Mijostyn,
I also find the Reed T5 and Schroder LT approach interesting. Both arms are based on the Thales circle geometry and achieve close to perfect tangency and do so without an offset angle that creates skating force. At least theoretically, the Reed T5 is a touch more elegant because the movement of the arm pivot is achieve by using a laser sensor to detect loss of tangency and a motor to then rotate the arm base to move the pivot point, while the Schroder LT requires the arm to pull the base into a new position. Of course, the simplicity of the Schroder approach is a big plus for that design. If I were to change my arm (Vector 3), I would certainly consider both of these arms. About three years ago, I talked to a Reed representative at a show and found out about the bad news on the T-5--it costs something like $18k. |
There appears to be some confusion about the cause of skating force. If the cause is primarily the cantilever being not tangent to radius of the record (i.e., cantilever to perpendicular to a line drawn from the spindle to the point of contact, then skating force would be near zero at the two null points and would reverse direction as the null point is crossed; that does not happen. Skating force comes from the stylus dragging in the groove. That force is largely along the line of the cantilever (very slightly to the left or right of directly on line when not at the null point). Of course it also varies from directly along the cantilever when playing different parts of a waveform, but still, it is primarily along the line of the cantilever. The drag is pulling the cantilever. If the cantilever is pointed directly at the tonearm pivot, there would be no left or right bias to such pull. But, that is not the case because the headshell (and therefore the cartridge) is at a very substantial offset angle and points far to the right of the pivot. This is the cause of the skating force.
You can demonstrate this for yourself by pretending your arm is a tonearm. Put your elbow on a table (this is the tonearm pivot. Hold your arm and wrist straight out and now pull on your middle finger (this is the equivalent of drag pulling on the stylus and cantilever). Your arm should go nowhere. Now bend your wrist at an angle (like the kink in your tonearm) and pull straight back on your middle finger. Your arm will move inward, just like a tone arm does when tracking a record.
Ledermann's technique is not meant to exactly simulate skating force drag. It is something he developed as a proxy for roughly estimating the amount of antiskating force that should be applied. ALL approaches are just rough compensation, which is better than nothing, but far from perfect. It is also very easy to apply. He makes the case that using test records to find the point where the cartridge mis-tracks in one channel to determine whether to apply more or less antiskating force optimizes the setting only under extreme conditions, but, those conditions happen very briefly so the rest of the time one is applying too much antiskating. I think his approach makes sense. A similar approach is suggested by a cartridge manufacturer (I think it is Lyra) in their set up instructions. They recommend looking at a cartridge from the front and then cuing the arm down to a groove near the center of the record; at the moment of contact, the cantilever will briefly skew inward or outward; if it skews inward more antiskating should be applied. |
I don't want to get into whether friction is higher or lower on a blank record vs. in the groove--there are too many variable to consider. For one thing, I have not seen mention of the fact that, when playing a modulated groove, the stylus is being dragged at a MUCH greater velocity (the path of a modulated groove is much longer per inch of record circumference) and there is inertia of the stylus moving side to side to consider as well.
None of this matters as to whether Ledermann's approach of using the blank side of a record is valid. He acknowledges that this does NOT duplicate actual playing conditions, but, his experience has shown that it results in a decent proxy for a correct setting. He has a lot of experience with examining stylus wear so he knows a thing or two about what achieves even wear. |
It is a bit confusing that some posters are saying that overhang or offset angles "cause" the skating force. The WallyTools videos posted here that shows that, under specific conditions, skating forces can even be in the opposite direction, add to the confusion because they do not attempt to explain why this is so. Overhang and offset angles don't directly "cause" skating forces, but, they do create the conditions that give rise to skating forces because they create the geometry that causes the force of friction (drag) to pull the arm in one direction or another.
The drag developed at the point of contact of the stylus with the wall of the groove is pulling along a line that is a tangent to the points of contact of the stylus. Whether this tangent is in the same direction as the cantilever or to the left or right of this line is dependent on the alignment of the cartridge at that particular moment. For a reference point, imagine that a spherical stylus, when viewed from directly above, is a clock face with the axis of the cantilever bisects the 6 o'clock and 12 o'clock position. When that stylus is at a point on the record where the contact is at the 9 and 3 o'clock position, the drag is directly along the line of the cantilever. Will this result in skating force? The answer is: it depends. If the alignment is such that the cartridge is perfectly straight (no offset angle to the headshell and cartridge), at that perfect tangency point on the record, there is no skating force because that drag is pulling straight back to the pivot point of the tonearm and that pivot is resisting that drag. But, if the alignment is a conventional alignment, such as Lofgren B, at the point of perfect tangency, there is a substantial skating force, because the drag along the same line as the cantilever is pulling at a point to the right of the pivot point (because of the offset angle) and the pivot cannot completely resist that pull. On the setup with a cartridge facing straight forward with no overhang, at points outside of the perfect tangency point on the record, the points of contact will not be at the 9 and 3 o'clock position, but something toward the 10 and 4 o'clock position; this means that the drag direct (tangent to these points) is aimed to the right of the pivot and so there is skating force. At the point inside the point of tangency, the points of contact are toward the 8 and 2 o'clock position, so the direction of drag is to the left of the pivot an the skating force is now in the opposite direction. The reason there is ALWAYS a skating force in the conventional direction with conventional alignments (e.g., Lofgren) is because these alignments minimize the deviation from perfect 9 and 3 o'clock contact (always less than 2 degree deviation) which is MUCH less than the fixed offset angle; this means that the drag is ALWAYs to the right of the arm pivot point even if does deviate over the diameter of the record. |
Air bearing linear tracking arms can be made to have reasonably low moving mass, but, they lack the mechanical advantage of a fulcrum and pivot of a conventional arm, meaning that for any given effective mass, they do impose a lot more force on the cantilever to drag the arm to a new position than a conventional arm imposes in order to swing the arm around the pivot point. This is an issue even if friction is zero.
