New acrylic platter for the SL-1200/1210

[Marco]

Hi Chris,

I think it could be very good , Arthur wouldn't be doing it otherwise would he ? i suspect its something he's been looking at for some time.

Indeed! It's only because Arthur's involved that I'm giving this platter any credibility

One other thing , it shouldn't really need any mat i would have thought , but i read somewhere that Arthur recommends the Achromat even on old PT's .

I agree, and when I have the platter, that's something I'll try - both with and without an Achromat. It's all interesting stuff!

Marco.

[YNWaN]

YNWaN. My Voyd is currently having a rest. (Only room for one functional TT here) When I set it up again I'll do some needledrops of records played on the acrylic & then the Lexan platter. It'll be instructive to see whether the difference is audible.

Ah yes, didn't realise you had both options - that would be most informative , many thanks.

[chris@panteg]

Ah yes, didn't realise you had both options - that would be most informative , many thanks.

Mark

FWIW , the difference is subtle but important IMHO , as it should be , if it was gobsmackingly huge then you would have to wonder if something's wrong with the design ?

[pure sound]

I do also have a 10mm Lexan mat for my SP10 which sounds good but it's thickness means that there isn't much spindle left if I choose or need to use a clamp.

Lexan is interesting stuff, dealing with shock very differently to acrylic. Drop an acrylic platter onto a concrete floor (Don't!) and its likely that a piece will chip off. Drop a Lexan platter and it will just bounce. The shock is dissipated more efficiently. Hence, I suppose why it is used for crash helmets and as vandalproof glass in bus shelters etc.

It does have a different characteristic when records are played on it to acrylic, but rather than try to describe it I'll do some Needledrops when the opportunity arises.

The bubble filled achromat material also has different properties to acrylic and also supposedly converts vibration to heat more efficiently than acrylic does.

[chris@panteg]

I do also have a 10mm Lexan mat for my SP10 which sounds good but it's thickness means that there isn't much spindle left if I choose or need to use a clamp.

Lexan is interesting stuff, dealing with shock very differently to acrylic. Drop an acrylic platter onto a concrete floor (Don't!) and its likely that a piece will chip off. Drop a Lexan platter and it will just bounce. The shock is dissipated more efficiently. Hence, I suppose why it is used for crash helmets and as vandalproof glass in bus shelters etc.

It does have a different characteristic when records are played on it to acrylic, but rather than try to describe it I'll do some Needledrops when the opportunity arises.

The bubble filled achromat material also has different properties to acrylic and also supposedly converts vibration to heat more efficiently than acrylic does.

Hi Guy

I know about Lexan's ability to resist shock , all car headlight lenses are now made from Lexan , i tried to break one once with a lump hammer , it just flew off and out of my hand , it was very funny watching my project manager trying the same

[Mike_New]

Hi Folks.

Some observations on the use of a non-metallic material for the Platter of a Direct Drive System such as the SL1200.

I have been following this thread for some time now and have remained silent and in the background as I read all the observations, and absorbed the euphoria, mainly prompted by the snake oil salesmanship of others.

However, I have been prompted by some to make a few observations. Also I would state here that I have been aware of the following issues for some time but have not thought it appropriate to raise them, in the interests of not giving the opposition a chance to hang their hat onto something that in the context of the standard SL1200 is not a massive problem.

I do hope my friend Arthur is reading this, as it could save him wasting his time!!! This will probably come as a bombshell to all those who follow this website and another one! And to those subscribers to other websites that I no longer have access to! And who have opined about the use of Plastic be it Acrylic or Delrin and whether Museum or Fish grade (Aquarium), with such superlatives as the following:

"Nothing like anything anyone has seen before ( think Clearaudio Statement ) and integrates properly with the direct drive PLL."

I investigated the use of acrylics and other materials before deciding on Aerospace grade Alluminium for the foregoing reason And this is not totally my own theoretical treatise on this subject. Other audiophiles, on other forums, who are devout anti SL1200ers have put forward their own justifications for not embracing the SL1200, based on the following:

Firstly, absolutely no one; out of all the supposedly technical experts who like to contribute their opinions; has given any thought to this: What do you think is bolted to the underside of the platter, a lump of Swiss cheese??

No. it is a very powerful ferrite Isotropic Magnet, Radially magnetised with a large number of poles. I will not say how many, I will leave it to the ‘temperature expert’ genius to see if he can work it out. Yes I do know, and I have plotted the polar fields it produces.

Now, to an extent the very intense flux fields emitted radially outwards and upwards, by the magnetic rotor are shielded by the design of the ‘Direct Drive Magnetic Motor System’ However, in any system such as the SL1200 Direct Drive motor, some flux leakage can and will occur.

However, the SL1200 platter does consist of a 3mm layer of metal albeit most of the mass is made up of bonded rubber! This metal platter has the effect of most definitely and positively screening the cartridge from any flux leakage from the motor magnet. The reason being is that the “permeability’ of Alluminium, which is a paramagnetic material (as opposed to a diamagnetic one) is sufficiently high when compared to air that it does have a considerable flux shielding affect. Copper is even much better!

