What is a Unipivot?

 It might seem obvious that a true uni-pivot arm has only one pivot point. In one sense that is true, as an arm may have one pivot when it is floating with no vertical tracking force (VTF), but in use the arm cartridge combination has two pivots because the stylus is the other one. So in that sense the arm is really a uni-axis system, the line joining the pivot and stylus. Or another way of looking at it is that it is really two unipivots in series, arm pivot to cantilever bearing then cantilever to stylus. However, uni-pivot is what we are stuck with.

And that's when things get more complicated because a lot of people would prefer a uni-pivot to be a bit more like a "normal" tonearm. While usually admitting the freedom of movement is great and the stylus can then do its thing, the freedom for the axis to rotate makes people edgy for various reasons, mainly that "normal" tonearms don't do that.

Adding anti-skate at anywhere not on the stylus/pivot axis will reduce this rotational tendency, by providing a contraint on movement.  And adding a device in the form of an auxiliary bearing or a sliding surface will do the same, as will adding damping.  Some uni-pivots have filaments to constrain rotational movement. Some have the pivot point at an angle or even horizontal, with devices to retain them in place. Some have magnets which exert forces to stabilise the arm, or tension a wire.

None of these arms are uni-pivots. In reality they are constrained bearing designs like "normal" tonearms, able to move only in the horizontal or vertical plane. They use various methods to allow that movement while controlling (that is, constraining) rotational movement. Even damping fluid is like a frequency dependent bearing. It allows movement at some frequencies but not others.

In other words, they work just like any gimbal arm does.  If a traditional gimbal bearing arm had an an extra set of bearings to allow rotation of the armtube and cartridge, it would be a unipivot  as long as all the bearing axes coincided.

The bottom line is: if a uni-pivot can't rotate on its stylus to pivot axis, it's not a really a uni-pivot. It actually has a number of extra bearings. But as long as there's a big and obvious sharp point that's usually enough for the publicity material.

What is Zero Offset?

 

There have been a few recent murmurings regarding zero offset tonearms, together with related talk about the advantages this brings regarding the issue of anti-skate.

So, what does zero offset mean? Put simply, the zero offset concept refers to a tonearm with the cartridge mounted such that the stylus and cantilever, (and usually the headshell and armtube) form a straight line with the arm pivot. 

This contrasts with a typical arm in which the cartridge is offset at an angle to a line from pivot to stylus which, combined with overhang, maintains the angle between the cantilever/stylus and groove (the Horizontal Tracking Error or HTE) to within a couple of degrees at all points in the arc. This angle (HTE) should not be confused with the angle between the Pivot, the Stylus, and the Groove tangent (let's call that angle PSG) which is what leads to skating forces. In a conventional arm this angle PSG is around 23 degrees.

For example, the classic DJ turntable has the zero offset characteristic, as does a linear tracking (tangential) tonearm, with its arm tube at 90 degrees to the groove radius.

The difference between these two cases is that while both have zero offset cartridges, only the linear tracker has zero overhang at all points of its travel because it tracks a radius. The DJ arm, and any other similar pivoted design,  like the ViV Lab, may have zero offset,  but, because they are pivoted, they must move through an arc. If that arc intersects the spindle, the arm has zero overhang. Otherwise it has either positive overhang like most pivoted arms, or negative overhang, i.e. underhang. 

Most zero offset arms are set up to have underhang so that there is a point on the arc where the armtube is at 90 degrees to the spindle, and the stylus is sitting on a radius. This a null point. 

It is important to note that it is a null because the horizontal tracking error (HTE) is zero - the stylus and cantilever are tangent to the groove (as it is at both nulls in a conventional arm). This is also the only point at which the angle between the pivot, the stylus, and the groove tangent is zero (angle PSG) and it is dependent on where the arm is mounted. There are no skating forces at this point, unlike at the nulls in a conventional arm where the angle PSG is typically still over 20 degrees.

In other words, unlike a linear tracker, as the cartridge moves away from this single null, the angle between the pivot, the stylus, and the  groove tangent  (angle PSG) increases, and this leads to an increase in skating force. It also leads to an increase in angle HTE which increases distortion.

So as the arm swings across the record, it describes an arc of around 30 degrees. If the null is around 70mm from the spindle, then there will be an increase in skating force and tracking error as the stylus moves away, both outwards and inwards from this point, and the error will be more at the outer edge than at the runout.

