Mikelavigne: "the very best arms i have heard are the Durand Talea 1 and Talea 2 in my system, in other systems, and at shows. i would also add the Continuum Cobra to these 2." Not to be nitpicking but I just want to point out the Cobra is not really a traditional unipivot because it has a secondary pivot/ball bearing, a sapphire "swash plate" that supports the second spike, much like a training wheel on a kiddie bike. Overall, it has TWO contact points, unlike a traditional UNI-pivot design. The Talea, from my own understanding, does not have a secondary bearing. The Cobra has no azimuth rocking at all. It belongs to a genre that includes tonearm like the Basis Vector, Continuum Copperhead, Graham Phantom, Nottingham Space arm, Holborne, SPJ, and perhaps precious few others. The Graham's secondary bearing is magnetically supported so it's compliant system which is a subgenre within a genre. If you think about it, this group of tonearms are closer in concept to DUAL pivot design like some knife edge bearing arms like old SME and SAEC, or dual spike arms like higher models from Origin Live--essentially two points sitting on a horizontal ball bearing. The Cobra and others are really a hybrid between pure uni-pivot and dual-pivot. A 1.5 pivot?? :) Oh, the Simon Yorke tonearm uses a teflon sleeve over the bearing post acting as the secondary bearing but does not even use a ball bearing. Very unique and brilliantly simple. It's debated among designers whether having some compliance or "wiggle" room in azimuth motion is a good thing. Basis' Conti argues that's not a good thing, hence his Vector design. But a rigid coupling is adding and extra contact point is not a good thing to Bob Graham. Which one is better is up to the user to decide but I sure enjoy the choices we can make. Personally, it's fun for me to think about these things and I have dog in this fight. Tonearm theories are so fun! I hope over time we will have enough data from users in the future to describe these tonearms' sonic traits relating to their designs. ______ |
I meant to say "it's fun for me to think about these things and I DON'T have dog in this fight." I apologize for all the bad spellings and syntax.
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Unipivots however do not rule the roost by any means when it comes to freedom in the motion of the bearings. Besides air bearing, unipivot does have the least friction and freedom in motion. I don't care what fancy gimbal bearing you have you cannot beat a needle on a dimple. On top of that, the bearing is preloaded by mass so I don't see how you can have bearing chatter and not to mention adding a drop of oil or lubricant in the reservoir. The problem with unipivot is, obviously, not about lack of movement but TOO MANY planes of movement, namely in the azimuth or torsional motion. Micha Huber of Thales tonearm boasts about the quality of his Swiss made bearing but admits it's still not as low friction as a unipivot. So let's not bring Department of Homeland Security into this. Let's just deal with the real issue of a unipivot. There are many ways to deal with the azimuth rocking of a unipivot. Traditionally, designers place the counterweight or outrigger/side weights below the pivot point. Much have been written about this so I won't repeat here. In recent years, designers started to use a secondary bearing to assist the main bearing and sometimes, completely eliminates azimuth rocking which also render it no longer a true unipivot and it might not SOUND like a unipivot but I don't own a Basis Vector, Continuum Cobra & Copperhead, so I can't tell. As a unipivot user myself, I can sympathize with Mike's sentiment about the its "freedom to wiggle" that creates its sonic character whether that's an advantage over gimbal bearing or not is something debatable. my perspective is that the most significant percieved and discussed weakness of a unipivot is actually it's biggest advantage, which is the freedom to wiggle. it is the micro and nano wiggling following the groove unimpeded that gives it the advantage over a fixed/gimbaled bearing pivoted arm which on the micro and nano level cannot follow the groove as well. Since Talea uses magnet to control azimuth rocking, as I am told, I would have to place it in the same genre with the Graham Phantom. It's an interesting development in tonearm design. The traditional mass below pivot point of stabilizing has a weakness in dynamic due to its pendulum affect and I am curious about the dynamic performance of arms like Talea or Phantom. Mike can report that to use. As far as I have seen, (and microscopic motion being the nature of LP reproduction) only a gimbaled arm can have the same kind of azimuth accuracy. Gimbal arm does not guarantee azimuth accuracy. The Triplanar's way azimuth adjustment is placed before the offset angle at the headshell, unless the worm gear is angled accordingly--approximately 23°--that adjustment will affect VTA. Bob Graham brilliantly uses two side weights angled 23° at the bearing housing to prevent that VTA change while changing azimuth. Same concept in the Vector, Cobra, and Copperhead. Smart. Again, a quasi-unipivot tonearm like Cobra, Copperhead, Cobra and precious few others, that use a rigid secondary ball bearing does NOT exhibit any azimuth motion at all. So let's not lump all of them together. At the end of the day, all tonearms have some sonic traits that please you and some others don't, just pick your cup of tea or poison. ______ |
