I agree with you Dan_ed. However - the eddy current brake approach as shown by J.C.Verdier isn't all that costly. All you need are 2 old 15" woofer dirivers magnets which never suffered from shock or extreme low temperature. Thats about all you need to get those 3 features mentioned above all in one.
BTW - idler drive was introduced by and for the broadcast service turntables first. Kind of heir from the days of the grammophone. For good reason. It was of paramount importance that the platter had full 33 1/3 rpm after less than 1/2 turn. You need it for broadcast. For timing the tune played next. When direct drive had grown to full mature (by mid 1970ies the latest) the idler drive TT all vanished from broadcast stations.
Idler drive needs extreme care in execution to supply good results for turntable application. It however always gives some problems as the idler wheel itself is a source of direct noise transmission to the platter.
The idler drive is the direct counter-approach to the belt drive - one favours direct coupling between motor and platter to have very direct and immediate control over speed. The other favours as little influence and as little coupling between motor and platter as possible to minimize any possible vibration and speed shift in the motor being transmitted to the platter.
If the time frame till stable speed is actually reach is of little to no importance, - a very heavy platter coupled by string (= little grip) and driven by a very good motor will give the most stable speed for a turntable. Huge inertia combined with "slip coupling" or a kind of "cumulative coumpond motor drive" (read: very good motor coupled via string to a platter with little grip). If correctly done, it will take fairly long to get to stable speed, but once there, the speed will be extremely constant and little changes in the motor have no effect on the speed of the platter due to the - wanted" slip/low grip of the string.
Of course, this is one of many approaches in today and yesterdays turntable design. It is however the technical engineers approach if absolute stable speed to the prime goal. And if the time taken to reach this stable speed is neglectable.
The whole scenery is worth musing about. However this approach does ask for fairly expensive components (= high quality = expensive motor and very large mass in platter and extremely precise manufacturing and tooling) and huge weight in platter. Nothing that can come cheap. |
Thanks for qualifying your position, Dertonarm. I understand now that you are referring to a specific approach taken by Verdier. That approach as you have outlined it does seem to be unique, but costly and hard to control from a manufacturer's point of view. As you documented there were other issues with this approach, so perhaps it wasn't all that to begin with. Seemed to be a good idea.
The eddy current breaking is a solid, proven approach. The problem is that it still seems to be beyond the financial means of most of us, so we are left with belts or idlers. To Chris's point, the non-compliant mylar belts and lower torque motors do sound very good. This type of belt along with a decent control mechanism can provide speed stability that is better than most belt drives and I believe it is very close to your average idler. I suspect this is because we are starting with a no-cog DC motor and the mylar provides a much tighter coupling to the platter than any stretchy belt. But that is going off on a different topic.
If nothing else this discussion should show that the designer must take the entire drive chain into consideration, regardless of whether the bearing or motor is chosen first. They still have to work together to produce a speed stable platform. |
As to my knowledge all La Platine Verdier sold after 1995 do already feature the ball supported bearing. So those can't actually "fall down", as the vertical position is already determined by the ball bearing and no longer by magnetic force. The Platine verdier was imported and introduced to the USA fairly late (it was originally a DIY-project presented in french L'Audiophile magazine (with detailed schematics and description how to built) in the late 1970ies and the first offical built retail version was tested in summer 1980 in a german magazine). I do not know, whether there were any Platines delivered to the USA via the offical importer before 1995. |
Thanks. Very interesting, indeed. I do recall hearing rumors about La Platine platters "falling down", due to loss of magnetic field strength, but none of my (two) friends who own the table have had that problem. There are a few other brands that now sport magnetic suspensions, but as far as can tell from photos, none of those has magnets of nearly the same size as found even in the current La Platine. Nor are the platters as massive. |
Lewm, two magnets rotating in a horizontal = planar sphere do indeed produce an eddy current field.
