Azimuth and zenith etc.

 Azimuth and zenith Where did these terms come from? And why?

I've never understood why a simple term like Horizontal Tracking Angle which is descriptive of the angle a cartridge makes to the groove has become "zenith" in some people's view.

Ok, maybe Horizontal Tracking Angle is a mouthful, so lets call it HTA. At least it means something relevant to cartridge set up.  Zenith is a meaningless term in this context. I suspect it was a term used by someone, a manufacturer or reviewer, maybe, looking to obfuscate, or distinguish their spiel from the usual obvious descriptors thereby associating a new term with their product or theory. 

The zenith is the point in the sky directly above you. Zenith angle is a term derived from this astronomical use, and used now, for example, in describing an angle which a line perpendicular to a solar panel makes with the zenith when the panel is pointing directly at the sun (when the sun is at the zenith ie overhead relative to the panel.  To use it with reference to cartridge alignment is total nonsense. 

So what's to blame for this? 

Well, someone decided that we needed a term to describe the angle, when viewed head on, that a cartridge makes with  the record surface, (or alternatively, the angle the stylus  is offset to the vertical due to misalignment in set up or manufacture.)  They called this angle  azimuth rather than, say, cartridge tilt angle or radial offset angle.  I suspect this was derived from tape head alignment use, where the tape head is angled with respect to the tape surface. 

To use the term for hifi use, the correct corresponding candidate would be HTA - the angle the cartridge makes with the groove, not the stylus or cartridge relative to a notional plane sited on a radius perpendicular to the record surface. However, we are now stuck with it. That doesn't mean we have to use the terminology instead of a more sensible and clearer terminology.

Which brings us back to zenith. Whoever decided on this was obviously intent on maintaining some sort of naming consistency with azimuth without any further thought. I suspect someone will coin altitude for SRA (Stylus Rake Angle)  or VTA  (Vertical Tracking Angle). 

Then all that's left is nadir, which is the point we will then have reached.

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

RP1-CR9

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) was 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

 Some pics of an arm that was on 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. 

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