Bearings: Unipivots, Ballraces and Knife Edges

Historically, arms mostly used a plain bearing for lateral movement, with needle and cup for vertical. This is typified in arms fitted to portable record players. Better decks had small ball races for the vertical movement. Later, most arms used ball races for both, such as many Japanese arms like FR,  and arms by Breuer, SME, Mission, Zeta, Triplanar, and others.  Some arms used watch-like jewel bearings, like the Technics EPA-100.

Decca used a unipivot in the arm they designed to complement their cartridges. Hadcock,  Transcriptors, and Keith Monks were others using this type. The Hadcock used a small three-ball angular contact bearing.

SME, of course, used a knife edge for the vertical movement in their original design as did SAEC in Japan. And Origin Live use a dual pivot (two unipivots) for vertical movement.

There are  also arms like the Well Tempered and Schroeder which use a filament to suspend the arm.
If you register on the brilliant Vinyl Engine site you can browse the huge database of arms in the library and use the excellent calculators.

So what are the advantages and disadvantages of each type? It might seem that one or other design is best, but it is never so straightforward. Here are just a couple of cases.

Let's look at  bearing friction. Unipivots and suspended arms generally have the lowest friction in the vertical and horizontal planes, followed by knife edged bearings or dual pivots (which have two unipivot bearings for vertical movement, kind of like two super narrow knife edges which are constrained).  Cup and cone and small ballraces are next. Typically, unipivots require only a few milligrams of force to move them, and most good quality arms need less than 25mg. The Technics EPA-100 with the jewel bearings was claimed to need around 5mg.

So this would appear to be a non-issue if such tiny forces are involved, only 1% or so of tracking force in the worst case. In fact, you could say friction might be a good thing as it adds damping, and damping is often seen in unipivot and other types of arm, although damping should be adjustable and evenly applied across the record and is probably best applied separately, eg by altering the viscosity in a damping bath. Friction in the bearing is not so easily adjusted.  But things are never straightforward.

Typically, when measuring friction, the difference is quite marked between the force needed to start something moving and the force needed to keep it going once it has started (static versus dynamic friction).  Tonearm bearings usually need almost the same force to start as to keep moving. But if a bearing is notchy or sticky in places this difference is increased dramatically. In the vertical plane this leads to variations in downforce, and, horizontaly, to variations in antiskate.

This is where the low friction unipivot/dual pivot has an advantage. It is not susceptible to notching. However, low friction can be one of its disadvantages because any force acting on it might also move it. This I discovered with my RP5, and is typified by the effect of the wiring on many unipivot designs, where the torque exerted by the wiring acts to turn, twist or push the arm in varying directions as it moves through its arc. The closer the wires are to the pivot and the finer they are, the better (as in any arm). Having the wires exit at a distance from the pivot adds leverage to any springiness in the wires and adversely affects tracking.  More on testing vertical friction here.

The knife edge is also almost friction free. Its disadvantage, most easily seen in the Goldring arm as fitted to the GL75, is that there is a substantial torque exerted on the bearing when the arm is rotated. In the GL75 the bearing carriers are prone to damage. Knife edge bearings should be made of hard material. They were optimised in chemical balances where they remain static. Like the dual pivot, they need to be balanced such that there is equal loading on each side of the bearing.

Both the above designs are often criticised because they don't feel as solid or as rigidly mounted as an arm with ball (or cup & cone) bearings for the vertical movment. However, if you think about it, it is only in the matter of rotational freedom that this perception occurs. 

The reason for this perception is the typical crude test for bearings, which is to try and twist the arm to detect play. But if there is no play, the bearing must be loaded and therefore subject to some friction. This lack of play is both a good and bad thing, because an overloaded bearing dramatically increases friction. 

For a given weight of arm, the unipivot load is simply that weight. It acts on what is a very small radius, perhaps 0.05mm, or less, so that the friction has little effect. In a ballrace bearing design the radius is perhaps 3mm and there is the additional preload of the bearings. Some designs such as Technics, have low friction due to the bearing radius being small and the loading low. 

 While low friction is desirable, smoothness of movement is also important, ie the variation in friction. In ball races, the design of the bearing and the way the balls are held contibutes to this. 

However, all the advantages of good bearings are lost if the wiring of the arm interferes with its movement.

A sliding SME base

 Here are photos, kindly sent by Chris, of his RP1 on his Thorens.
It is using an SME adapter, ostensibly for Ortofon arms, which comes from Hong Kong.
The ebay link is here. I have also seen them advertised in clear anodising (silver finish). Another black option is here

It looks good and isn't too expensive. As a finishing touch, an option is to paint the silver knob black, or get a gold plated one.

Arm/Cartridge Resonance.

