Tonearm - straight or curved

[Holst]

Take a deep breath … Apparently, there’s a bit of more complex geometry than the easily visualized effective length/swing arc going on here. While swing arcs of both type pivoting tonearms of equal effective lengths are identical. The tangential relationship of the stylus to the groove (that ever changing axis off 90 degrees as the record tracks across the record) is influenced whether its offset (cant) is at the tube end or back 4 to 5 inches.

Take another deep breath… This, in turn affects, not only the tracking behavior of the arms but also each of the two type’s dependency on anti-skate. The straight arm needing much less than the S-curved offset (Rega users will attest to this). Some have it that a 12” straight-arm can perform well enough (with a spherical stylus) to dispense with any skate compensation completely.

Now, I started this with “apparently” because I am spatially challenged and I need wood mock-ups, compass arcs and not words to visually anything beyond the simple plane geometry. So I’m not convinced about this, nor am I, in any way, prepared to defend it. But this is out there and I’m trying to keep my mind open it. I’ve asked for illustrations.

- Mario

I had a book once, in the 70's, that was basically geometry for Audio salemen. It was put out by Pioneer. Thus said, a tonearm with a pivot at said right angle, as in restorer-john's example would be pushed to the right, pivot's eye view on a straight arm. On an "S" arm or an "L" arm the stylus would be pulled forward across the record (Picture it in your mind, I don't have the graphics anymore) thereby negating or lessing the need for antiskating devices.

Damn, this hurts my brain.

Tonearms are nice, records are good, me like tonearms and records.

Records, records, records......

There, I feel better.

This again was used as a sales tool, by Pioneer, who sold turntables, with "S" shaped arms, that had antiskate devices. Did I mention the PL-71 was pretty. We had one in the store, "S" shaped arm. I took home the Technics SL1100a, I got a discount for working there, it had an "S" shaped arm.

The headache is going away.

As restorer-john said, the "S" isn't just to look pretty, I've always really likes "S" arms. I'd like to own one again.

The SME vertical axis (SME 3009 SII) is approximately 30 degrees displaced from perpendicular. Even with such grossly misaligned bearings, the effect on azimuth due to a change in stylus height of 1mm is less than 0.25 degrees. While this effect may be worthy of consideration when choosing between tonearms using aligned or misaligned bearings, the effect on azimuth of an alignment error of a few degrees (which would be typical of selecting an alternative linear offset to the recommended value) is absolutely negligible.

But it is very obvious that other types of arms didn't just happen either. Somebody put a lot of thought into it.

If it works, it's good.

The SME 3009 SII is pretty too.

 

[jeffn]

my 2cents.
The stylus can't know what shape arm is connecting it to the pivot. The tonearm could be wrapped in knots and it wouldn't matter.

BUT I'd think that a straight arm could suffer more from resonant frequency problems than an odd shaped arm (curved or otherwise).

 

[goldear]

I remember reading an article on this subject more than 20 years ago. If I am remembering properly (no guarantee there, I'm afraid), the S shape was designed to mimize the amount of azimuth error throughout the swing of the arm.

All S and J shaped arms are designed to be perfectly tangent at two points within the arc (if the cartridge is alligned properly). But the number of degrees of error between those two tangential points is slightly greater with the J, than it is with the S.

That said; great arms can take on all shapes, and other factors in the design really seem to make a much bigger difference to arms sound than does this one

Holst: Great diagram :thmbsp:, even if it didn't quite answer the question at hand.

 

[Mopic5]

GoldEar wrote:

"That said; great arms can take on all shapes, and other factors in the design really seem to make a much bigger difference to arms sound than does this one..."

I completely agree with you there, GE.
If, at some point, we were to accept that stylus tracking or skating were in some part affected by their offset geometry - (something unproved and undemonstrated in this thread up to this point) - then it's possible that "other factors in design" might be considered good tweaks to those offsets. And this doesn't even broach that part science, part mystical art of cartridge matching and whether this factors into it along with mass compliance.
The second shoe to drop here, which somebody just as curious as SailOne - the opener of this thread - is bound to ask is, "Conical, spherical, elliptical and Shibata... Why so many stylus tip designs and which works best with which arm?"

Now my head is spinning.
"Gee Mr. Wizard, which cartridge are you going to hook up to that EMT today?"
"Doesn't really matter Timmy, just sit back and catch this riff from Mr. Bojangles."

 

[restorer-john]

I think when you are working with a system that is inherently so flawed and full of variables, all these innovations to solve 'issues' often end up creating or making obvious hitherto obscured characteristics. Let's face it, the LP is an ancient technology, but romantic in its ability to beguile- partly because of the wonderment in seeing a piece of plastic slowly turning with an industrial diamond trundling accross the surface and producing a creditable facsimile of the original recording. I enjoy LP's for what they are and the turntables of yore are fun to play with, regardless of the arm they use. My preference is 's' as I have heaps of headshells and cartridges to swap around.:thmbsp:

 

[Mopic5]

Well said, John!

