GB2419025A - Tone arm - Google Patents

Abstract

A tone arm 10 for use with a counterweight (62) where the tone arm comprises an arm 12 and a first bearing 14 having flexing sheets or films 46 and (48) for permitting a pivoting movement of the arm in a vertical plane in use. Blocks 34, 36, 24 and 28 clamp the flexible films. Using the flexing means, rather than a ball bearing, does not cause rattle that would come from a worn ball bearing. A second bearing 16 has a spindle 40 in a sleeve 42 having a ball bearing 46 or (47').

Description

24 1 9025 - 1

TONE ARM

FIELD OF THE INVENTION

The present invention relates to a tone arm and to a method of manufacturing a tone arm.

BACKGROUND OF THE INVENTION

0 In the opinion of many audiophiles the playback of sound recorded on vinyl still represents the most realistic and faithful reproduction of the original.

The reproduction of sound from vinyl is a mechanical process in the first instance. Each record is cut with a spiral groove comprising modulations, the is amplitude and frequency of which correspond to the original sound. A turntable holds and spins the record relative to a stylus that is mounted in a cartridge on a tone arm.

The stylus is caused to move relative to the cartridge by the modulations, and this oscillation is converted into small electrical signals (typically by electromagnetic induction) in the cartridge. The small electrical signals are sent to a pre-amplifier, so then to a power amplifier and finally the amplified signal is converted to sound by loudspeakers.

Ideally, all movement of the stylus caused by the modulations would be translated into electrical energy. Ilowever, no vinyl record is perfectly flat and as undulations and warps are present to varying degrees on different records. These undulations and warps must be accommodated by the tone arm without inducing unwanted electrical signals in the output. Therefore the tone arm must comprise an efficient and durable bearing system to permit substantially free movement in the vertical plane, firstly to accommodate these undulations, secondly so that the performance of the cartridge is not impaired, and thirdly so that the stylus may be placed on and removed from the record by a user.

Usually pivoting of the tone arm provides the movement in the vertical plane.

The length of most tone arms between a pivot point and the stylus is between about 3s 230mm to 240mm. For such tone arms the required amount of movement in the - 2 - vertical plane is small. Typically about +0.25 of movement up and down is sut'ficient to accommodate undulations in records in serviceable condition.

desirable amount of movement to safely emplace and remove the stylus from the surface of the record without damage is about 5 . s

The bearing system of the tone arm must also permit substantially free movement in the horizontal plane so that the stylus of the pickup cartridge may traverse the surface of the record, following the spiral groove. The required amount of movement is not more than about 30 .

Any free play of the tone arm other than the horizontal and vertical movements described above and any resistance to those movements is highly undesirable. Such free play has a serious deleterious effect on the ability of the pickup cartridge to retrieve accurately the information stored in the modulations in the walls of the groove of the record. Depending on the particular bearing system the free play can be manifested in rotation of the tone arm about its longitudinal axis whereby the vertical alignment of the stylus is lost, and in translation of the tone arm relative to the record.

Various bearing arrangements have been proposed to accommodate the horizontal and vertical movements mentioned above. One such arrangement is known in the art as a "unipivot". As the name suggests this arrangement comprises a single pivot point for the tone arm that permits both horizontal and vertical movement.

Ilowever, a single pivot point also permits rotation of the tone arm about its 2s longitudinal axis. To inhibit such rotation the centre of gravity of unipivot tone arms must be below the pivot point. However, it is not possible to substantially eliminate this rotation.

The applicant believes that it is ideal if' the bearing system provides separately for the vertical and horizontal movements.

Such bearing arrangements have been proposed. Arguably the most common tone arm bearing system today comprises a number of ball races that allow independent movement in the horizontal and vertical planes. A tone arm is provided with a pair of journals that co-operate with two sets of ball races to permit movement - 3 in the vertical plane, and is also provided with a spindle that co-operates with another two sets of ball races to penmit movement in the horizontal plane. Such tone arms are sold in the UK by Kega Research 1,td and SME Limited for example.

Whilst ball bearings are very useful for applications where many tunes are to be accommodated, they are inherently unsuitable for application in tone arms where the required degree of movement is relatively small and there is a requirement to tolerate vibrational stress. In particular, when a record is played, some of the energy of the movement of the stylus (caused by the modulations in the groove) is 0 transmitted to the tone arm and results in vibration of the arm. If the vibration is not inhibited it has a serious adverse effect on the ability of the stylus and cartridge to reproduce correctly the sound stored on the record.

In tone anms that utilise ball bearings, the vibration causes wear that allows free play and rattle between the tone arm and the bearings. This is because the vibration is transmitted by microscopically small points of contact between the balls and the inner and outer races. Furthermore, since the movement of the arm in the vertical plane when playing a record is very small (typically +0.25 ), in the case of the bearings permitting vertical movement those points of contact move so little that wear due to vibration is localised. Any wear of the ball bearings severely impairs the ability of the tone arm to perform correctly. Rough handling of such tone anms can also cause damage to the ball bearings.

