[neobop]

From the TNT review of the Septum arm...
"OK so if we apply this test to a tonearm we'll get an ellipsoid around the arm's centre of gravity - approximately around the pivot of the arm. Now project the ellipsoid forward to the stylus so an ellipse surrounds the stylus and you'll have a representation of all the forces acting on the stylus, and by the way, to the arm cartridge combination. See diagram.

The perfect arm would have the ellipse as a circle with the stylus tip in its centre. That means that the stylus will move equally easily up, down and left/right. Neutrality is complete.

If the ellipse is flat then the system is not neutral, it is easier for the stylus to go up and down than left to right. In this case the combination of the arm's effective mass and cartridge compliance will give various frequencies of resonance rather than one which is easier to control. This is the design weakness of all air-bearing parallel-tracking arms, the resistance to up and down movement will be much like a bearing tonearm, but side to side will be much more difficult as the totality of the arm must move - around 20 times more difficult than vertical movement. Depending on the eccentricity of the record (no record is perfectly centred) the arm movement can take any direction and speed each time adding a different resonance. At best such arms can be symmetrical, at worse just crazy - by that I mean that to have an ellipse so distorted with the stylus way off centre - it means that all movements are different, adding different resonances, different behaviours etc."

Pierre Lurne also believes that the arm has 80% influence on the sound and the TT only 20%....

Based on the possible SQ from both linear and pivoting arms, I wonder if this isn't taking theory to a level beyond the requirements of a cartridge tracking a record groove.  If the resultant resonant frequencies (horiz & vert) can be tuned to an appropriate or complementary range, perhaps the difference in vert and horiz resistance is of little consequence?
This brings to mind a post on Audiogon about tuning the Terminator air bearing linear arm by adjusting the vert eff mass.  Apparently it works.

I think the arm contributing 80% would only apply to tables at a certain quality level, and not anything close to budget or entry level tables. 
neo

 

[xsb7244]

In regards to Pierre Lurne, the Audiomeca Turntable Belladonna with Septum Tonearm on Ebay sold for $5733.00 on AUG 7 2013.

[dlaloum]

Quite a bargain, given that list price for that combo was $20,000 I believe.... (I think I saw it mentioned as $15k for table and $5k for arm)

There is also an Audiomeca J1 on a British site for around UKP2000

Both are good value relative to new prices....

[neobop]

Suspension or non-suspended.

From the TNT article:
http://www.tnt-audio.com/sorgenti/belladonna2_e.html

"Conventional turntables mount their main bearing in one of two ways. First they can be fixed to a subchassis, which in turn is suspended (or supported) on springs/elastic in order to isolate the working parts of the turntable from the outside world. These springs are tuned to certain frequencies (usually 1-5Hz to avoid being excited by record warps) and energy from platter/motor system that is not managed by the subchassis can be transformed into low frequency vertical movement where it will do least damage to the sound. This is why the setting-up of suspended turntables is such a black art - as anything but a perfect vertical bounce will provoke all kinds of parasitic movements (wobbles again) which again are uncontrolled and chaotic and can reach well into the audio spectrum. Anyone who has spent a jolly afternoon trying to get an LP12 to bounce cleanly will sympathise.

The snag with almost all such designs is that no matter how well set up there will always be some parasitic movements, poor mass distribution, non-standard arms with different weights, tired springs, odd springs and just bad design make things worse. When you add the sideways pulling of the motor on such a system you can see what a nightmare it is to get right."

There's a picture of a cross section of the table, underneath the quote.  It's like the chassis is balanced like the platter and the springs are not taking the weight. 
neo

[dlaloum]

No springs...

[neobop]

To my way of thinking, no springs is preferable to an imperfect set of springs, and almost all suspensions are imperfect.  On my first high end audio job I learned how to set-up and adjust a Linn.  I even owned an LP12.  The springs should be adjusted so the table bounces straight up/down.  This is checked by pushing straight down on the platter at a point equidistant from each spring.  On the LP12 this is on a straight line between the arm pivot and spindle where the label ends or lead out groove would be, or roughly 50 to 60mm from the spindle.  (It's been a long time and I don't remember exactly, but when adjusted properly you'll get a straight bounce from anywhere near that point between spindle and pivots.) 