The Platine Verdier folks and dealers will hate me for this .......anyway:
The current La Platine does feature somewhat lower quality magnets (compared to the old Focal magnets used till 1990/91 - that particular magnet was no longer available when Focal changed to the "6-tablet-magnet" - design invented by J. Mahul for the 15" woofers in early 1991) - thats why they promoted the ball to be inserted in the top bearing shaft hollow. To stabilize the vertical movement of the platter (in mid-90ies production was a tendency to instable magnetic field and often in loss of magnetic force causing many Platines in europe (and I suppose elsewehere too) to "oscillate" (= being unstable in height of platter)). To solve this problem the "top ball bearing shaft" was introduced (well, the hollow was there before, so they just put in the ball - smart move). That particular problem never occured with pre-1991/92 Platines. However the eddy current brake effect is no longer as dominat as it was in the original version with much better and more homogenous magnets. |
Dertonarm, Are you saying that the vertical magnetic suspension of the La Platine per se simultaneously offered a form of eddy current braking of the rotation of the platter in the horizontal plane? If so, why would that not also be a feature of the current version of the La Platine? (I am going to check my physics books to verify that such a phenomenon would occur, but for now I accept the principle, if that's what you meant.) |
Hi Dan_ed, nobody else aside from Jean Constant Verdier did combine these three effects with one very simple device (two magnetic rings of large diameter (in fact former Focal 15" woofer magnets pre-1990 vintage):
- a true eddy current brake on very large diameter to stabilise the movement
(which in fact is not really bearing friction, but acts in that way WITHOUT any noise added).
- vertical bearing elevated by magnetic force.
- further vibration damping on large surface-diameter of turntable platter (underneath) by magnetic force.
That is all I meant.
I have yet to see another turntable offering these three technical aspects with one simple design feature.
The thread was about bearing friction being applied to stabilize movement. Not comparing turntables regarding their "sound".
Here is one TT doing this AND combining the measure to achive some very nice side-effects.
I am certainly not going into any discussion about sonic pros or cons on this or that turntable.
I would be fruitless anyway.
This is - as always in audio - a subjective field alltogether.
My comment is about unique technical features being combined.
In no way do I want to promote the Platine Verdier here (especially so, as the current model no longer do indeed feature these 3 merits in the way the vintage model does).
There are several TT designs with good points and decent performance around.
All have their merits - all have their flaws.
Thats all. |
Ok, Dertonarm. I'm just wondering what you meant when you posted this. The only turntable so far which did it right (i.e. - correct application of "friction" to stabilize movement AND to provide additional damping to the platter) is/was the old venerable Platine Verdier in its original form (pre-1992) of bearing (without the ball support). Nobody else has gotten it right? |
Hi Dan_ed, I do not use the Platine Verdier. Furthermore I do not see that I have refered in any way to the sound of the Platine Verdier.
Did I ?
My comment was about a construction feature of the Platine Verdier which was used in this turntable indeed for the very first time in this particular context.
BTW - I have listened to about every commercial turntable released on the market the past 32 years (and a few non-commercial TTs too...).
However - you would never read any comment from me regarding sonics about any current market product ever. |
Dgarretson,
I addition to the practical difficulties that Dertonarm pointed out there are some good theoretical reasons against using a flywheel at the motor. In all motors motion is produced by magnetic attraction. When there is no drag the
magnetic fields in the stator and rotor will be perfectly aligned. When drag is applied the magnetic fields pull apart and the attraction creates torque. This is a very desirable characteristic in that torque is almost instantly created in response to a load perturbation. I say almost instantly because the rotational inertia in a motor delays the delivery of torque. Any delay in torque delivery adversely affects the motors ability maintain constant speed because the corrective torque is being applied after the fact. The longer the delay the worse the problem becomes.
Belts, particularly stretchy ones, have the same effect. They filter cogging, but also delay delivery of torque. Filtering cogging is a good thing, but it comes at a price. In many cases the fix is worse than the problem. Much better to just start out with a motor that has low cogging.