This is not the case with linear arms that employ a conventional pivot and a sensor that detects when the arm is out of linear position and then turns on a motor to move the entire arm assembly. But, as with every design, the motorized arm version has its own shortfalls, such as, vibration from the mechanism getting into the arm, lack of overall rigidity and mechanical grounding of tonearm vibrations).
The very short arm on some linear trackers (e.g., the ClearAudio arm), may give rise to another problem--a change in record thickness would mean a bigger change in VTA with a short arm than a longer arm.
The Schroeder LT is not a tonearm on a string design (I've helped set up and listened to a Schroeder arm-on-a-string and it is a good arm). It has conventional pivots, but also an innovative mechanism that moves the arm, including the pivot point, in a semi-circle to greatly reduce deviation from perfect tangency while not causing skating forces. Because it is using the drag of the arm tracking the groove to move the pivot, I don't know if it increases friction seen by the arm. The Reed T-5 uses a sensor to operate a motor to move the base of the arm to achieve the same kind of result as the Schroeder.
The bottom line is every type of arm has its pluses and minuses, and I haven't heard any type that I thought was obviously superior to another. |
lewm,
You are describing the Moerch anisotropic arm I described above. I think the model is the DP8 or DP9.
I have no idea if 25 gms is too high, I just know it is WAY higher than is common with conventional arms. In the past, some users of air bearing arms reported snapping the cantilevers on their cartridges (Walker owners, I believe), but, who knows whether there is some other cause for such problems (its not like user error is a rare phenomenon). |
An effective mass of 25 gms is very high. Whether or not this translates to a real problem is another matter. As lewn notes, some arms deliberately have a higher horizontal mass to improve bass response, but, it is usually not that high. Bass notes are often cut monophonically because cutting bass stereophonically would mean excessive changes in the depth of the groove. A high effective horizontal mass means that the arm will not swing side to side from the needle tracking the wide modulation of the groove (the full groove swing will be translated to movement of the cantilever instead of some of the movement lost to the movement of the arm).
|
It is NOT ideal to have the cartridge moving. You want the cartridge position to not change so that the full motion of the needle swinging side to side is transferred to the generating element of the cartridge. If the cartridge moves, the amplitude of the signal reaching the generating element is reduced. That is why high effective mass (high inertial mass) reduces the tendency of the cartridge to move in response to large bass modulations.
Moerch makes an "anisotropic" tonearm with large outboard counterweights located at the vertical pivot. Because of this location, the weights contribute little to vertical mass, but, they increase horizontal mass for the purpose of improving bass response. I've heard this arm and it does have a bigger bottom end than typical arms. I also thought the bottom end of air bearing arms seem particularly full, but, I have no way of attributing this to the high mass.
I personally agree with you that excessive horizontal mass is quite undesirable. This probably puts a strain on the cantilever/suspension of the cartridge and might even cause uneven stylus and groove wear. I have never heard the Shroeder LT or the Reed T-5 arm, but, I really do like the engineering concept behind those arms. They seem to be the best way to maintain proper azimuth without causing other problems. |
mijostyn,
I agree that if horizontal mass is high enough, it raises concern with very low frequency resonance. I was just pointing out what is the theory behind the anisotropic arms. As long as the bass frequencies are above the low frequency resonant point, those arms should work as intended, at least in theory. The theory also only works if the bass is mastered in mono so there is no vertical modulation for those frequencies. Some mastering engineers say they do blend bass to mono, but, others say they do not.
What is your personal experience with either the Schroder LT or the Reed arm? Do you know how much the LT costs? I once inquired about the Reed, but, I lost interest when I found out it cost about $18,000 (two and a half years ago). I am assuming the LT is not cheap; my friend's Schroder arm was quite expensive (but exquisitely constructed). |
Inner grooves are the hardest part of the record for a cartridge to track because the groove modulations (wiggles) are compressed into a smaller diameter and become more extreme in shape. Wrong antiskating can contribute to problems with tracking, so in that sense you could say they "cause" the problem. When grooves are highly modulated more skating force is developed, and therefore, more antiskating would be desirable. But, as Peter Ledermann points out, increasing antiskating to optimize high modulation playing would mean that, for the vast majority of the time (when modulation levels are lower), antiskating is set too high. That is why he discourages using test records to set antiskating for equal distortion in both channels at high modulation levels. |