To produce a tone arm utilising ball bearings, it is necessary to use high quality ball bearings with extremely high precision in all parts if free movement in the two required planes is to be achieved, but undesirable free play is to be avoided.

Accordingly some pre-tensioning of the ball bearings is necessary in pursuit of this delicate balance. Even then, skilled hand assembly of selectively matched components is required if a tone anm of the highest quality is to be built. Despite this, use of the tone anm over a period of time will eventually result in bearing wear and rattle. When this happens it is not possible to replace the ball bearings and manufacturers of high quality tone arms often offer replacement of the entire tone arm at a discounted price.

Other desirable properties of a tone arm are: that it has adequate mechanical - 4 strength and rigidity; that it provides for secure mounting of a pickup cartridge at its moving end; that it has an appropriate and preferably adjustable effective mass to suit the compliance of the stylus of the pickup cartridge; and that it provides for adjustment of the tracking force of the stylus on the record being played.

GB I 339 042 (Nicholls) discloses a tone arm in which the vertical and horizontal movements are permitted by thin flexible hinge portions forming part of the arm. One thin flexible portion defines a horizontal plane and provides for the vertical movement of the tone arm. T he other thin flexible portion defines a vertical 0 plane and provides for the horizontal movement ofthe tone arm. The arm is moulded from synthetic plastics material and it is stated that the arm is of such low weight that a counterweight does not need to be used to obtain a stylus tracking force of between 3 and 5 grams. Assuming that the tone arm has negligible mass the tracking force (or down force) applied by the stylus onto the record would be equal to the weight of the cartridge. Thus present cartridges, which have weights between about 4.2 grams and 24 grams, could not be used with such a tone arm because the required tracking force is equivalent to the force applied by between about 1.2 grams and 2.5 grams under gravity.

Furthermore by making the thin flexible portions part of the arm rotation of the tone arm about its longitudinal axis is not inhibited, and the rigidity and strength of the arm is compromised. Nicholls also mentions that the thin flexible portions provide substantial acoustic damping. At the time GB I 339 042 was filed, acoustic damping of tone arms was thought by some to be desirable. It is now generally accepted that such a provision is undesirable as it destroys the abilities of the tone arm to provide adequate mechanical strength and rigidity and to permit no movement of the cartridge save in the two required planes.

Despite the teaching of Nicholls, the majority of the tone arms actually manufactured and sold at present (and over the last 20 years) either use ball bearings or are of the unipivot style. There remains a long-felt want in the industry for a simple bearing arrangement, particularly for the required vertical movement, that addresses the problems of durability and of rotation about the longitudinal axis whilst preserving the desirable properties of the tone arm including strength and rigidity. - 5

SUMMARY OF THE INVENTION

Preferred embodiments of the present invention are based on the insight by the applicant that the required degree of movement at the pivot point of a tone arm to s accommodate undulations and warps of records in serviceable condition is extremely small and that by provision of suitable means designed specifically for this movement the quality ol tone arms can be dramatically improved. Typically such movement will be of the order of about 0.5 up and down, and most frequently about 0.25 up and down corresponding to movement of about I mm up and down.

According to the present invention there is provided a tone arm for use with a counterweight, which tone arm comprises an arm and a first bearing having flexing means for permitting a pivoting movement of said arm in a substantially vertical plane in use. Undulations and warps cause the tone arm to pivot, the pivoting movement being accommodated by flexing of the flexing means. Since the required degree of movement at the pivot point is so small on the scale of the tone arm, the applicant realised that a flexing means would offer substantially no resistance (i.e. restoring force) to movement over the required range by appropriate selection and arrangement of material(s) comprising the flexing means. In this way one of the most important criteria of a tone arm is met: there is substantially no resistance by the bearing to movement of the arm. Furthermore, the first bearing is resistant to wear and will not suffer chatter after extended use. A greater range of movement can also be accommodated by the flexing means for emplacing and removing the tone arm onto and from a record. For this greater range of movement it does not matter if the 2s flexing means offers resistance to the movement (i.e. a restoring force) as the stylus would not be in contact with the record. (he flexing means may comprise a film of material, such metal or plastics. In one embodiment the flexing means is a polyester film.

Preferably, said first bearing inhibits translation of said arm. In one embodiment all points of the tone arm are inhibited from moving in parallel directions through equal distances, whilst pivoting movement in the vertical plane is permitted.

Advantageously, the arrangement is such that, in use, said first bearing means - 6 - inhibits rotation of said arm about its longitudinal axis.

Preferably, said first bearing means is positioned substantially outside a body of said arm. In this way functions of the arm can be disassociated from the functions of the first bearing. For example the arm can be made substantially rigid.

Advantageously, a part of said first bearing means is displaced from a substantially vertical plane containing the longitudinal axis of said arm. In this way rotation of the arm about its longitudinal axis can be inhibited.

Preferably, said flexing means is flexible in a first sense to enable said pivoting movement, but is substantially inflexible in a second sense to provide said translation inhibiting and/or said rotation inhibiting.

is Advantageously, in use, said flexing means bears the weight of said tone arm in said second sense. This provides a substantially rigid support for the weight of the tone arm whilst permitting the necessary pivoting movement.