That is of course exactly how the bearings in the Triplanar are built. Notice the word "bearings" is in plural... This type of argument is a logical fallacy known as a red herring. All this fallacy talk is making my head hurt. What I want to say is I wish the azimuth adjustment on the Triplanar is done in relation to the offset angle. If the headshell has an offset angle, the azimuth adjustment mechanism should have an offset angle so it would not affect VTA. The Vector's second spike is placed with an offset angle just like the headshell. Same thing with outrigger weights on the early Graham. The Phantom's magnet sticking out of the bearing housing is angled 23° for a reason. It is a simple fact that once set, the azimuth will not/cannot oscillate on a gimbaled arm as it is held in locus. I did NOT say there's azimuth oscillation in a gimbal arm. It requires more set up care if the design of the azimuth adjustment disregards the relationship between azimuth and VTA in a tonearm with offset angle. I'm just trying to get the LPs to sound as close to the master as I can. Congratulations on finding the perfect tonearm while I look up what is a logical fallacy. Oh, I suppose you tried every tonearm in existence. _______ |
I don't know why people get so worked up on this gimbal vs unipivot debate. Raul, you really take the cake. I have both kinds of tonearm and, again, I have no dog in this fight. I enjoy tonearm design and it's fun for me to think about these things but I'm no dogmatist. I maintain audio to me is a hobby not a religion. Let me start with the positives of a unipivot design. A tonearm needs to move in at least two planes, horizontally and vertically. A unipivot can do that easily with very little friction and no bearing chatter in a single fixed bearing point, which to me is a very nice advantage. It also forces the resonance to travel in one direction, into the bearing and into heat. But every rose has its thorns... It also by nature exhibits torsional movement that affects the azimuth during play. If all records are perfectly flat , perfectly same thickness, and perfectly centered, there should not be azimuth rocking even in a unipivot arm. The same with a car on a perfectly flat straight road then the car would not even need steering. Since records are not perfect, the arm has to hold the cartridge to travel the grove of mountains and valleys. In a "controlled' unipivot design that allows what Mike called some "wiggle room" is not necessarily a bad thing to some designer. Just like cars have suspension for uneven roads with the occasional bumps and nasty potholes. I am not defending this is exactly the case but I at least allow this possibility. In the quasi/pseudo-unipivot with non-compliant/rigid secondary bearing like the Cobra, Copperhead, and Vector, the "unstabilities" that Raul refers to does not even exist. Modern unipivot tonearm designers are well aware of the azimuth instability, hence the emergence of new breed of unipivot tonearms in combating this problem. They stick to unipivot because they believe the positives outweighs the negatives. All they did was to spend the time, resource, and effort into addressing the issues at hand. And what's wrong with that? Happy listening! -------------------------------------------------------------------- P.S. For a good read on the topic, let me quote passages from Dick Olsher's now classic review of Graham 1.5 tonearm from 20 years ago, that its points are still valid today. It starts with the role of the tonearm in an analog system to the Graham solution. Obviously Graham didn't solve all the problems with the original design otherwise he wouldn't proceed to design the Phantom. The perfect tonearm
The role of the tonearm has been compared to that of the enclosure in a loudspeaker. In this analogy, think of the bass driver as representing the cartridge. The first important point is that it is impossible to assess the driver's performance without considering its interaction with the cabinet. The cartridge/arm combination should be viewed in the same light. The arm's effective mass should be compatible with the cartridge compliance to produce an optimal low-frequency resonance. Just as enclosure wall flexure and resonances may color a speaker's reproduction, so can arm resonances influence the overall frequency-response and time-domain behavior. Arm resonances, both lateral and torsional, should be minimal and well-damped.
From the perspective of the cartridge, the arm is essentially a "monkey on the back." As the stylus negotiates delicate groove modulations, the cartridge has to literally drag this monkey, kicking and screaming, down the groove spiral. Bearing friction at the arm pivot, sufficient to impede the motion of the cartridge, gives rise to distortion because frictional forces along the groove wall increase as a result. Thus, low bearing friction is an automatic prerequisite for a good arm. For a magnetic, velocity-characteristic cartridge, the differential velocity between the stylus and cartridge body gives rise to the output signal. Should the arm rattle the cartridge, the signal's amplitude and the system's frequency response will both be affected. This can happen when the arm bearings are loose and "chatter." Unfortunately, for conventional bearings of the gimbal or ball-race design, the requirements for low friction and tightness (no chatter) are contradictory; some compromise must be struck between the two. In other words, the tighter the bearings, the greater the friction.
The dynamic behavior of the arm is critical to overall performance. Real-world records are eccentric and warped. Trying to negotiate such a record subjects the arm to lateral and vertical accelerations. By far the most serious practical problem is that of negotiating a small-radius warp. As the stylus starts to climb the uphill side of the warp, the cantilever is compressed upward, which may significantly increase vertical tracking force. This is bad enough in itselfincreased VTF accelerates record wearbut the cantilever may be displaced upward to the extent that the cartridge enters the twilight zone of nonlinearity: either because of suspension overload or operation in the fringe of the magnetic field.