 

[ozmoid]

"Conical, spherical, elliptical and Shibata... Why so many stylus tip designs and which works best with which arm?"

If you accept that the alignment of the stylus is slightly "out" as the tonearm travels through its arc ("twisting" in the groove), then geometry would indicate a uniformly conical stylus would be best, and give the most consistent performance, as the stylus itself would be unaffected by the horizontal "twist" of the headshell in relation to the groove.

In reality, the conical shape seems to be at the bottom of the pile.

 

[Nat]

The reason for arm shapes is not geometry -- alignment of the stylus is completely determined by offset angle and by length, both of which are independent of shape. (That description of non skip arms is only talking about those horrendous DJ tables that don't have any offset -- by straight arm they actually mean straight, with the cartridge in line with the arm, which creates pretty significant tracking error.)
The reason for S shaped arms is twofold: given the need for the cartridge being offset, it may be easier to create a strong joint between headshell and arm if the arm comes into the headshell straight on (this is particularly obvious with the SME/japanese headshell arms, but its also arms like the Alphason). And bending a tube makes it less prone to vibrating like a reed, so the curves may lead to better resonant behavior.
On the other hand, the extra length required by the bend adds to the mass of the arm, and this creates other resonance problems, principally that of the interaction with the compliance of the cartridge suspension. So particularly for high compliance cartridges a straight arm has real advantages.
The bearings is either type of arm can be set up at any angle the manufacturer wants -- usually they are set up to be perpendicular to the off set angle so that when the arm moves up or down, the cartridge maintains the same azimuth, but sometimes arms have the bearings perpendicular to the the arm itself -- this doesn't make sense to me.
The other consideration is simply a practical one -- a lot of specialist arms are made in small numbers, often by very small companies, and not having to have a bending jig for arm tubes is a savings, allowing more spending on some other aspect of the arm.

 

[REDone]

Thanks Nat .. we are getting somewhere .. So Why does the offset angle vary between manuf's arms ? is this caused by variation in arm length or by a different set of calcs & equations?
I would have thought maths would have arrived at a constant unvariable angle

Anyone know how to calculate the offset angle required for a given arm length?

 

[energyandair]

Hello everyone, I have this question gnawing at me regarding the merits of tonearm shape. I see that some TTs have an s-shaped arm and some have a staight arm. WHY??? What's the difference???

I did a search for some answers here, but could not find anything relevant, so if there is already a thread on the subject it would be great for someone to point me to it and then everyone can just ignore me.

Thanks for your help and collective knowledge.

As others have said, good and bad arms have been made with both straight and curved arms and other factors such as bearing quality and how well the particular design is executed are far more important.

With a straight arm if all things are equal (and they never are), it is possible to achieve more rigidity for a given weight and, manufacturing is a bit less complex and hence a bit less costly.

With a curved arm, it is possible to set up the headshell so that it can be tilted from side to side to adjust for azimuth with less effect on the rest of the set-up than would be the case with a straight arm. (this adjustability is however only provided on relatively few arms)

Most arms have gimbal bearings. For these, the center line of the bearings attached to the arm itself should ideally be parallel to a line between the center of the record and the stylus but this can be done (and usually is) with either straight or curved arms.

If an arm has a monopoint bearing or is suspended, then it has to be balanced laterally (or it will tip to one side). This is easier to do if the arm is straight and all such arms that I have seen have been straight. Once set up this way, then arm movement should be fine.

My guess is that for most (but not all) arms, fashion has been the dominant consideration in deciding whether it should be straight or curved.

If you want to learn a lot about tonearm geometry, do a search of Vinyl Asylum for posts by John Elison and Brian Kearns.

David

 

[thisOne]

GoldEar wrote:
The second shoe to drop here, which somebody just as curious as SailOne - the opener of this thread - is bound to ask is, "Conical, spherical, elliptical and Shibata... Why so many stylus tip designs and which works best with which arm?"
QUOTE]

First of all this is turning out to be a fascinating discussion and I'm really excited that, as a relative newbee I finally asked a question of interest to someone...

Secondly, I like the new moniker Gold Ear came up for me, I wish I thought of it when I joined AK, since sailing is my other expensive hobby...

Thirdly, the question I would like to throw in the mix is: whch arm is easier to set, and align, etc

I actually asked the original question because I got myself a Marantz 6370Q, which came damaged in shipping and was supposedly "fixed" by a tech I paid good money to (I'm electronically challenged, but that tech is dead to me now...). My TT was just not tracking the qroove (skipping all over even on new records). I spent days making all kinds of alignments till I finally took apart the pivot mechanism and discovered that if I raised it slightly everything fell in place and the TT magically played perfectly (I still don't know what I did, but I'm not touching it, except to play a record).