Preferably, said flexing means comprises a flexible material connectable between said arm and said first bearing, the arrangement being such that, in use, said pivoting of said arm causes a part of said material to flex. The material may be in the form of a sheet or strip for example. In one embodiment the sheet of flexible material comprises Mylar).

2s Advantageously, said sheet is mounted between a first part of the first bearing and a second part of the first bearing leaving said part exposed to accommodate said flexing.

Preferably, a plane defined by said sheet in an unflexed condition is oriented substantially vertically in use.

Advantageously, said plane is oriented substantially perpendicularly to a plane containing the longitudinal axis of said arm. Alternatively said plane is oriented substantially perpendicularly to a plane containing the longitudinal axis of a cartridge 3s mounted in a headshell of said arm so that the azimuth of the cartridge is not at all - 7 altered by the rise or fall of said arm.

Preferably, the tone arm further comprises at least two sheets of flexible material.

Advantageously, the tone arm further comprises a second bearing for accommodating rotation of said arm in a substantially horizontal plane in use.

Preferably, said second bearing comprises a part having a first surface for lo supporting the weight of the tone arm and a second surface for guiding said arm in rotation, wherein said second surface has an area greater than an area of said first surface.

In one embodiment said part comprises a spindle mountable in a sleeve.

Advantageously, said sleeve comprises a blind bore.

Preferably, the tone arm further comprises a bearing means at a blind end of said blind bore, through which bearing means the weight of said tone arm is transferred to said sleeve and that assists rotation of said spindle in said sleeve in use.

Advantageously, said bearing means comprises a ball and said spindle comprises a substantially flat surface for abutment with said ball.

Preferably, said bearing means comprises an angular contact ball bearing race and said spindle comprises a surface adapted for co-operation with said angular contact ball bearing race.

Advantageously, said spindle has a length of between approximately 15mm and 50mm.

Preferably, said spindle comprises a part of said first bearing means. The part may be a cross member connectable to another cross member by the flexing means.

Advantageously, said spindle is removable from said sleeve whereby different - 8 tone arms may be supported in said sleeve.

Preferably, the tone arm further comprises friction reducing means for facilitating movement of said second surface. s

Advantageously, said friction reducing means comprises oil, low viscosity oil or other lubricant. Low viscosity oil is preferred as over acoustic frequencies it is substantially solid.

lo Preferably, said arm is substantially rigid along its length.

Advantageously, one end of said arm is adapted for mounting a counterweight and the opposite end of said arm is adapted for mounting a cartridge.

Preferably, the tone arm further comprises a counterweight and/or cartridge.

According to another aspect of the present invention there is provided for use with a tone arm, a f rst bearing means having any of the first bearing means features as set out above.

According to another aspect of the present invention there is provided for use with a tone arm, a second bearing means having the second bearing means features as set out above.

2s According to another aspect of the present invention there is provided a turntable adapted for use with a tone arm as set out above, which turntable comprises a sleeve for receiving a spindle part of the second bearing means.

According to another aspect of the present invention there is provided a method of manufacturing a tone arm, which method comprises the step of mounting on an arm a part of the first bearing means as set out above.

Advantageously, said part comprises a first cross member connectable to a second cross member by said flexing means. 3s

_ Q

Preferably, the method further comprises the step of mounting said second cross member on said first cross member.

Advantageously, the method further comprises the step of mounting a part of the second bearing means on said first bearing means.

Preferably, said part comprises a spindle insertable into a sleeve that is mountable on a turntable or the like.

lo According to another aspect of the present invention there is provided a method of manufacturing a bearing means for use with a tone arm, which method comprises the step of injection moulding said bearing means from a plastics material.

I'he bearing means may comprise any of the first bearing means features as set out above. In one particular embodiment, the bearing means comprises a part for mounting an arm (e.g. by friction fit), a flexible part for providing the pivoting movement in the vertical plane in use and a means for mounting the bearing means on a spindle (e.g. by friction fit) that provides movement of the arm in a horizontal plane in use. By injection moulding all of these features in one part the bearing means can be quickly and cheaply manufactured from one material. According to another aspect of the present invention there is provided a tone arm kit comprising at least one arm and a bearing means as set out above. In this way a user may have a single bearing means mounted on their turntable and a number of readily interchangeable arms, each arm matched to a respective cartridge t'or example.

BRIEF DESCIPTION OF THE FIGURES

Fig. I is a schematic side view of part of a tone arm in accordance with the present invention; Fig. 2 is a schematic front cross section of the tone arm in Fig. 1; Fig. 3 is a schematic front cross section of an alternative bearing for use in the tone arm of Figs. I and 2; Fig. 4 is a schematic side cross section of the second bearing of Fig. 3; and Fig. 5 is a schematic side view, partly in cross section, of the tone arm of Figs. 1 and 2 in use with a turntable and record.

DETA1LED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to Figs. I and 2 a tone arm generally identified by reference numeral 10 comprises an arm 12, a first bearing 14 for permitting movement of the s arm 12 in the vertical plane, and a second bearing 16 for permitting movement of the arm 12 in the horizontal plane.