Therefore, because skating forces are generated by the PSG angle not the headshell offset or lack of it, then there will be forces acting on the stylus which will try to rotate the arm. On a straight arm  with underhang this will mean that the arm has a force acting on it inwards or outwards at points on the arc depending on where the null is positioned. 

While this error (the PSG angle) is less than with conventional geometry, at the outer edge it is still around 20 degrees. The horizontal tracking error (HTE) will also increase to this level rather than the 2 degrees or less of the conventional arm.

This means firstly, a zero offset arm will need less skating compensation than a typical arm with overhang but it is incorrect to say it has zero skating forces. Secondly, it will generally have more distortion at the beginning and end of side than a typical arm. Thirdly, as with any arm, there will be less error as effective length increases. Fourthly, a spherical stylus may work better than a fine line stylus. 

You could experiment with zero offset if you have an arm with a detachable headshell, You can buy a cheap replacement headshell and drill new holes or extend the slots such that the cartridge can be mounted in line with the pivot. You may have to make or adapt an arm board to enable the arm to be positioned to obtain a null with the stylus at a radius of around 70mm, though this isn't super critical given you are accepting the additional distortion that comes with this set up as a trade off with a reduction in antiskate.

To calculate the mounting distance for a zero offset arm, with any chosen null, square the arm's effective length, add the square of the null, then take the square root of that sum.

I'm no longer doing repairs and refurbishments

 Just a note to say that as of now I will no longer be doing any rewires, repairs or refurbishments of tonearms.

I am happy to supply spares if I have them, and information should it be requested (eg drawings) and, of course, I'm always happy to hear from RP1 owners.

Best wishes,

John

Gold signature No. 015

 

 

Up for sale on ebay (mounted on a Source turntable) Odyssey Gold Signature number 015 from 1992.

It looks in excellent condition with a gemstone on the headshell, an amethyst. 

It is a 9" effective length. 

More info on the Gold Signature here

And on the Source here

More pics to follow.

Chrome RP1-xg (RP1-Cr12)

Here are some photos of the RP1-Cr12. 

As a standard J shape, with fixed block headshell, like this:

or with SME style headshell connector, like this:

Some ended up like this, with a 9" armtube:

Or like this, with a strange S-shape 12" tube (which allowed using the original headshell: it was easier to bend the tube than make a new headshell with a different offset angle. Plus it looked kind of neat.)

These were the last arms of the original RP1 design. Now the only thing I'll be doing is servicing existing arms. Any future new arms would be a revision of the RP1 or even a new design completely, assuming I ever were to do that.
 

So it's finally, definitely, maybe, the end of the line for the old Odyssey RP1 - perhaps...!

But, as always, please let me know of any used arms which might be for sale, or if you wish to buy an arm, or are looking to have one repaired or modified, and I'll see if I can help. There may be arms available via sites like ebay or audiogon and other forums. I may know of people looking to sell (although that doesn't happen frequently, if at all).

Email odysseytonearms at gmail dot com.

RP1-XG on ebay

 For sale on ebay in the USA:

Someone has made a nice box with some perspex supports for an RP1-XG. The arm looks like it has been rewired, probably by me, as it has the straight through wiring. But I have no more info than that. Its manufacture dates from the Odyssey Engineering period, before Source-Odyssey. 

The ad can be found at 

https://www.ebay.co.uk/itm/325193056057?hash=item4bb7043f39:g:QG0AAOSw2Lpig9Qn

RP1 on a Micro Seiki DQX 1000

 A lovely photo of an RP1-xg2 on a Micro-Seiki. And the LP is the Verve label. What's not to like? Thanks to Gonzalo.

 

Latest Update

As mentioned previously, I was in the throes of renovation and redecoration of our flat here in Glasgow and converting the attic into a study/workshop space.

That has now come to an end, but I have decided that I will not be making any more tonearms. I am still open to do repairs depending on what they entail, and rewires or other servicing. 

I shall continue to post occasional photos as they turn up and add tonearm information as and when I remember it (or get round to it).

I will always be open to give help and advice on arms and turntables, so do get in touch with any questions.

Email address remains as in the contact details.

Meantime enjoy your music,
John

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 even 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.

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 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 can be 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, it needs 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 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.