So back to my second question, which shape arm is easier to set up, in your collective opinion...

Again, Great Thread, and fascinating discussion!!! Thanks

 

[vinylengine]

So Why does the offset angle vary between manuf's arms ? is this caused by variation in arm length or by a different set of calcs & equations?

Once you've decided on the null points you want to align to (and manufacturers have used many different points over the years), the effective length of the tonearm will determine the overhang and offset angle (not to be confused with the angle of the headshell).

Anyone know how to calculate the offset angle required for a given arm length?

Again, it depends on the alignment you want? If you want to work out the offset angle and overhang for Baerwald alignment there are free tools available to do this, otherwise this excellent post by BKearns shows the equations that are involved:
http://www.audioasylum.com/audio/vinyl/messages/62570.html

 

[vinylengine]

So back to my second question, which shape arm is easier to set up, in your collective opinion...

I can't speak for the collective but I don't think the shape of the arm makes much difference to the ease of set up, although the removable, standard fitting headshells on some S-shaped arms make cartridge fitting and azimuth adjustment easier.

 

[energyandair]

Again, it depends on the alignment you want? If you want to work out the offset angle and overhang for Baerwald alignment there are free tools available to do this, otherwise this excellent post by BKearns shows the equations that are involved:
http://www.audioasylum.com/audio/vinyl/messages/62570.html

or use:

John Elison's Alignment Spreadsheet

It is based on the same equations and enables you to see the effect of setup on stylus alignment as the arm moves across the record.

David

 

[Nat]

Agreed that Vinyl Engine has good material on tonearm geometry. What you will find is that there are several different possible geometrys, all of which offer different combinations of arm lengths, offset angles, null points (the (usually two) points in the arc of the stylus where there is no misalignment), and average and peak misalignment.
The problem is that with a pivoted arm, the stylus traces a curve over the recorded area, whereas ideally it ought to follow a straight line -- a radius. With the correct offset, the angle of error can be small, but it exists. The longer the arm, the gentler the curve, so error is minimized, but at the expense of mass and possibly more flexure in the arm. Manufacturers have to balance a bunch of variables when they design tables, and the arm length and geometry are among those variables.
I had assumed that most of these issues were fairly well agreed upon until I started trying to use a Dennesen Soundtractor to set up an early Technics SL 1200 -- its impossible to set the arm up correctly with the Dennesen without having the cartridge at a significant angle in the headshell -- Technics used a different geometry, which is ok, but incompatible with the Dennesens geometry. I remember a thread years ago slamming Pmounts are being wrongly designed for exactly this reason. The slam was wrong, but you had to know that one size doesn't fit all to recognise this.

 

[goldear]

The reason for arm shapes is not geometry -- alignment of the stylus is completely determined by offset angle and by length, both of which are independent of shape. (That description of non skip arms is only talking about those horrendous DJ tables that don't have any offset -- by straight arm they actually mean straight, with the cartridge in line with the arm, which creates pretty significant tracking error.)
The reason for S shaped arms is twofold: given the need for the cartridge being offset, it may be easier to create a strong joint between headshell and arm if the arm comes into the headshell straight on (this is particularly obvious with the SME/japanese headshell arms, but its also arms like the Alphason). And bending a tube makes it less prone to vibrating like a reed, so the curves may lead to better resonant behavior.
On the other hand, the extra length required by the bend adds to the mass of the arm, and this creates other resonance problems, principally that of the interaction with the compliance of the cartridge suspension. So particularly for high compliance cartridges a straight arm has real advantages.
The bearings is either type of arm can be set up at any angle the manufacturer wants -- usually they are set up to be perpendicular to the off set angle so that when the arm moves up or down, the cartridge maintains the same azimuth, but sometimes arms have the bearings perpendicular to the the arm itself -- this doesn't make sense to me.
The other consideration is simply a practical one -- a lot of specialist arms are made in small numbers, often by very small companies, and not having to have a bending jig for arm tubes is a savings, allowing more spending on some other aspect of the arm.

You make some excellent points. I don't complete agree about the equivilence in terms of Geometry, as I beleive that the the S is supposed to have slightly less azimuth error throughout the arc than does the J. But since I can't prove this fact, it is possible that you may be correct.

I especially appreciate the distinction you made between J shaped and truly "straight" arms. Many people in this thread are using "Straight" and "J shaped" terms interchangably, but these are not the same! Most of the arms that people are thinking of here as straight are actually "J shaped"

For reference, I am attaching a picture of a truly Straight tonearm (stolen from somebody's avatar). Please notice that there is absolutely no offset to this design! Unlike both S and J shapes which are tangent at two points along the arc, a straight arm only is tangen at one point. These types of arms have a huge amount of azimuth error, which is probably why the DJ forum referred to them as being "skipeddy".