The arm 12 (shown partially in Fig. 1) is made from stainless steel tube of 12.7mm (0.5") cross section and 260mm in length. The arm 12 is substantially rigid 0 from one end to the other with no portions that allow one part of the arm to move relative to another part of the arm. At one end of the arm 12 there is fixed a headshell (not shown) in which a pickup cartridge (not shown) is mounted. At the opposite extremity of the ann 12 there is mounted a counterweight (not shown) that surrounds and is a sliding fit upon the arm 12. A grub screw (not shown) running in a threaded hole in the counterweight perpendicular to the axis of the arm 12 is provided to enable the counterweight to be fixed in position on the arm 12 when the correct balance has been established to apply the desired down t'orce to the stylus of the aforementioned cartridge. The first bearing 14 is mounted to the arm 12 210mm from the extremity of' the arm 12 to which the headshell is fixed and comprises a first cross piece 18 made from stainless steel fixed to the arm 12 by silver soldering. The first cross piece 18 is of 40mm length and mounted at its midpoint to the arm 12, its longitudinal axis being substantially perpendicular to the longitudinal axis of the arm 12. The first cross piece 18 has two outer side surfaces 20 and 22 that are parallel to one another and that are oriented substantially vertically in use. A top surface 23 of 2s the first cross piece 18 is substantially perpendicular to the two side surfaces 20, 22 and is oriented substantially horizontally in use. 'the top surface 23 is 6mm in width.

The side surface 20 is 5mm in width and the side surface 22 is 6mm in width. A lower surface 24 is inclined at an angle of 10 between the two outer side surfaces 20, 22. Adjacent either end of the first cross piece 18 is a respective bore 26 passing between the two side surfaces 20, 22.

The first cross piece 18 also comprises two separate stainless steel fixing blocks 28 of complimentary dimensions. In use, each fixing block 28 is releasably mounted on the first cross-piece 18 by a respective bolt 30 in the bore 26. The lower surface 32 of each fixing block 28 is inclined in an opposite sense to the lower - 11 surface 24 such that, in use, the first cross member 18 and the fixing blocks 28 meet substantially along a line that runs parallel to the longitudinal axis of the first cross piece 18.

The first bearing 14 also comprises a second cross piece 34 and associated fixing blocks 36 and bolts 30, all of the same dimensions as the corresponding first cross piece 18 and fixing blocks 28. The second cross piece 34 further comprises a transverse bore 38 for receiving a spindle 40 of the second bearing 16. Furthermore the second cross piece 34 is oriented oppositely to the first cross piece 18 such that lo the inclined surfaces face one another and define a first space 42 of gradually decreasing width. The fixing blocks 28 and 36 are also oriented oppositely such that their inclined surfaces define a second space 44 respectively of gradually decreasing width. The arrangement is such that the first space 42 and second space 44 provide for a pivoting movement of the first cross piece 18 relative to the second cross piece 34 substantially in the vertical plane in use.

The first cross piece 18 and fixing blocks 28 are connected to the second cross piece 24 and fixing blocks 36 by two strips of Mylar 46 and 48. Each strip of Mylar is 12mmx8mmxo.2smm (length x width x thickness) and is mounted at either end of its length across the two cross pieces 18, 34 by clamping with the two fixing blocks 28, 36 and bolts 30. 'I'he mounting is such that a part 50 of each of the Mylar strips 46, 48 remains exposed. The exposed part 50 of each strip is 8mm wide and 0.5mm high. This height is such that the material permits substantially free angular movement of the arm 12, but does not permit any significant movement along its axis. Details of the physical properties of the Mylar film can be found in data sheet H-37232-3 published by DuPont Tcijin Films, to which attention is directed.

As mentioned above the second cross piece 24 is mounted on the spindle 40 of the second bearing 16 for providing rotation of the arm 12 in the horizontal plane in use. The second bearing further comprises a sleeve 42 of stainless steel having a blind bore 44 of 4mm diameter for accommodating the spindle 40 that is 35mm in length. A stainless steel ball 46 of diameter substantially equal to the diameter of the blind bore 44 is held at the blind end of the bore by the spindle 40. The spindle 40 is of a slightly smaller diameter than the blind bore 44, in this case by approximately - 12 0.01mm. An annular space 49 and a space 51 around the ball 46 are substantially filled with Yamaha Valve Oil, a low viscosity oil, ordinarily intended for use on musical instruments. The second bearing 16 is mounted to a plinth 52 of a turntable (not shown).

Referring to Figs. 3 and 4 an alternative embodiment of a second bearing generally identified by reference numeral 16' is generally similar to the second bearing 16 with like numerals indicating like parts. However, the lower end of the spindle 40' is of substantially conical shape having a point that lies on the 0 longitudinal axis of the spindle 40'. In place of the steel ball 46 there is provided an angular contact ball race 47' that receives part of the conical portion of the spindle 40'. The weight of the tone arm 10 maintains the spindle 40' in contact with the ball race 47', even in the event of some wear of the ball race 47'.