In addition to the theoretical arguments there is plenty of empirical data that indicates that low inertia motors tend to sound better. That has been my experience.
BTW: Certus turntables use an eddy current brake. The braking force is much greater than what a belt drive mechanism can keep up with.
Chris |
A flywheel can very well create problems if not done the right way.
It is only suitable to motors featuring a solid axis shaft and a very good and rigid bearing. The flywheel - if done the right way - add considerable amont of inertia to the motor shaft and to the whole moving system. Therefor it requires solid construction. Especially if the flywheel is directly attached to the pulley or part of it (which is the only way to use the inertia moment of the flywheel to smooth out most imperfections of the motor itself). The smaller motors envogue right now do not fare very well with flywheels.
The small motors common today have 3 big advantages. Even if all turntable-manufacturers using them do list some other advantages, it all comes down to these 3 only....:
1. they are inexpensive.
2. they are inexpensive.
3. they are inexpensive............ |
On a related point, one thing I've been wondering is why not integrate a flywheel with the motor pulley. For AC motors in particular this would have the advantage of smoothing out cog noise at the motor source, rather than pass it on to the platter(or an outboard flywheel) through the drive system. I know some of the older heavy-duty high-speed motors have some built in flywheel effect; it's surprising that this approach is not taken with some of the smaller motors in current vogue. |
This is an interesting discussion, thanks to all. It seems to be the case that some other manufacturers take a quite different approach. Nottingham, for example utilizes an extremely low torque motor, so any kind of breaking is out of the question.
I have no idea myself what are the merits of any particular design choice. All I can say is that I have, and like, my Basis Debut table and I have, and liked some of the others mentioned. |
Dertonarm,
the only table? Really? Perhaps you should try to listen to more 'tables. ;-) |
The only turntable so far which did it right (i.e. - correct application of "friction" to stabilize movement AND to provide additional damping to the platter) is/was the old venerable Platine Verdier in its original form (pre-1992) of bearing (without the ball support). Its a true eddy-current brake applied on a large diameter and fairly strong. The large diameter provides excellent damping on the turntable platter itself (which however was only a side-effect of the vertical bearing being suspended by magnetic force). Somehow archaic in execution, but very effective indeed. |
Lew,
Chris Brady's Certus DD designs are said to implement this principle very thoroughly, as does his Verus rim drive, both due to the the motor design.
His belt drives used bearing design and choice of lubrication to achieve a similar effect (as do the similar designs from Galibier and Redpoint). Bearing noise on any of these is nonexistent in any system I've heard, including mine. Motor noise transmitted through the belt, while very low, is more audible than that.
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Come to think of it, Win, I see your point. I observed the herky jerky movement of my SP10 motor without the platter. It almost threw itself off the benchtop where it was sitting. So clearly, the mass of the platter smoothes out the operation. Further, the mass of the platter is constrained to within certain limits, if a servo dd table is to operate well. Of course, that is quite different from an eddy current brake, but the principle is perhaps similar. I just had not thought of it that way. |
Lew,
Yes, there are direct drives that do it, although it may be a happy accident for some. By centering mass in the platter, some direct drives avoid excessive noise from their motors. As a result, they also introduce a better way to handle inertia. By my way of thinking, using inertia properly is key to the conversation, so in that sense, a lot of direct drives succeed. |
I don't know of any direct-drive tables that deliberately use this strategy, except possibly in their electronics. Can anyone think of any? |
From a physics perspective, using the bearing drag to introduce a well defined load for the motor is certainly sound as nicely explained by Dougdeacon and others above.
The DPS differs from some of the other tables mentioned above: the bearing friction is significantly higher than on the other implementations since the DPS uses a light platter and doesn't have the benefit of the inertia of some of the above heavy mass tables. The reason behind the high friction is that, in comparison, any friction introduced by the needle (needle drag) is neglible. From my own experience, the DPS is one of the few tables with a light platter that does have any of the associated speed variations and in many ways sounds like a high mass design (of course the base is fairly high-mass anyway as it contains two layers of lead).