A quick note on arm/cartridge resonant frequencies:

When the mass of the arm (as seen by the cartridge at the headshell) reacts to the springiness  of the cantilever, there is a low frequency resonance. This can give rise to vertical, horizontal and, in the case of unipivots, rotational (about the armtube axis) oscillations.

This of itself is not necessarily problematic. Much depends on the amount of energy that is in the system. If the resonant frequency is within  a low energy range of frequencies, it is unlikely to be excited. Generally this means a frequency below the lowest audio signal either on the record  or in the air, and above the highest frequency transmitted through the turntable suspension or support.

Record warps range from 0.5Hz to approximately 8Hz, depending on their nature, and, depending on arm design, mainly affect vertical resonances, while record eccentricity affects the horizontal.

Generally, record warps are the most obvious problem source if an arm cartridge combo has an excessively low resonant frequency,  easily seen in excessive woofer movement, but also most easily sorted - through not playing warped records, or using some form of clamp to reduce the amplitude of the warp.

More subtle are resonances induced by low amplitude ripples on the record surface, suspension harmonics or large bass signals, which can affect bass response and imaging, especially with some unipivots.

The three resonant modes are not necesarily the same. The horizontal usually being the lowest, although with unipivots the rotational mode frequency will depend on the distribution of mass about the pivot. Also the resonance  characteristics will be more or less peaky depending on arm features and design, which affect how resonances are damped.

The turntable design is also important. If it is a sprung suspension design, then it has its own built-in isolation from structure borne vibration. If it is a solid plinth design, then it is prone to transmitting vibration from its support unless mounted on an isolation platform or other form of energy absorber (which effectively turns it into a sprung suspension design).   Cones are principally stability devices and transfer vibration in both directions.

RP1 on a Lenco

Here's a pic of Nick's arm on his re-plinthed Lenco with double platter.

RP1-xg2 Tonearms

Here's a nice photo of an arm that was on ebay. Check out the HiFi Hangar in Hampshire.
They specialise in vintage stuff and have an archive gallery of arms and other gear.

This was another on ebay,  in Italy:

 

Science and Objectivity

Scientists and Objectivity

"Scientists should be objective. If everyone approached things by scientific method we'd get somewhere." Peter Baxendall.

The problem here is that if everyone was totally objective about everything then there would be no new ideas because ideas and theories are subjective items. They don't exist in the objective world until someone thinks them up and speaks or writes about them.

There are no "laws of nature" until someone comes up with them, and they last only as long as it takes someone else to come up with new ones. The old ones are then ignored, forgotten if they conflict with the new, or are incorporated into the body of knowledge if they don't.

Science is not objective; science is subjective, first and foremost. And throughout the ages it has suffered because scientists could not accept new ideas. They could accept new facts, and fit them to existing theories somehow, but accepting new ideas was difficult. That is caused by a lack of confidence. Not confidence about facts or theories, but self-confidence and being able to be wrong.

Many people (of both the scientific and non-scientific persuasion) are intent on getting the facts, by which they often mean as getting the right answer to a problem. But this can lead to a state of mind which tends to rely primarily on the facts that they themselves have discovered, which they feel allow them to promote theories to explain phenomena as being some kind of final statement, or at least a stepping stone to The Truth.

What is really necessary is to be able to produce a theory which fits the facts and yet still be able to accept it is wrong. In other words, have confidence that you are wrong even though your solution appears to be the correct one.

Unfortunately there are those who, because their theories fit the facts, assume they are right. And because they are confident this is so, they then have to endure a conflict within themselves when either the facts change, or a new theory, which contradicts their existing one, also fits the facts.

Hifi is a classic case of the push and pull between subjectivity and objectivity.  Advocates of various good sounding pieces of equipment often assume that this is due to a particular feature even though other items without that feature sound good to others, despite apparently inferior objective measurements, thereby leading to conflict such as that between belt and direct drive enthusiasts.

Which leads us back to subjectivity again, so really we don't get anywhere at all when value judgements are involved, as in the anti-skate debate. In that case, there are facts and opinions, and the conflict arises because the facts appear to contradict the opinions. What happens then is that some people hold that, because of the facts, opinions on sound quality must be wrong. Others hold that because of their opinions regarding sound quality, the facts are irrelevant.

It's all very like politics, where politicians love to think they are dispensers of the The Truth, and are happy to quote facts when its suits them and condemn Fake News when it suits them.

The reality is that facts change with new information,  and all news is fake, given that the complete, whole story is never told. The best you can do is find out as much as you can for yourself and make up your own mind.

Personally, I just let people have their opinions regarding whether or not something sounds good. There are no absolutes, not in hifi nor in life.

RP1-XG Mk 1 Instructions

RP1-XG Mk 1 instructions, for those who have got an early arm and lost theirs, or bought one without any set up details.