Referring to Fig. 5 the tone arm 10 further comprises a cartridge 60, which is a Koetsu Black that is of the moving coil type. The tone arm 10 also comprises a counterweight 62 for obtaining the necessary tracking force on the stylus of the cartridge. set of cables 64 is mounted within the arm 12 that connect the cartridge with a pre-amplifier (not shown).

Before use, the arm 12 is supported by a support (not shown) on the plinth 52 of a turntable 53. When the stylus of the cartridge 60 is to be placed on a spinning vinyl record 68 the arm 12 is lifted and the Mylar strips 46, 48 flex allowing the arm 12 to pivot and the stylus to be lifted above the height of the record 68. The arm 12 is then moved in the horizontal plane assisted by the second bearing 16 to position the stylus over the lead-in portion of the record 68. The stylus is placed on the record 68. The arrangement is configured such that the longitudinal axis of the arm 12 is substantially parallel to the upper surface of the record (assuming it is perfectly flat).

The counterweight 62 is adjusted such that the stylus has an "effective mass" so as to apply a tracking force to the record 68 equivalent to 2 grams. Thus the arm 12 has a moment about the pivot point provided by the first bearing 14 that tends to bias the arm 12 onto the record 68.

When the stylus encounters an undulation in the record 68 the arm is either moved from the horizontal upwardly against the moment (lifting the effective mass of - 13 the tone arm), or moves downwardly under the moment (under the weight of the effective mass). The frequency of this movement depends on the size and number of undulations, together with the speed of rotation of the record (33-1/3rpm/3.5 red so).

Usually the frequency of such undulations lies between about 21z and 41z, although s higher frequencies cannot be ruled out. Any resistance offered to this movement may cause a relative movement between the stylus and cartridge 60, inducing a signal in the output. Further, any such resistance adversely affects the application of the correct tracking force. Thus it is imperative that the first bearing 14 offer as little resistance as possible to this movement ofthe arm 10.

I'he first bearing 14 accommodates movement of the arm 12 and cartridge 60 in the vertical plane due to these undulations. In particular the Mylar strips 46,48 flex to allow this movement. Usually the movement of the arm 12 to be accommodated is approximately +0.25 from the horizontal, although greater movements cannot be ruled out on particularly warped records and on shorter length arms for example. The length of the arm 10 between the pivot point of the first bearing 14 and the stylus is 237mm. Thus +0.25 of required movement corresponds to a required arc length of' movement of the stylus in the vertical plane of approximately I mm up and down from the neutral position. In the applicant's experience this is ample movement to play a record in serviceable condition.

Assuming that the upper edges of the exposed part of the Mylar strips 46, 48 undergo the same +0.25 of movement from the vertical and are not extended and compressed on either side respectively by the movement, this corresponds to an arc length of movement of the upper edge of each strip of approximately +0.002mm. It as will be appreciated that this is an extremely small movement on the scale of the tone arm 10. I;'urthermore, the Skylark strips 46, 48 offer substantially no resistance to movement over this range, either to initiation of movement by a warp or during movement across the warp itself. Over this range the restoring force exerted by the Mylar strips tending to return them to their verticalequilibrium position is so small as to be negligible, at least as far as their application in the first bearing 14 is concerned.

Furthermore, due to (1) the orientation of the strips 46, 48 in a plane substantially perpendicular to the longitudinal axis of the arm 12, and (2) the substantially incompressible/inextensible nature of the Mylar strips 46,48 in that plane under the forces exerted by warps on a record, rotation of the arm about its longitudinal axis is inhibited. This is further limited by the displacement of the strips 46,48 away from a vertical plane containing the longitudinal axis of the arm 12. The Mylar) is resistant to fatigue as the range of movement needed in the vertical plane is such that it remains well within the elastic limit.

The exposed part 50 of the strips provides a sufficecnt amount of material to permit the stretching and compression of the two sides respectively of the Mylar.

However as this is limited to a relatively small portion of the Mylar the radius of curvature of the flexing portion is kept large, thereby inhibiting movement of the stylus forward or backward in the groove of the record, which is otherwise l o undesirable.

The modulations of the record groove representing the music apply forces to the stylus cantilever. These forces act against the moment ol' inertia of the arm 12, cartridge 60 and counterweight 62. Inevitably some of these forces are not absorbed by the coil mechanism in the cartridge and are transferred to the arm 12. Dependent on the spring constant of the stylus cantilever, the moment of inertia of the tone arm 10, the magnitude of these forces, and the resonance characteristics of the arm 10 and all of its components, the cartridge 60 and arm 12 will vibrate relative to the stylus over a range of frequencies. This induces unwanted electrical signals in the output.

The applicant has noticed that this vibrational energy is not sufficient to cause any noticeable extension/compression of the Mylar strips 46, 48 in their plane of orientation at these frequencies. It is believed that this is because the Mylar strips 46, 48 are substantially incompressible/inextensible in that plane at these frequencies under the magnitude of the forces exerted by the arm 10. Thus, wear of the first bearing 14 is inhibited and the problem of chatter suffered by ball races is eliminated.