I have not noticed any detrimental effects for the DPS, especially not added noise. Several friends have commented that the DPS is in fact easily one of the quietest tables in our group with the blackest background (comparing to TW Raven AC, Platine Verdier, VPI Scoutmaster, Michell Gyro, etc.).
The DPS is IMO one of the most well designed and thought out tables out there. Everything is extremely carefully matched and optimized; e.g. Willi tried several different platter thickness and weight to optimally match the loading of the motor. If you are looking for a table with a slightly more compact form factor the DPS should be on your list. On the downside the design is not for someone who likes tweaking - the top base has to be replaced for a tonearm and it already comes with all the isolation and platforms you may ever need. |
Free spin equals loss of control, in my opinion.
That's the best way to sum it up! Dodgealum, there is no noise issue. Because a bearing is designed with some drag does not mean that it is being created by allowing things to rub together. For the 'table designer this is much like matching a load with an amp. The motor will be able to control things much better if it is doing work against the load. This is what the bearing drag is for. |
So I see that there are a number of tables that exploit this design principle in one way or another. Can anyone comment on the issue of bearing noise? Also, most of these tables are large and heavy. I'm using a wall mount shelf and therefore require a table that doesn't weigh a ton. Any suggestions as to models I should explore? |
Hey Doug,
Call me Win. I'm trying to outlive the "mosin" moniker, but without much luck. ;)
Anyway, Chris did give a great explanation, and there are various implementations of it. Even sheer mass at the platter can help when the bearing arrangement is designed with braking in mind, but there are other implementations. Properly used, the motor itself can go a long way to controlling the spin. Also, Garrard (as you already know) used a grease bearing very effectively as a dynamic brake. The list goes on, but the most important thing is to make sure the turntable has no runaway effect, or that it is not bogged down anytime during its operation. Free spin equals loss of control, in my opinion.
Best,
Win |
Basis, Galibier, Teres, Redpoint, Mosin's Saskia, Garrard, probably many others. Chris Brady wrote an effective explanation of how a carefully chosen amount of steady-state drag (from the bearing, an eddy current brake or otherwise) can minimize the effect of variable drag events (i.e., stylus drag). Check the Teres website, it's on there somewhere. The concept is that if the motor is working against a high steady load, the proportional value of any stylus drag event is reduced. That makes it easier for the motor/coupling system to overcome. If a table shows the motor/coupling system little or no load, each stylus drag event is proportionately much greater and therefore more likely to be audible.
Having heard most of the tables mentioned above, my ears agree that the theory has merit, always depending on execution of course - as Dan said. It certainly works with Mosin's table, brilliantly.
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Basis, Galibier are two that I've noticed having a bearing with more drag than one might otherwise expect. I expect there are others but these I have first hand experience with. If you spin these bearings with no platter they don't turn very many times. Add the platter and they spin a few more times before stopping. I would also think in the case of these two bearings that it is the tight tolerances and oil viscosity that provide this resistance.
I think of this as being a bit different than eddy current breaking, but it is hard to consider the bearing a table uses without also considering how the platter is driven. (BTW, in addition to Mosin, I understand Teres is using eddy current breaking in their DD tables and maybe in the Verus drive. But I'm not sure about the Verus.) As one can imagine, motors with differing degrees of torque may work better with a bearing with a bit more or less drag. It gets even more interesting when different belt materials are added to the system.
In contrast I recall stories about Walker tables and how they will spin a long, long time on that magnetic field bearing. I don't know what Walker uses for a motor, but I would expect that it is a very low torque motor.
The point I'm trying to make is that you have to take the drive system on any table as a whole. There are many designs possible to get to a solution. |
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It's not a new idea. The Garrard 301 used an "eddy current brake" in much the same way. The degree of braking was used to fine adjust speed, but it also stabilized the speed. Don't know of any other "modern" turntables that do this, though. |