 Odyssey Precision Tonearm:  model RP1-XG


Fitting Instructions

Your Odyssey tonearm should at all times be treated with care. On no account should any parts be forced or moved with excess pressure, especially where the bearings may be damaged.
If you have a problem or a question regarding the operation of your tonearm, contact John Gordon who will be only too pleased to offer help and advice.
Parts can be kept clean of fingermarks by wiping gently with a soft cloth or cotton bud which may be dampened with pure alcohol or pure petroleum spirit (cigarette lighter fuel). Liquid metal polishes should be avoided.

You may care to have your tonearm installed by a technician to ensure it is correctly set up. He will be pleased to fit the tonearm and install your cartridge (but, unfortunately, usually only if you pay him...)
If you arrange for your tech to fit the tonearm, please omit the instructions concerning installation and concentrate on those concerned with adjustments.

Please only adjust or unscrew fasteners which are mentioned as being adjustable. Do not attempt to unscrew any others or damage may result to the arm and a repair may be costly.

Before unpacking the tonearm, read through these instructions and refer to the diagrams. In order to make yourself familiar with the name of each part and where it is located, the General Arrangement Drawing (figure 1) and its list of part numbers should be studied carefully.

Carefully unpack the tonearm. Take care not to lose any parts. You should have the following additional parts and assemblies: counterweight & sleeve, supplementary counterweight, tools, base plate, lifter assembly, cartridge fasteners & fingerlift and base fasteners.
2.    To fit the arm to your turntable you will require an adaptor plate or arm board with the standard SME cut out and fixing centres, which can be supplied by your dealer. Alternatively, for the 9" arm, a 35mm diameter hole can be drilled in the arm board at a radius of 215mm from the spindle. The base plate  should therefore be used to mark the position of the 4 base fixing screws for drilling.
3.    Make sure you position the arm so that it does not overhang the front or rear of the deck or that the lid does not hit it.
4.    To detach the base from the main pillar assembly, loosen the two screws on the rectangular base plate.
5.    The tonearm base plate (16) should then be firmly fixed to the turntable arm board. Use four 25mm x M2.5 (or 6BA) screws with washers and nuts. The base plate should be installed such that the logo and name face towards the front. The eccentric base (14) main pillar (6) assembly can now be fitted to the base. Do not fit the phono leads at this stage.
6.    When handling the main pillar assembly try to avoid unnecessary rotational movement and in no circumstances allow up and down or side to side rotation of more than half a turn in order to avoid possible bearing damage or damage to internal wiring.
7.    Ensure the stylus guard is fitted to your cartridge to protect the stylus, install the cartridge under the fixed headshell (2) using two 25mm or two 19mm screws. In fitting the cartridge connectors it may be necessary to use small pliers to positively grip the connector tag and avoid straining the wire.
8.    Then clip the arm tube into the arm rest (8).
9.    The counterweights (7) normally supplied with the 9" arm are 80g, 60g, with a supplementary weight of 40g..
10.    Items like heavy or lightweight fixing screws, may alter the choice of counterweight.
11.    Select suitable counterweights. Slide the weight onto the sleeve and temporarily lock in position with the screw, then fit the sleeve onto the arm/counterweight support (4).
12.    Balance out the arm, until it is floating above the platter and set the tracking force using a stylus balance. A rough adjustment can be made by moving the weight on the sleeve, and a fine adjustment by rotating the sleeve and counterweight.
13.    Then remove the stylus guard and fine tune the tracking force.
14.    Do not allow the stylus to rest on the platter unless the tracking force has been set within the manufacturer's recommended range.
15.    The lifter (9) should be fitted and its height adjusted with the screw. Note that if the arm has been stored or transported in cold weather, the lifter will need to be operated a number of times to eliminate any air locks and ensure it is working correctly.
16.    Check that when viewed from the front the stylus is perpendicular to the record surface. This may be adjusted by loosening the screw  at the top of the arm counterweight support, and rotating the arm tube until the correct orientation is obtained, then tightening the screw.
17.    To initially set the horizontal cartridge alignment you will need an alignment protractor of your preferred geometry, but not an arc. First loosen the screws on the rectangular base plate then rotate the offset base (14) to move the whole arm assembly towards or away from the turntable centre such that the stylus can align with the grids on the protractor which you have  mounted on the turntable platter. When the stylus is resting on the marked points, the cartridge body should be square to the grid lines. 
18.    You may also have to angle the cartridge in the headshell to do this. If this is the case, use the base adjustment until the error angle is equal at both null points, then rotate the cartridge slightly to eliminate the error. With the adjustment complete, tighten the screws.
19.    There are a number of sophisticated set up devices available to fine tune this parameter, and it is worth looking at the posts on the Odyssey blog to help familiarise yourself with the concepts. Or just leave it to a dealer or knowledgeable friend.
20.    The arm height (VTA/SRA) can be varied by loosening the screw (13) on the eccentric base (14) and then raising or lowering the main pillar (6). When the desired VTA is obtained the screw should be tightened. A recommended setting is with the arm tube level, or sloping slightly down towards the headshell by 1-2 mm.
21.    The lifter height may require adjustment after the VTA has been altered.
22.    The position of the arm when at rest can also be adjusted while adjusting VTA by loosening screw (13) and rotating the pillar and arm rest (8) to the desired position. The rubber O ring on later models can be positioned to constrain the pillar and prevent a sudden drop.
23.    Set the bias force by screwing the bias weights (11) to the required setting, then locking them by turning them in opposite directions towards one another. A test record can be useful for this, failing which set the weights in the centre of the screw as a default.
24.    The point of maximum effect of the bias force can be varied by loosening screw on the bias mount and moving the bias assembly in or out. The recommended setting is such that the arm carrying the bias weights is horizontal when the stylus is tracking the inner grooves of the record.
25.    Fit your phono leads to the DIN socket on the underside of the main pillar and ensure that they do not cause any drag on the turntable suspension. Check that any clamps or clips on the turntable for the purpose of holding the phono leads are securely tightened.
26.    Check that the turntable and armboard are level and the turntable is free to move on its suspension, if it is of that type, and that all screws are tight.
27.    The separate earth lead which is normally wired together with the phono leads should be connected to the earth terminal on the pre-amplifier unless this creates an earth loop and causes hum. Depending on the turntable and cartridge earthing arrangements, an earth wire may be required from arm pillar or turntable chassis.
28.    Having set up your system to your satisfaction, sit back and forget about it and enjoy the music.