It will be noted that the first bearing 14 provides a pivot point for the arm 12 that is displaced from the body of the arm 12 itself i.e. it does not lie within the arm 12. This has the advantage that the function of the first bearing 14 (i.e. to permit movement of the arm 12 in the vertical plane) can be de- coupled from the functions of the arm 12 (inter alill to provide rigidity and resist vibration in the range of frequencies from approximately 2011z to 20KHz). Furthermore the function of the counterweight to provide adjustable tracking force for different cartridges/records can be accommodated without al'l'ecting the functioning of the first bearing 14. In - 15 particular, the arm 12 can be made substantially rigid i.e. with no portion moveable relative to another portion.

Arranging the Skylark strips 46, 48 substantially perpendicular to a plane s containing the longitudinal axis of the arm 12 has the advantage that the weight of the tone arm 10 can be substantially rigidly supported by the first bearing 14 substantially without any compression or buckling of the Mylar), whilst at the same time permitting pivoting movement of the arm 12 in the plane containing the longitudinal axis. It appears that any compression that does take place under the lo weight of the tone arm (usually not more than about 0.3kg for the heaviest tone arm, cartridge and counterweight combination) is negligible so far as operation of the tone arm is concerned. In this way the strips 46, 48 provide a first plane in which the weight of the tone arm 10 can be substantially rigidly supported substantially without compression of the Mylar, and a second plane in which the arm 12 can pivot substantially without resistance over the required range.

I;urthermore the relatively large overall length and width of the strips 46, 48 permits them to be clamped securely to both the first cross piece 18 and second cross piece 34 over a relatively large surface area, in this case approximately 36mm2 respectively. In this way the arm 12 is mounted firmly on the arm base 52, with very limited possibility of unwanted movement. This is contrary to prior arms that are mounted via sets of ball races on an arm base. 'I'he bearings of the ball races only provide point contact of very low surface area with journals on the arm and must be adjusted precisely to ensure that they are not too tight or too loose. 2s

The first bearing 14 comprising the Mylar strips 46, 48 provides inter alla the following functions/advantages: (1) flexes to permit movement of the arm 12 in the vertical plane, substantially without resistance over the required range; (2) has no contacting parts that are required to move relative to one another to permit pivoting movement; (3) provides substantially rigid support for the weight of the tone arm 10; (4) inhibits movement of' the arm in a forward and backward sense in the horizontal plane; 3s (5) Inhibits rotation of the arm 12 about its longitudinal axis; - 16 (6) de-couples the functions of the arm 12 from the function of the first bearing 14 whilst providing a mechanical comection between the two that is resistant to play; (7) permits the and 12 to be used with a counterweight for adjusting the tracking force on the stylus for dil'ferent types and models of cartridge; (8) provides a very simple, robust and reliable construction that is easy and quick to manufacture; and (9) if the Mylar strips are damaged they may be replaced readily by a user and at low cost.

The second bearing 16 permits the arm 12 to follow the groove in the record 68. The range of movement required in the horizontal plane to play a standard 12" record is not more than about 30 . The weight of the arm 12, cartridge 60 and counterweight 62 is supported on the spindle 40 and transferred via the ball 46 and sleeve 42 to the arm base 52. The groove of the record 68 moves the arm 12 inwardly toward the centre of the record. The inward movement of the arm 12 is passed to the second bearing 16 by the connection with the first bearing 14. The spindle 40 moves about its longitudinal axis supported by the ball 46. The thin film of oil in the annular space 49 provides lubrication for this movement. At audible frequencies this film of oil is substantially rigid thus inhibiting wear to the bearing, lateral free play and vibration. The ball 46 provides a convenient way of permitting rotation of the spindle without having to perform very precise machining and provides an easy and quick way to ensure that the point of rotation lies substantially on the longitudinal axis of the spindle. Furthermore, the point contact between the weight-bearing part of the second bearing 16 help to reduce resistance to rotation of the spindle 40.

The length of the non-weight bearing surface of the spindle 40 and sleeve 42 helps to ensure that the spindle 40 is held substantially in the vertical plane. Since this part of the spindle 40 and sleeve bear no weight, their surface area of contact can be much greater without unduly inhibiting rotation. Testing of the second bearing 16 carrying a load of 0.35kg established that the spindle 40 is readily moved in the sleeve 42 by a force equivalent to 10 milligrams (i.e. 9.8XIO-sN) applied at a perpendicular distance of 0.22m from the longitudinal axis of the spindle. Applying the same test to the second bearing 16' with ball race established that the force was reduced to less than 5 milligrams (i.e. 4. 9x 10->N).

Whilst the tone arm 10 is in operation, the drag force exerted by the record 68 on the stylus is very small. The drag force varies according to the extent of modulation ol'the groove in the record. However, even in the loudest parts of the music (when the groove is heavily modulated) this force does not exceed about 4.9X10-5N (equivalent to the force exerted by 5 milligrams under gravity). Thus there is negligible lateral force (i.e. in the horizontal plane in use) transferred to the second bearing 16.

