Ok, move the SME arm forward you change the off-set angle yes.
Again, if you are running on the pin-point of the stylus, it has NO influence at all, at what angle that pin point rubs on smooth vinyl as a pin has no unequal sides --- unless that is only the case in my universe...
The change in friction force caused by a change in VTF of that pin-point is of course of MUCH greater influence and goes hand in hand higher VTF asking (all things being equal, which in the groove they are not...) for higher anti-skate compensation.
Also, the over-hang for a given arm is a ~ fixed parameter and as such it can actually be disregarded, fair enough.
There the variable is the VTF / friction force => stylus geometry.
As to the "off-set" it is the same as with the over-hang i.e. fixed also for any particular arm geometry and therefore also can be disregarded.
Axel |
This has certainly evolved into an interesting thread, and an educational one despite the several contradictory claims which have been made.
I've just done an experiment to try to bring it all down to a more practical level. As I mentioned in my earlier post, I have always set up anti-skating on my mm/mi cartridges so as to produce no visually detectable right or left deflection of the cantilever when it is in the record grooves, compared to when it is not in the grooves (which has always meant straight ahead, with my cartridges).
Using the high output Grado Reference Sonata which I presently have installed (compliance 20, vtf 1.5g, elliptical stylus), I carefully looked for cantilever deflection at approximately 12 different points distributed across the full length of a lightly modulated (chamber music) recording.
At no time did I see the slightest deflection.
The cartridge is mounted on a 1980's SOTA Sapphire turntable with Magnepan Unitrac tonearm. Anti-skating is set for about 2/3 of the value recommended in the tonearm manual for the 1.5g vtf (35 shot pellets in the bucket, instead of the recommended 52). As a point of interest, I had previously had a Grace F9E Ruby mounted in that arm, set up by a very good dealer using an oscilloscope, test record, and Lissajous pattern. Their setting was also about 2/3 of the recommended anti-skating for the 2g vtf used with that cartridge (52 pellets instead of 79). And I have found a similar 2/3 factor to be about right on some other turntables/arms/mm cartridges I have set up in the past.
When I have done these setup procedures in the past, modest deviations from the anti-skating settings I settled on, such as using 30 or 40 shot pellets instead of 35, produced clearly perceivable deflection.
It seems to me that this experiment, indicating no perceivable deflection at any point across the record, supports the following conclusions, as pretty much stated earlier by Neil (NSGarch):
1)While skating force may vary from one point on the record to another, the range of variation is very small compared to the nominal value of the skating force.
2)The considerable cantilever deflection that does in fact result with these or similar cartridges from using zero anti-skating force would seem simply based on common sense to be undesirable.
3)Common sense would seem to suggest that the same two conclusions apply to moving coil cartridges, except that with these cartridges the differences resulting from different anti-skating settings (including 0) may be more difficult to perceive, because of their stiffer suspensions, lower compliance, and (possibly) different stylus shapes.
Can we all agree on this, from a practical standpoint?
Regards,
-- Al |
Al,
I'm fine by that, it does not contradict any of my own findings.
Axel |
Axelwahl
I have a simple test for you.
If possible, adjust your arm to zero overhang and check the skating. It will have changed very slightly (due to the change in offset angle) whereas if I read your theory correctly you would expect no skating at zero overhang.
Mark Kelly
|
Thank you Mark,
but you are missing the friction force in your test suggestion, present in variable degrees due to variable VTF / stylus shape / friction, as mentioned.
Therefore there will *always* be a skate force present even at 0 mm over-hang :-)
There will be MORE skate force with more overhang, my initial argument and proven by test.
The influence of the VTF => friction is (as I said also) of much more influence i.e. over-hang may therefore be ignored as be off-set, since they are a non-variable parameter (in theory at least) according to a specific arm geometry.
The test: lift the stylus of the vinyl and you'll have 0 skate force where ever the arm is :-)
Greetings,
Axel |
Axelwahl
Go back and read my post 15 above this one.
Mark Kelly |
Mark,
am I having a different understanding of what is off-set?
It may explain perhaps some crossed wire...
"Off-set" in my vocab is the angle that a head-shell / cart /cantilever is mounted out of the true / straight line with the tone-arm-wand i.e. 0 deg = no off-set.