Acrylic is practically useless in this regard and would be almost equal to air. (would some one out there like to check on this!)

The whole subject of magnetic flux and relative permeability is not the subject for this forum. For those who wish to follow it up I can suggest:
“Electricity and Magnetism” A degree in physics. C J Smith and Arnold. (My edition was 1959)

It follows therefore, does it not, that a platter machined from a solid blank of aluminium 16mm thick and bonded with a 3mm plate of annealed copper is going to be unbelievably vastly superior to a lump of Acrylic, even if Dave Cawley is selling it....!

This flux leakage when interacting with the coil of a good quality sensitive cartridge will result in modulation of the collected signal.
How does this happen. Given that the record and the platter and therefore the Magnet and its magnetic poles are rotating at 33rpm any leakage flux will be cutting the coils of the cartridge, at the rate of the number of poles, which the magnet has, per revolution.

Now here comes the crunch!! If at any particular point in real time the cartridge is transducing a pure note or complex waveform at the point when one of the flux fields of the magnet is cutting the cartridge coils, then presto you a have a fantastic bit of modulation and resultant distortion.

Now here comes the interesting bit, the position of the particular section or fraction of audio that could be affected, in relation to the flux fields of the magnet as they pass under the cartridge, is going to be dependant on the radial position that the record is located onto the platter. In other words any perceived modulation or otherwise distortion, will never be constant or repeatable for evaluation or correction purposes!!! Just a random degradation of the sonic experience predicated on the audio content of any particular record and it’s position as placed onto the platter.

And this is only going to cost you about $800.

As I have stated earlier in this analysis, I have plotted the flux fields and carried out a few simple evaluations of my own, all of which lead me to the conclusion that the use of any non-metallic material is going to greatly but randomly degrade the performance of a direct-drive turntable, such as the SL1200.

Indeed it should be intellectually intuitive to any one who has just the merest modicum of intelligence; that not having some effective flux screening between the powerful magnet and the very sensitive cartridge must surely be asking for problems.

It is I believe, because of the foregoing observations, that the copper mats are so effective on the SL1200 series of turntables. Besides providing extra mass, they provide an order of magnitude higher shielding against any flux leakage that may just occur. Remember we are talking sub-micro volts in the cartridge coils and phono amplifier electronics.

In conclusion I do not believe that the use of acrylic in this instance for a direct drive turn table is a particularly smart one. Sure, it is a lot less costly to produce than my alluminium/copper approach, and is about $100 cheaper to boot!

Some may enquire that almost all the rubber band-driven turntables use some form of acrylic platter. Exactly!! Because the vendors of these units must realize a decent profit to stay alive, and acrylic is dirt cheap compared to aluminium and copper. The motors on these units are high torque AC or DC off-the-shelf units, with adequate shielding, most often this could be Mu-metal (the ultimate flux screening material).

Further, they are usually placed about 500mm from the centre of the platter, far enough away that any flux leakage would never be a problem.

[chris@panteg]

Hi Mike

I've asked Dave about the screening of the platter , and well , you can see his answer !

I've also asked Richard (Vantage Audio) about this platter and look forward to reading his thought's . FWIW my feeling is the OEM does screen out the magnetic field of the magnet , but i don't regard myself as an expert on this TT , just looking for a straight answer ! And wanting to learn something .

[Marco]

Hi Chris,

i don't regard myself as an expert on this TT , just looking for a straight answer ! And wanting to learn something .

Indeed.

Trouble is, you'll never get a straight answer from Dave C, simply because his modus operandi is to shroud everything in an air of 'mystique' and secrecy in order to portray the image that his knowledge of the SL-1200 is somehow 'unique', which of course adds to his delusion that he's a Technics T/T 'guru' - and the 'world's leading one', at that...!

Don't ask him too many questions, though, as he'll simply ignore them, or selectively answer some, and eventually trip himself up! When you scratch below the surface and expose his technical naivety, like Mike has just done, however, the truth comes out.

Mike - an excellent post. I'll comment more after lunch

Marco.

[chris@panteg]

Hi Chris,



Indeed.

Trouble is, you'll never get a straight answer from Dave C, simply because his modus operandi is to shroud everything in an air of 'mystique' and secrecy in order to portray the image that his knowledge of the SL-1200 is somehow unique, which of course adds to the delusion that he's a Technics T/T 'guru' - and the 'world's leading one', at that...!

Don't ask him too many questions, though, as he'll simply ignore them, or selectively answer some, but eventually trip himself up! When you scratch below the surface and expose his technical naivety, like Mike has just done, the truth comes out.

Mike - an excellent post. I'll comment more after lunch

Marco.

Fortunately for me i do know someone who does understand this TT and i have 100% faith and confidence in the guy

[Marco]

Indeed - Richard does, as does Mike (and I have 100% confidence in him)!

Isn't it funny how I knew what material the new platter was made from before Dave could inform folks on his own forum!!

What does that tell you?

Marco.