0 Tone arms intended for the audiophile market use ball races that require highly skilled labour to set up for minimum free play whilst providing minimum resistance to movement of the arm in the required planes. This manufacturing process is time-consuming and expensive. For example the leading tone arm manufactured and sold by Rega Research Limited under the 'Irade Mark "RB 1000" is advertised as taking thirty times longer to produce than another arm sold by that company under the Trade Mark "KB300". It is believed that this time goes into selecting ball bearing races by hand and by ear, and then precision fitting the bearings so that there is as little resistance as possible to movement of the arm in the vertical plane, whilst the arm is held as rigidly as possible in all other respects. Conversely, the tone arm 10 may be manut'actured much more quickly and cheaply than tone arms of this type on the market at present and yet offer improved performance and durability. In particular, the first bearing 14 of the tone arm 10 does not require precision manufacture and can be easily assembled substantially without specialist skills.

Manulacture of tone arms in accordance with the present invention may include the following steps: cast the arm 12, head (for holding the cartridge) and first cross piece 18 in one piece; cast the cross piece 34 with bore; insert spindle 40 into bore of cross piece 34; affix the two Mylar() strips to the first and second cross pieces; insert ball 46 into the sleeve 42; oil spindle 40 and insert into sleeve 42; mount resulting assembly on an arm base 52 (the arm base can be fixed subsequently by the user or retailer to a plinth that is either separate or integral with a turntable); mount wiring for connecting cartridge to interconnect; mount cartridge (may be done by manufacturer, retailer or user); and mount counter-weight (may be done by manufacturer, retailer or user). lS

Both the first and second bearings eliminate the need for substantial precision-machined metal housings to accommodate ball races thus permitting the tone arm and the base containing the vertical bearing to be reduced in size and weight. The weight of some tone arms intended for the audiophile market is so great that they may not be used on turntables wherein the turntable assembly is sited on a sprung sub chassis so as to isolate it from extraneous vibration. The size and weight of a tone arm in accordance with the invention may be such that it is suitable for use

on any turntable.

lo The simplicity and case of manufacture of the second bearing is apparent. It has been mentioned above that the second bearing would permit the ready interchange of different tone arms.

The first bearing calls only for the provision of suitably rigidly mounted flexible strips arranged in the vertical plane. Such flexible strips might be formed as part of or moulded into a plastic housing of suitable strength and rigidity. Such a moulded part might incorporate any or all of parts 18, 28, 34 and 36 mentioned above. In any such moulded part or parts provision could be readily made for mounting (by press fitting or by the use of suitable adhesive or otherwise) of either or both of the arm 12 and the spindle 40 or, of course, the spindle of any desired bearing arrangement that permits horizontal movement of the tone arm.

The mass and polar moment of inertia of a tone arm should ideally be matched to the characteristics of the pickup cartridge in use. 2s

Tone arms manufactured in accordance with the invention could be made at such low cost that a range of interchangeable arms could readily be offered for sale (for example in kit form or as spare parts) and thereby offer the audiophile the facility to use any type of pickup cartridge in an ideally suited tone arm.

It will be apparent that different materials will be suitable for use as the Mylar(l) strips 46, 48 provided that such strips meet the flexing function set out above. As a guide to this functional requirement, the following test was performed on the tone arm 10 described above to quantify what is meant by the flexibility of the material: a force equivalent to 7 milligrams under gravity (i.e. about 6.86X10-5N) - 19 applied either upwardly or downwardly at the position of the stylus causes an angular displacement of approximately ().5 . For the purposes of the experiment the counterweight was set so that there was zero tracking force i.e. the arm 12 was balanced about the vertical pivot point when the force was applied. Such different materials may comprise for example metal foil, a wide variety of plastics or any other material that can be arranged in use to flex parallel to a first substantially vertical plane, and be substantially inextensible in a second substantially vertical plane at the forces and frequencies exerted by the tone arm during use in that second plane. Such functionality may be obtained by a combination of different materials. Non-metallic lo materials are preferred however as these are less likely to suffer brittleness and work hardening through repeated flexing.

It will also be apparent that different materials will allow a variation in the dimensions of the exposed part 50 to achieve the same "'unctions. In particular a stiffer material will allow the length of the gap (corresponding to the vertically oriented dimension in use) to be increased, and vice versa. For a given material increasing the width of the exposed part 50 (corresponding to the horizontal dimension in use) will increase the resistance of the bearing to movement of the arm 12 in a vertical plane. For a given width and length of material, a stiffer material will permit the thickness of the material to be reduced.

If desired the position of the transverse bore 38 may be arranged to place the axis of the spindle 40 in the same vertical plane as the strips of Mylar 46, 48.

The first bearing 14 may comprise any number of strips of flexible material providing the function of the invention is maintained.

In another alternative the first cross piece and second cross piece may be in the form of one piece moulded from plastics with a flexible portion therebetween The shape and size of the l'irst cross piece 18 and second cross piece 34 is in no way critical, so long as they are sufficient strength and rigidity and provide suitable fixing surfaces for the strips.

Whilst the preferred embodiment has been described in relation to a moving - 20 coil type cartridge, the invention can be used with any type, for example magnetic, dynamic, piezo-electric, and capacitance.

