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 the arm is really a uni-axis system,  pivoting along the line joining the pivot and stylus. This axis can move freely in all planes. 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 the 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 movement in the rotational frame, and some have the pivot point at an angle or even horizontal with devices to retain them in place.

None of these 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. 

In other words, just like any gimbal arm does, often using similar types of cup and cone bearing to the traditional uni-pivot. 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 if 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 uni-pivot. It actually has a number of extra bearing surfaces, sometimes four or more. 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 the advantages of zero offset tonearms, together with related talk about the advantages this brings regarding the issue of anti-skate compensation.

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 spindle. 

For example, the classic DJ turntable has this characteristic, as does a linear tracking 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 so that there is a point on the arc where the armtube is at 90 degrees to the spindle, which means the stylus is sitting on a radius. This a null point. The stylus and cantilever are tangent to the groove at this point and only this point, and it is dependent on where the arm is mounted.

In other words, as the cartridge moves away from this single null, the angle between the pivot, the stylus, and the  groove tangent  increases (let's call it angle PSG), and instead of zero overhang like a linear tracker, this leads to underhang.

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 tracking error as the stylus moves away from this point and the error will be more at the outer edge than at the runout.  The stylus will underhang as opposed to overhang the spindle.

Because antiskate forces are generated by the PSG angle not the cartridge offset, 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.

This means firstly, i t will need less compensation as tonearm length increases. Secondly , it will need less compensation than a typical arm with overhang. Thirdly, it will have more distortion at the beginning and end of side than a typical arm. Fourthly, a spherical stylus may work better than a fine line stylus.

 

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

RP1-CR9 for sale

Here's an arm which was custom made in 2016 by me.

It's a 9" chrome and satin aluminium model based on the RP1-CR style, and . I have some pictures from when it was being packed to send to the owner:

There were very few like this. Actually fewer than the Gold Signatures.

The pic below shows it in the process of being packed in its box.

There is a post I blogged here

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.