As to the various measured forces caused by friction between various materials I have no issue with at all.
In fact, and often, the faster the speed the LESS the friction force e.g. when an object starting to plain on water, rather then being dragged through it is some extreme case in point.
If a tiny tip of a stylus is 'dragged' over smooth vinyl, I can see that no measurable difference in friction force would be the case.
However, no friction force *NO* skate force!
It is this friction that wants to pull the stylus tip with it. Since it can't, due to the arm pivot holding it back, it will do the next best thing and pull the arm in line with the center of rotation. This would also be the shortes distance from pivot to center of rotation, right?
Like a pendulum being pulled (eventually) to the shortest distance from pivot to the center of gravity.
No gravity force, the pendulum will remain where ever is happens to be.
Axel |
Mark, my rudimentary understandig says line contact has less force on the groove wall so shouldnt less force lead to lower friction and hence lower skate force, but I cant reconcile this with your Amonton law breakdown stating lower pressure implies higher friction, although wont some of the breakdown be ameliorated by the cantilever making the diamond less hard compared to the vinyl? |
Axelwahl
I think I have confused you by trying to simplify things. The angle I referred to as the true offset is the angle between the groove tangent and the line between the stylus and pivot. The frictional force acts along the tangent so this is the angle between the frictional force vector and the restraining force vector. The true offset angle varies with groove radius for ordinary pivoted arms and the offset angle of the arm is an approximate average of the true offset angles.
I can see how this is confusing and I should probably have used a different term.
Perrew
You are confusing force and pressure. The force is set by the VTF and the groove angle, so the sum of the forces on the sidewall is SQRT 2 times the VTF (for 90 degree groove). The pressure is this divided by the contact area. Larger contact area = lower pressure for a given VTF.
Mark Kelly
|
Mark, so increasing the VTF would increase the pressure and the skate force would decrease, all else equal? Wouldnt this imply that high VTF means anti-skate can be ignored? |
Mark,
ja now fine, and so we have even more discrepancies since I, for the sake of TRUE simplification, been talking over and again about BLANK vinyl, and the stylus point riding on this smooth surface --- AND THEREBY taking any of this groove tangent stuff OUT OF THE EQUATION.
There IS a skate force WITHOUT any groove! Therefore NO tangent, what so ever comes into play as I tried real hard to get across.
Taking things into the groove, with all this stuff Perrew is on about is no good, if the basics are not cleared up, yes?
So, again, NO friction force, NO skate force, period.
Tangent has NOTHING to do with it at this point, *if we are NOT in any groove*!
As such all this talk about tangent, or off-set, etc. only serves to cloud the basics of the skate force issue --- unless you are much more deeply into the details then at this point was my understanding.
As such, and going back to the SIMPLE, *no groove*, model Dertonarm is NOT right in assuming a zero skate force at a 0 groove tangent angle.
I can prove that, as soon as there is friction, there is a skate force.
His (Dertonarm's) point is such only of any relevance, if at 0 tangent groove angle -- and as he assumes --- there be less friction force than at any other angle.
If this be so in the first place (and I have no explanation why it should be so!), it be so minute a difference, as to have no measurable effect.
Therefore off-set is as little part of the PRACTICAL equation as is Over-Hang. If one groove side is traced a micro-millimetre later (or earlier) then the other --- what be the increased friction due to that?
None! for a spherical stylus, and none for most any other stylus as well, even it the sides be a sharp as it gets.
It will make a mess of the signal, oh yes, but that is another discussion all together i.e. correct cart alignment.
Axel |
Axel, the often cited law of friction by G. Amonton (who kind of re-discovered it 2 and a half centuries after its original "inventor" Leonardo) as the main "law" being the skating force in phono playback assumes, that the bearing of the tonearm is not able to *completely support* the resulting force towards the inner groove. This force is a result of the offset of the tonearms "head" and therefor the cartridge and its cantilever. This offset implies a force that would - if not compensated by bearing or lateral balance - swing the tonearm's "offset part" (the "right side" if view from the front towards the cartridge/tonearm head) downwards because it needs to find a stable position in gravity.
Thus resulting in a horizontal force on the inner groove wall.
So far so good.
And I agree with this of course.
