Hi Mike,
Just to pick up on a few points from what you've written:
Quote:
I do hope my friend Arthur is reading this, as it could save him wasting his time!!!
I've asked Arthur to comment, and he will do so in due course. I know his answer is sure to be enlightening.......
Quote:
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..."
Well that's a typical example of Dave getting rather over-excited in an attempt to be seen as 'the special one'... "Nothing like anyone has seen before?" - You mean an acrylic platter like that used on many other T/Ts currently on the market?
The platter looks nice enough, but I don't see anything radical in its design such as "nothing anyone has seen before". So, as Michael Winner would say: "Calm down, dear!" :lol:
Quote:
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.
It follows therefore, does it not, that a platter machined from a solid blank of alluminium 16mm thick and bonded with a 3mm plate of annealed copper is going to be unbelievably vastly superior to a lump of acrylic.
And:
Quote:
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.
So my decision to have my own platter bonded with 3mm of annealed copper is vindicated? Nice one! :)
My theory for doing so was based on nullifying the effect of any eddy currents being generated by the motor unit, where in fact the main priority was actually screening the cartridge effectively from any flux leakage from the motor magnet. Or is that in fact one and the same?
Quote:
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 come 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.
Very interesting.... I wonder if DC, in his infinite wisdom, as the self-confessed Technics T/T guru extraordinaire, had considered those facts?
Quote:
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 dependent 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.
Or that one? ;)
Quote:
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.
Yup - even a technical numpty, like me, can understand that!
Do we know, however, if Arthur has fitted any form of shielding, e.g. a copper (or Mu-metal) plate, on the underneath of the acrylic platter?
It's possible, as other than Arthur (and wonder-boy from Dartmouth) I don't think anyone's seen what the underneath of the new acrylic platter looks like.
However, such shielding would simply be acting as a bandage. If acrylic really is the wrong material from which to construct a platter to use on a direct-drive turntable, then any incorporated shielding would simply smack of 'locking the barn door after the horse has bolted'...
Far better not to have let the horse bolt in the first place!! However, knowing Arthur, he must've considered this.......?
Quote:
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.
Ah, now that makes sense! :cool:
Marco.
