the cartridge. Cartridge mounting to the headshell is unusual in using two sets of holes, instead of slots, in the headshell. This is to ensure that cartridge offset angle is accurate (adjustment is then reduced to overhang only, easily done with the Townshend 'Elite Alignment Gauge'). Viewed from above, a straight line joins stylus, paddle, arm tube and bearing, providing maximum resistance to twisting and bending under stress. The headshell is fixed to the arm by means of a scarfed joint, which offers not only very high torsional stiffness and strength close to the cartridge mounting position, but also a large area of contact with the arm. The joint is held with three high tensile pins and high strength epoxy. The arm tube uses thin-walled stainless steel as its basic material. Stainless steel is heavier than the more commonly used aluminium, but its specific stiffness is similar and it can be made into thin-walled tubing of larger overall diameter. This increases rigidity for a given mass. Importantly, its speed of transmitting sound is very high and it is this that determines the upper frequency limit at which the arm can be considered rigid. To increase this limit even further, the arm is significantly shorter than usual, a compromise with tracking error, which nevertheless hardly increases tracking distortion. To further increase stiffness, the arm pipe is rigidly clamped by two high rigidity screw clamps, by a coupling tube to the arm stub which has its centreline in the plane of the disk and the horizontal bearing axis. The arm assembly is fixed by a screw which allows for easy removal of the arm-pipe assembly to facilitate cartridge mounting or to cater for those who wish to swap cartridges on a regular basis. The inside of the arm is filled with a low density polyurethane foam which damps arm resonances and, in conjunction with the front-end damping, reduces arm-induced colorations to the barest minimum. Although the front-end damping trough considerably reduces the requirements of an arm, experience has shown that the entire arm, and especially the bearings, must still be of the highest possible quality (in other words, working without a trough places impossible demands on any arm, especially on the bearings). The Excalibur uses high grade, large diameter ball bearings, which are very strong. . . the horizontal bearings have a maximum working load of 0.3 tons! The point of using such bearings is to give maximum mechanical coupling between arm tube and arm mounting. Normally, bearings are lubricated with relatively light oil and tightened as far as possible without impeding movement, but this results in a mechanical coupling which is at best point-to-point and at worst highly temperature sensitive. In the Excalibur, the bearings are adjusted so that under normal conditions there is a very small amount of play detectable; this ensures that under extremes of temperature they will not start to seize, nor will friction vary with age. However, at signal frequencies (which is what counts), high performance oil provides excellent coupling across the bearing, just as it does in the damping trough, ensuring that bearing rattle cannot occur. This arrangement also gives low bearing friction. In effect, the bearings "lift off” due to the oil running between the bearing surfaces by capillary action, eliminating metal-to-metal contact which, in any bearing, is the major source of bearing noise and wear. The dual counterweights at the rear of the arm are slung low to keep the centre-of-gravity close to the plane of the record surface and the mass as close as possible to the intersection of the bearing axes, thus keeping the moment-of-inertia and hence effective arm mass as low as possible. A wide range of cartridges may be correctly balanced, statically and dynamically, by means of these counterweights. Tracking force is applied by calibrated coil spring to ensure that the centre of gravity of the arm/cartridge assembly remains at the point of intersection of the vertical and horizontal axes. This is vital to ensure that the least amount of sound degradation is caused by external vibration. If the centre-of-gravity is not at the intersection of the bearing axies, translational vibration will cause a turning moment and hence a sound when the arm base is forced to translate due to external disturbance. Bias compensation is provided by a calibrated adjustable magnet mechanism which gives precise, friction free force for perfect bias operation. The arm bearing assembly may be raised or lowered and then securely locked in place to set the optimum vertical tracking angle (VTA). The internal Excalibur arm wiring is by four bundles of four-strand ultra-thin Enhanced Deep Cryogenically Treated (EDCT) pure enamelled copper wire plus a ground wire. These wires are twisted together and exit the rear of the arm pipe and ark over to the back of the turntable where they are terminated in a miniature plug with gold-plated pins. This plug connects to a socket mounted on a small junction-box which is attached to the rear of the turntable. The box is fitted with phono connectors or optional additional XLR balanced connectors. Two EDCT low level fine twisted-pair Isolda interconnects fitted with phono connectors or XLR connectors plug to the junction box and thence to the phono stage. A thumb screw/4mm banana socket earth terminal is provided for proper grounding to chassis earth. The arm is completed by a convenient, reliable arm lift which removes the risk of damage to records caused by hand slippage. In keeping with Townshend Audio's pursuit of the perfect disc replay instrument, Excalibur is the ideal complement to The Rock V. With or without the trough, it has audibly lower levels of coloration and vibration artefacts than other arms, and it has been designed to work reliably, quietly and unobtrusively for many years. If you care enough about sound quality to buy a Townshend Audio turntable, you owe it to yourself to complete the package with an Excalibur.
If you wish to use your own arm, then a double outrigger and paddle
assembly is available to realise the full benefits of front-end
damping. See fig 9.
Fig 9 Double outrigger and paddle, Goldring Cartridge fitted to Rega arm. One part of the outrigger is placed on the top of the headshell and the other part below, above the cartridge and the lot rigidly bolted together. The assembly must be arranged so the paddle, stylus and arm bearing intersection are in a straight line to reduce unwanted and unnecessary turning moments. Suitable arm-plates are available drilled for all common arms.
Completing the partnership with a top flight moving coil cartridge
and the purpose built TA732 Townshend Audio Phono stage feeding
the Townshend Audio amplification and Glastonbury range of speaker
system produces a sound off vinyl that experienced critical listeners
describe as astonishing. The lack of the usual gramophone noise
is the most striking aspect. There is no rumble, minute surface
noise is only noticeable during silent tracts at very high volume
and there are virtually no clicks. The music, however, is entrancing!
Words cannot describe the aural experience – that’s
a job for your ears! Book a demonstration now. Specification Performance
Arms
Mechanical
Electrical Colour - Mirror stainless steel case, brushed aluminium front and rear panel
Physical Dimensions - 52 x 42 x 7
cm (20 x 16.5 x 3 inches) |
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