This is true for most pivot tonearms.
But not for all I think.
Now what IF the bearing is able to COMPLETELY SUPPORT the resulting force.
It is obvious, that most uni-pivot tonearms and knife edge bearing tonearms (among others) can NOT completely support this force.
However - a rather long effective (12" is fine .... of course 16" would be better) tonearm with resulting LESSER offset and a left side lateral balance can (at least in empirical observations.....) almost (if not 100%) completely support that force, as it is compensated by lateral counterforce.
A completely balanced FR-66s with its lateral balance correctly adjusted and on dead level TT shows no skating force in the 2 zero points of the tangential curve.
2.7 VTF with a stylus even "sharper" than a line contact.
A SME 3012 with bronze knife bearing does show heavy skating even in the 2 zero error points.
Both tonearms adjusted for same tangential curve and running with the same cartridge (FR-7) on the same table.
Maybe this way I can illustrate why i still think, that the model ... maybe .... is a bit more complex as the initial anti-skating model we are used to.
BTW - I vividly remember that the anti-skating devices in former Thorens, some EMT and Dual turntables (among others of west german manufacturers of years gone by) showed at least 2 different anti-skating scales: - one for elliptical stylus, one for conical......... |
Perrew
I cannot see how you can get to that conclusion.
Axelwahl
I love how you can take a simplification and twist it until it becomes a complexity. Your jump from "no groove" to "no tangent" is unjustified.
The frictional force vector is in the direction of relative motion between the stylus and the vinyl. When the stylus is following a groove, this direction is tangent to the groove curvature. When the stylus is not following a groove, this direction is tangent to the scribed arc of the stylus on the vinyl.
The rest of your post rests on this false dichotomy so it also falls (except for the bit about our befuddled friend DT not knowing what he's talking about. That I agree with)
Mark Kelly
|
Mark you said, P=F/A and "The breakdown takes the form of a pressure dependent coefficient of friction; the coefficient decreases with increasing pressure "
Still an example with your choice of tonearm would be helpful. |
Ok I can see it now.
The coefficient of friction does indeed reduce with pressure but what we haven't established is that the pressure is a linear function of VTA. For that to be the case contact area would have to be constant and it isn't.
I'm not even going to try to explain why not, I have a reasonable grasp on Hertzian contact theory but not good enough to explain it to you. Google Hertzian contact theory and you might see why.
Mark Kelly
|
Mark,
+++ The frictional force vector is in the direction of relative motion between the stylus and the vinyl. +++
>>>Yes!
When the stylus is following a groove, this direction is tangent to the groove curvature.
>>>Yes!
When the stylus is not following a groove, this direction is tangent to the scribed arc of the stylus on the vinyl.
>>>Yes!
+++
I completely agree with all the above.
I have no idea where you see a: "false dichotomy"
in my statements of: No friction = no skate force, no groove = no "off-set" in the groove (well there is NONE).
The only other angle is that angle between pin-point stylus to pivot and the pivot to centre of rotation. This angle seeks itself to be closed as soon as a friction force is applied.
If indeed it is what you keep referring to, we have no argument only an issue with terminology.
"Off-set angle" is by definition (in my universe and also at least SME's) the angle the head-shell and thereby the stylus is "off-set" in relation to the tone-arm-wand. This "off-set", (specific to each tone-arm) should produces two null-punkt (0 deg tangent angle) spots on the vinyl after correct alignment.
This tangent (tracing) angle is a constantly changing angle whilst the tone arm moves toward the centre of rotation, or end of record.
I am starting to think you are using "off-set" in place of this variable "tangent trace angle"?!
Sen wat we hav here is se problem mit de "Begriffsbestimmungen", oh mein Gott!?
Failure to communicate?
Axel |
Axelwahl
You said no groove = no offset. That's not correct.
The tangent to the arc of the stylus traces on the vinyl does not pass through the tonearm pivot. Therefore there is a separate vector pasing through the same point (the stylus contact point) which does pass though the tonearm pivot. The first vector is the stylus frictional reaction force vector, the second is the tonearm restraining force vector.