 

[Holst]

The other consideration is simply a practical one -- a lot of specialist arms are made in small numbers, often by very small companies, and not having to have a bending jig for arm tubes is a savings, allowing more spending on some other aspect of the arm.

A lot of supposedly different pivoted arms can be traced to a very few factorys. The tuly small companys seem to make unipivots.

On the other hand Nat mentioned the highly advanced Aphason, the HR100S was originally produced by it's inventor Mike Knowles in his garage.

As mentioned above, part of the reason for differences in tonearms could be called fashon. "S" arms do tend to be higher mass arms and they were at their peak when a high end cartridge meant a V15, a top line AudioTechnica or several other other fine high compliance types.

Designers then started to build low mass high performance arms like the Grace 707, Hadcock, Maywire and others that would show those carts prowess over vinyl.

The audiophiles embraced the Moving Coil cartridge, but then the Cartridge and Mounting Hardware Total Mass/Dynamic Compliance resonance compliance calculations became the big deal.

And while someone did a lot of wonderful math to come up with a brilliant formula to mate cart with tonearm, the result was a lot of low compliance carts being attached to a lot of low mass arms in the quest for a resonance point below the lowest sound produced by your speakers, yet above a footfall.

While in theory, it's a great combo, in practice, it's a bad idea. It led to terms like the tail wagging the dog, or the formula "low compliance + low mass = lathe" (that was in Stereo Review).

It seemed once it was realized that a moving coil worked better in a more ridged/beefy tonearm, the "S" had been chased off the market by the audiophiles.

I've asked this elsewhere on AK, but why aren't owners of those old 70's Pioneers and Marantz and others with "S" arms buying MC's like the Denons, AT and lower priced Benz Micros and going to town.

 

[merrylander]

I have been following this thread with interest, especially the post regarding John Elison's spreadsheet as it produces results matching those of my "Elite Electronic Improvements, LTD*" plug in alignment gauge. The gauge replaces the headshell of SME stye arms and allows you to determine the optimum overhang, i.e., the point where the stylus should be. You then place the headshell and cart in the gauge and set the stylus position.

Using this gauge I installed my Audio Technica AT31E MC cart in my SL-1600 Mk2 and low mass/high mass it seems quite happy. The other tonearms are straight Rega RB300 with an Audio Technica Signet TK5E and an Ariston Icon with an Ortofon OMB10. I don't count the shop Yamaha P450 as it is there solely to provide a sound source to verify phono inputs.

While there are sonic differences between MM and MC all TTs provide quality sound.

Rob

* Made by Elite Townshend Ltd, Surrey, England, picked up on a trip to London

 

[packrat]

... I don't complete agree about the equivilence in terms of Geometry, as I beleive that the the S is supposed to have slightly less azimuth error throughout the arc than does the J. But since I can't prove this fact, it is possible that you may be correct.
.....

He is correct, there is no geometrical difference in J,S, or straight arms. The mechanical differences in how they are put together determine whether an arm will produce azimuth error over the playing arc due to warps. You could build an arm shaped like a pretzel, and as long as the pivot-stylus distance, pivot-spindle distance, and offset angle are correct for the selected null points it will work as intended with 0 degree offset angle at the selected null points.

Not all S-shaped arms are high mass - the SME Series III is S shaped and is a very low effective mass design. The later SME IV and V are straight arms with a higher effective mass - counter intuitive to all the thumb rules presented about straight vs. S arms. The overall design matters, not the shape of one part.

The cartridge weight has a large affect on the effective mass - a 2.5 gram Ortofon OM-40 will produce a much lower effective arm mass than a 28.5 gram Ortofon SPU Classic no matter what arm you use! Put that SPU on anything and the effective mass of the arm jumps by about 20 grams compared to almost any other typical (8.5 gram) cartridge. The SPU works well on a J shaped SME 3009II or the Ortofon S shaped arms, or the straight armed 12 inch VPI unipivot, while the OM-40 (or Shure V15 IV, etc...) on the S shaped SME Series III or the straight armed Grace 707 or straight armed Infinity Black Widow. See any conclusions here about shape?

Forget about arm shapes! Focus on matching arm effective mass and cartridge compliance.

 

[pustelniakr]

I've asked this elsewhere on AK, but why aren't owners of those old 70's Pioneers and Marantz and others with "S" arms buying MC's like the Denons, AT and lower priced Benz Micros and going to town.

I have been promoting the Pioneer PL-630 with a Benz Micro MC Silver, ever since I first heard the combination. Superb! You do need a slight tail-drag tracking angle to hit the sweet spot however.

Enjoy,
Rich P