On the downhill side of the warp the cartridge begins to lose contact with the groove. The effective VTF is reduced, which increases distortion, but the ultimate danger is that of complete loss of contact and groove skipping. What's required here is a nimble arm, dynamically able to keep the stylus in the groove while negotiating a roller coaster.
A figure of merit for assessing a tonearm's dynamic performance is the ratio of VTF to effective mass: the greater the better. This (with an important caveat) gives the maximum acceleration in gravitational "g" units that the arm can withstand before leaving the groove. The effective mass for the Graham arm is about 11 grams. Thus, with a VTF of 2.0 grams, the maximum safe acceleration is 2/11, or 0.18g.
What we have ignored so far in the dynamical analysis of the arm are the effects of damping fluid and arm-pivot restoring forces. Damping is normally applied at the pivot of the arm in the form of a fluid. Used in moderation, damping is a good thing. It is not a magic potion that will somehow convert a poor arm into a good one, but it does help an already good arm perform even better by reducing the "Q" of any resonances. Used in excess, damping can backfire by reducing the dynamic capability of the arm. Damping fluid resists acceleration and exacerbates the problems encountered by the arm while negotiating warps.
Another negative complication involves the action of restoring forces acting at the pivot. On some arms the pivot is located above the arm's center of gravity in what is known as a "stable static balance." The analogy suggested by Bob Graham is that of a high-wire artist balancing himself with the use of a large pole which bends at the ends to well below the plane of the wire. The pole confers stability by lowering the center of gravity below the "pivot point," in this case the artist's feet, thus opposing the decentering of the center of gravity. Again, this makes it more difficult for the arm to navigate warps. Once the arm is knocked out of balance by the warp, the arm attempts to steer back to stable balance regardless of what the dynamic situation demands.
The Graham Solution
Let's look at how the Model 1.5 addresses the various criteria for the "perfect arm," starting with the pivot design.
It may surprise some of you to find that Graham has chosen to go with a "unipivot" bearing. Nevertheless, a unipivot has a lot going for it: First, it is the simplest design. Second, it pre-loads the bearing surface to zero tolerance, and is capable of yielding the lowest possible friction in a mechanical design.
Bob sent me a videotape of a test he conducted where he pitted an SME IV against an undamped Model 1.5. Both arms carried the same cartridges and were statically balanced. Both arms were displaced vertically downward the same distance at the start of the test. The idea was to see which arm bobbed up and down the longest, as this would indirectly reveal the degree of vertical friction in the bearings. The SME pooped out after about 30 seconds. The 1.5 kept going for two and one half minutes before Bob stopped the arm for fear of having me fall asleep. He claims that the arm actually continues its pendulum-like motion for over five minutes.
Third, bearing performance does not drift out of tolerance, as there is nothing to adjust. Graham uses tungsten carbide for both the bearing cup and pivot. Both elements are said to be polished to stylus-tip tolerances. With a typical 7-9gm cartridge mounted on the arm, Graham calculates the loading on the bearing point to be in excess of 100 tons per square inch. With this sort of loading there is no play in the bearing to interfere with the transduction process. Finally, a unipivot design makes it easy to add damping fluid around the pivot point. Viscous silicone fluid (about 0.75ml) is used here to provide damping in both vertical and horizontal planes.
Older unipivot arms, such as the Formula Four, used stable balance with the pivot point well above the center of gravity of the system. In contrast, the 1.5 places the pivot point in the vertical plane essentially at the center of gravity of the assembly, hence in neutral balance. The pivot point is in line with the longitudinal axis of the arm tube, main pivot housing, and counterweight. Two outrigger weights are positioned to either side of the pivot housing and slightly below the pivot point to provide lateral stabilityotherwise the arm could tip over to one side or the other because it does not favor a particular rest position. The short lateral levers connecting the outriggers to the pivot housing create a strong stable balance along the line connecting them, thereby resisting torsional motion and keeping the arm in the correct upright position.
The outriggers manage to lower the center of gravity of the assemblybut only slightly in the vertical planeand the arm operates essentially in neutral balance with minimal restoring forces. According to Bob, if the arm is lifted a full 0.5" off the record, the generated restoring force at the stylus tip is only about 30 milligrams. Because the outriggers are so close to the pivot point, their effect on the effective mass of the arm in the vertical plane is minimal. However, they account for most of the moving mass in the lateral plane.
These same outriggers are used to adjust the azimuth. The weights are moved in and out along threaded rods that provide a precise and stable adjustment. A closer look at these weights shows that they are displaced from true perpendicular in reference to the arm tube. This is by design, and assists in preventing the cartridge from twisting while negotiating record warps.
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Just because I don't have the specs for the friction measurement of a unipivot tonearm, all of a sudden I am a poster child of the subjectivist camp without even a "touch of objectivity"? Thanks for the flattery, Raul. I'm not that ambitious.
In audio, I am an atheist and polygamist. I like choices. If you want to be the high priest of the absolute sound, go right ahead.
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Dan_ed: "Hiho, are you beginning to see the religious nature of all of this?" Indeed. As Luis Bunuel would say, "Thank God I'm an atheist." _______ |