The angle between these two vectors is the angle I described as the "true offset" and which we are now going to call something else, hopefully slightly less cumbersome than "the angle between the stylus frictional reaction force vector and the tonearm restraining force vector". As explained above, the fact that the stylus frictional reaction force vector doesn't pass through the pivot is what causes skating. That's all there is. Nothing more, nothing less.
Mark Kelly
|
Axel
I've just read your post more carefully and my first response was too hasty.
I'm not familiar with the term "tangent trace angle" but by your description it is what I described as "true offset angle".
Indeed we may have been suffereing from terminological incommensurability.
Mark Kelly
|
Mark, regarding Hertz are you saying the contact area of the vinyl is expanding due to deformation with increasing force, (I dont see the stylus deforming), and therefore the contact area might expand more than the force and P becomes smaller even though F increases? |
Dertonarm,
I'm a bit lost with the concept "that the bearing of the tonearm is not able to *completely support* the resulting force towards the inner groove"
All that means to me the friction of the tonearm-bearing subtracts itself from the skate force as it is an opposed force?
If you tighten up the bearing that much so is equals the skate force, the arm will not be moved by the skate force anymore.
Where it fits into the understanding of it all I'm not sure right now.
The friction force will create a vector forces due to the arm held at the pivot and produce a resultant force - the skate force. Actually pretty simple, until you want to actually calculate what this resultant force would be.
All we know, it's pretty much proportional to the actual friction force --- that force which we usually don't know and keep guessing about.
Axel |
Perrew
No that's not what I'm saying.
I said I wan't going to try to explain Hertzian contact theory to you and I'm sticking to that.
Mark Kelly
|
Axel, that is in the general accepted model I mentioned.
To illustrate the point:
an uni-pivot tonearm with no lateral balance and an offset (be it in the way of a S- or J-shaped pipe or an offset headshell mounting area on an otherwise straight arm-pipe) "head" would - if viewed from the front towards the cartridge mounted - "swing" down with its right side over the long axis till it finds a position in gravity.
This "down-swing" over the right side does cause the force towards the inner groove wall (left....- viewed from the front).
Now - if this obvious tendency is (just as a hypothesis...) completely (even the technical books aren't very detailed here, but the general term used - even in old AES literature - is, that the bearing in general does not "completely support (address)" this torque movement) counter-balanced at or in the bearing, we would see no longer an additional friction at the inner groove wall which would be rooted in the offset.
This model would indeed require a design which does feature a lateral balance option which would be able to counter-balance the downforce initialized by the offset.
The "actual friction force" we see on pivot tonearms is (I am careful now...) "maybe" not only a matter of the offset.
As this force is in my 30 years long experience quite different in different tonearm designs (mounted with identical cartridge and stylus and VTF) I believe (I am careful again....) that there is still more to it than just the force initialized by the offset of the pivot tonearm's geometry.
All I want to suggest (carefully....) is that maybe there is more and that we - maybe - settled to soon with an explanation which - maybe - does not address all parameters.
I will get a Wacom touch board next week - then I can draw the whole model and try to illustrate the point by graphics.
Cheers,
D. |
Hi Pär,
the vinyl is indeed kind of soften and expanding its surface due to the heat of the stylus .
That is an old problem noticed by record companies as long back as the middle 1950ies. Thats why records do suffer from repeated playback (especially single tracks played over and over again in a row - which BTW is a (smaller) problem with CDs too....) - the vinyl has no "time" to "recover" (read: return into its original form and cooling down completely).
Thats why it is so important - aside from recovering the most tiny detail information engraved - that the stylus is as perfect aligned as possible.
A misaligned stylus does obvious create much more "problems" to the softened vinyl groove wall.
An while friction may be linear to VTF, the pure kinetic force on the groove wall is a result of down-force divided by contact area. The largest contact area (and resulting lowest force on the groove WALLS (plural..) in A GIVEN RECORD is in the groove-compliant VTA.
Means - even if not all agree on the sonic benefits of a certain VTA for a certain record, it is in this very situation, that we have the least "pressure" (VTF divided by contact area) on the walls of the record groove.
Cheers,
D. |
Ok,
more "off-set" friction.
I'm lost, but learned to be stuck, just being with the question :-)
More to learn as it seems. So thank you all for your patience and kind participation, massaging my lack of awareness in this matter.
Axel |