Remember: if you have any questions regarding your RP1 you can email:
odysseytonearms@gmail.com
You can also find more information on the website:
www.odysseytonearms.blogspot.com

Or telephone:
(+44) (0) 7948 215899      mobile



John C Gordon

Odyssey Mounting Dimensions

ODYSSEY RP1 - xg

Mounting Dimensions

The RP1 has an adjustable base, which allows the distance from the centre of the turntable spindle to the horizontal arm pivot axis to be varied. This compensates for variations in cartridge cantilever length and position, which change the effective length, and thus the offset angle, which necessitates an adjustment of mounting distance.

The notional geometry is Lofgren A DIN, (nulls at 63.5mm and 119.5mm) but Lofgren A IEC (nulls at 66mm and 121mm)  or Stevenson can be accommodated. The difference in angle is tiny, and so, due to the clearance between the cartridge screws and the mounting holes in the headshell and cartridge, there is normally enough adjustment to get the appropriate offset angle. If the new headshell is fitted, then setting the offset to any angle is simplified.

When mounting the arms, a 35mm diameter hole should be drilled in the arm board such that its centre is approximately 215mm (for the standard arm) or 295mm (for the long arm) from the centre of the turntable spindle. A spacer made from card can also be used: it should be 200mm long and placed between the spindle and the arm pillar. This 200mm distance is not critical as there is ample adjustment to obtain the appropriate pivot to spindle distance for any cartridge. The base can then be orientated

The base plate can then be positioned to suit the arm board position and also to allow for the maximum adjustment range of the offset pillar mount together with easiest access to the adjustment screws, either facing towards the centre as per SME, or parallel to the arm-rest. The 3mm diameter holes for the mounting screws can then be marked for drilling.

Alternatively, a standard SME arm board can be used, or any SME template, centred at 215mm (for the 9" arm), or 295mm (for a 12" arm), and rotated to the desired alignment. The slot can be replaced by a 35mm diameter hole.

Because the arm has the facility to adjust pivot to spindle distance at the base, the initial positioning is non-critical. See the article on SME geometry  for more information. An arc protractor should not be used, as the effective length is nominal and depends on the distance from the cartridge mounting holes to the stylus. Adjustment at the base allows for this.

The standard arm:
The standard arm is designed as a nominal 9" (230mm) effective length arm.

The long arm:
The long arm-tube options are now usually at a nominal effective length of 12"
(308mm) with a mounting distance of around 295mm and an offset of around 17.5 degrees. Custom effective lengths can also be made to order, and will require appropriate mounting distances.

Odyssey RP1-XG II In original packaging

Some pics from an old ebay ad showing the original packaging for the first revision of the RP1-XG.

Update

We are now settled in Glasgow's West End,  and are in the throes of re-decorating and house stuff.
Apart from  kitchen, bathroom, and the usual renovations to an old flat, I was converting the attic into a study/workshop space.
However I have not been in the best of health, and it currently looks like I shall now be occupied with all this well into 2018. Then perhaps I will be able to undertake some tonearm work again. But do get in touch with any questions.

Email address remains as in the contact details.

Meantime enjoy your music,
John