It's easy to see why any sideways movement would mess up performance.  Even though the arm is on the subchassis and moves with the platter, the platter is usually attached to a rubber band that will anchor any movement and make the platter rock and/or wobble.   "They" say that some treble modulations on a record groove are around a millionth of an inch and lateral movements will introduce extraneous forces on tracking which is an in-groove  balancing act to start out with. 
Many, if not most sprung tables have a fixed motor/pulley so even vertical movement is problematic.  If the belt doesn't shimmy up and down during vertical movement, it's still exerting the rubber band effect.  The fact is, even if a platter/ arm appears to have a perfectly vertical bounce, on a minute level it doesn't.   If the motor is mounted on the subchassis and moves with the arm/platter the anchor effect is eliminated, but any imperfect suspension movement will still compromise performance.

The reason suspended tables with an imperfect suspension can sound good, is that suspension often isn't, or is barely excited while playing a record.  Otherwise there's a smear and loss of information.  Defeat the suspension on such a table and it becomes obvious.
After the LP12 I had a Studietto, a suspended DD table.  It had 3 big springs between the base and the top plate.  These were the same springs used on the Studio, but tuned for the Studietto and the T5 linear arm.  I had a conventional arm on there and had to steal an optional Studio spring to balance the suspension.  Some people defeated the suspension to optimize, substituting rubber or sorbothane pucks for the springs.  Non-suspended tables have another set of problems dealing with vibrations, but it seems to me it's more straightforward.  If the same considerations of placement are given to a non-suspended table (not on top of a wobbly equipment rack) as a  sprung table, then extraneous energy must be dissipated in a direct manner, rather than through a partially intervening suspension.
neo   

[dlaloum]

Lurne's whole design focus is based on minimising micromovements of any sort, and where they cannot be minimised, to control them and focus them in a relatively harmless direction....

So he has gone from the J1 (descendant of the Goldmund Studio/Studietto) which is sprung but designed carefully to control the motion and ensure it remains vertical, to the current (?) Belladonna, which is effectively a solid plinth design....

His ideas are worth considering when optimising and modifying a TT and its platform!

[theophile]

Another issue is that PLL direct drive systems used to lock into a Quartz signal, are dependent on the ineraction between torque of the motor and mass of the platter...

If the platter is too heavy, it will speed up insufficiently and require additional pushes - making the whole process uneven, and introducing the potential for the "jitter" than some SL1200 tweakers describe (mostly when talking about PSU mods for the technics tables)

Same problem if the platter is too light...

The intricacies of determining how much latitude a specific motor and control system has for additional mass is a mystery to me - I am aware of the issue and its potential impact, but do not have the theoretical understanding, let alone the relevant measurement tools to investigate and tweak this aspect....

I think this is a high risk area, where a lot of tweakers have gone down the path of using heavy TT weights - with potentially subtle negative effects. (and due to psycho acoustics, it can be VERY difficult to pin down subtle effects...)

I have added a small amount of mass to my Revox platter in the form of plasticine on the underside as a minor damping treatment, and I use a BIB Clamp rather than a massy (messy?) weight.

I do like the idea and sound of the stability provided by huge inertia, but have issues with the idea of a 200lb+ turntable..... it somehow just lacks elegance.

bye for now

David

David I know that this applied to many, many turntables but it didn't apply to the Yamaha GT 2000 which could be used with either the standard 6.5 Kg aluminium platter or the 18Kg gunmetal platter without any adjustment at all.

Pierre Lurne is a genius and his insistence upon attention being paid to center of mass( center of gravity) is well worth consideration for many aspects of high fidelity reproduction.

[neobop]

Hi Theophile,
Welcome to the monkey house, glad you stopped in.  David hasn't been around much lately.  He seems to be occupied with family and whatever.  Hopefully he'll visit again, soon. 

Your GT 2000 is a wonderful table IMO, and not equaled by many of today's "top" tables.  For those who don't think metric, 18kg = 39.68 lbs.
The Goldmund Reference platter is 35 lbs. 
Here's something about the table:
http://www.thevintageknob.org/yamaha-GT-2000.html

http://www.thevintageknob.org/yamaha-GT-2000x.html

The stock platter is listed at 5.8 kg = 12.79 lbs.   The tonearm has 7mg of friction in both planes. 

I'm putting one of these on my Christmas list.
neo