As used in the appended claims "flexing" is intended to include bending and/or compression and/or extension of a material. - 21

Claims (37)

1. A tone arm for use with a counterweight, which tone arm comprises an arm and a first bearing having flexing means for permitting a pivoting movement of said arm in a substantially vertical plane in use.

2. A tone arm as claimed in claim 1, the arrangement being such that, in use, said first bearing inhibits translation of said arm.

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3. A tone arm as claimed in claim I or 2, the arrangement being such that, in use' said first bearing means inhibits rotation of said arm about its longitudinal axis.

4. A tone arm as claimed in claim 3, wherein said first bearing means is positioned substantially outside a body of said arm.

5. A tone arm as claimed in claim 4, wherein a part of said first bearing means is displaced from a substantially vertical plane containing the longitudinal axis of said arm.

6. A tone arm as claimed in claim 2, 3, 4 or 5, wherein said flexing means is flexible in a first sense to enable said pivoting movement, but is substantially inflexible in a second sense to provide said translation inhibiting and/or said rotation inhibiting.

7. A tone arm as claimed in claim 6, wherein in use, said flexing means bears the weight of said tone arm in said second sense.

8. A tone arm as claimed in any preceding claim, wherein said flexing means comprises a flexible material connectable between said arm and said first bearing, the arrangement being such that, in use, said pivoting of said arm causes a part of said material to flex.

9. A tone arm as claimed in claim 8, wherein said sheet is mounted between a first part of the first bearing and a second part of the first bearing leaving said part exposed to accommodate said flexing. - 22

10. A tone arm as claimed in claim 8 or 9, wherein a plane defined by said sheet in an unflexed condition is oriented substantially vertically in use.

11. A tone arm as claimed in claim I 0, wherein said plane is oriented substantially perpendicularly to a plane containing the longitudinal axis of said arm.

12. A tone arm as claimed in any of claims X to 1 1, further comprising at least two sheets of flexible material.

13. A tone arm as claimed in any preceding claim, further comprising a second bearing for accommodating rotation of said arm in a substantially horizontal plane in use.

14. A tone arm as claimed in claim 13, wherein said second bearing comprises a part having a first surface tor supporting the weight of the tone arm and a second surface for guiding said arm in rotation, wherein said second surface has an area greater than an area of said first surface.

15. A tone arm as claimed in claim 14, wherein said part comprises a spindle mountable in a sleeve.

16. A tone arm as claimed in claim 15, wherein said sleeve comprises a blind bore.

17. A tone arm as claimed in claim 16, further comprising a bearing means at a blind end of said blind bore, through which bearing means the weight of said tone arm is transferred to said sleeve and that assists rotation of said spindle in said sleeve in use.

18. A tone arm as claimed in claim 17, wherein said bearing means comprises a ball and said spindle comprises a substantially flat surface for abutment with said ball.

19. A tone arm as claimed in claim 17, wherein said bearing means comprises an - 23 angular contact ball bearing race and said spindle comprises a surface adapted for co operation with said angular contact ball bearing race.

20. A tone arm as claimed in any of claims 15 to 19, wherein said spindle has a s length of between approximately 30mm and 40mm.

21. A tone arm as claimed in any of claims IS to 20, wherein said spindle comprises a part of said first bearing means.

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22. A tone arm as claimed in any of claims IS to 21, wherein said spindle is removable from said sleeve whereby different tone arms may be supported in said sleeve.

23. A tone arm as claimed in any of claims 14 to 22, further comprising friction reducing means for facilitating movement of said second surface.

24. A tone arm as claimed in any preceding claim, wherein said friction reducing means comprises oil, low viscosity oil or other lubricant.

25. A tone arm as claimed in any preceding claim, wherein said arm is substantially rigid.

26. A tone arm as claimed in any preceding claim, wherein one end of said arm is adapted for mounting a counterweight and the opposite end of said arm is adapted for mounting a cartridge.

27. A tone arm as claimed in claim 26, further comprising a counterweight and/or cartridge.

28. For use with a tone arm as claimed in any preceding claim, a first bearing means having the first bearing means features of any of claims I to 12.

29. For use with a tone arm as claimed in any of claims 1 to 27, a second bearing means having the second bearing means features of any of claims 13 to 24. - 24

30. A turntable adapted for use with a tone arm as claimed in any of claims I to 27, which turntable comprises a sleeve for receiving a spindle part of the second bearing means.

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31. A method of manufacturing a tone arm, which method comprises the step of mounting on an arm a part of the first bearing means according to any of claims 1 to 12.

32. A method according to claim 31, wherein said part comprises a first cross lo member connectable to a second cross member by said flexing means.

33. A method according to claim 32, further comprising the step of mounting said second cross member on said first cross member.

34. A method according to claim 31, 32 or 33, further comprising the step of mounting a part of the second bearing means on said first bearing means.

35. A method according to claim 34, wherein said part comprises a spindle insertable into a sleeve that is mountable on a turntable or the like.

36. A tone arm substantially as hereinbefore described with reference to and as shown in Figs. I and 2 of the accompanying drawings.

37. A tone arm substantially as hereinbefore described with reference to and as shown in Figs. 3, 4 and 5 of the accompanying drawings.