Vintage DD turntables. Are we living dangerously?

Geoff, I would have thought that "EMI absorption" is but a more concise term for "low frequency magnetic field absorption".  Especially since my reading also tells me that the forte of mu metal as a shield is to block or contain EMI, rather than RFI.  As I understand it, mu metal is not much good with RFI.  Please correct me, if I am off base here.  

L07D owners, including me, espouse the use of a shield between its platter and the stainless steel "platter sheet", which seems to be Kenwood's term for "mat".  For a few years, I used TI Shield, when it was available from M Percy, in this application, and I reported that it did some audible good.  Then I bought an Ortofon MC2000 cartridge and mounted it on my L07D.  I immediately noted that the rotor of the L07D motor, being itself a large magnet, induced magnetism in the TI Shield, which in turn sucked my MC2000 down on to the platter, nearly collapsing its suspension, a very very bad thing.  This observation forced me to remove the TI Shield from the sandwich, and I have lived without it since then.  Apparently the placement of the magnets in the MC2000 (and maybe other Ortofon LOMC cartridges) makes it uniquely susceptible to this problem, because I never had experienced it previously with any other brand or type of cartridge.  I know that Dave Garretson used ERS cloth on his L07D platter, instead of TI Shield, and I wonder whether ERS cloth would produce the same induced magnet problem, probably not.  However, I always thought that ERS cloth might have the negative effect of impeding energy transfer between the platter sheet and the platter, on an L07D, which is why I have not tried it up to now.  Any comments appreciated.  ERS is not quite as good a shield as was TI Shield, as far as I can learn from my reading. (It's a very complex subject.)

On my TT101, I use an SAEC SS300 mat that was hanging around the house, which I tell myself adds some shielding effect between motor and LP surface.  I am using an Acutex LPM320III cartridge in an FR64S, on the TT101, and I perceive no problem that could be ascribed to EMI or RFI, so I don't worry about it.  In fact, that combo is sooooo surprisingly good that I am loathe to experiment further.

Lewm wrote,

"Geoff, I would have thought that "EMI absorption" is but a more concise term for "low frequency magnetic field absorption". Especially since my reading also tells me that the forte of mu metal as a shield is to block or contain EMI, rather than RFI. As I understand it, mu metal is not much good with RFI. Please correct me, if I am off base here."

actually mu metal absorbs magnetic field, that’s why they refer to mu metal as high permeability material, it allows magnetic fields to permeate the material. The correct mu metal for transformers is low frequency high permeability mu metal. EMI is electromagnetic whereas magnetic fields are not electric, strictly magnetic, I.e., gauss the magnetic fields are absorbed not blocked or shielded. Mu metal might actually be good for RF since it is a metal alloy, mostly steel, like a chassis frequently. But conductive materials like ERS cloth are generally of no use against magnetic fields.

Geoff, I will have to chew on that mouthful for a bit, but can we agree that EMI has the property of "frequency"?  Assuming you would agree, then in what way would EMI differ from "low frequency magnetic field(s)"?  Permanent magnets per se do not exhibit the property of "frequency"; you need a coil AND a permanent magnet and relative movement between the two, to generate a pulsatile field.  At least, that's my story.

Dear @lewm : That experiences you had with your Ortofon MC 2000 is " exclusive " of that extremely low output design: 0.05mv that needs very powerful magnets.

I owned 3 of those gems and in those old times Ortofon gaves a serious warning not to have any single metal ( obviously that non-magnetic materials as aluminum is no problem. ) near the cantilever/cartridge and that's why when other cartridge manufacturers puts inside the cartridge box a metal ( magnetic. ) screw to mount the cartridge Ortofon puts aluminum one.

I had that kind of experience but not with the TT platter but with the Shure VTF tool that in those times some production lots where not made it of non-magnetic metal. In my case I can't save my cartridge and send it to Ortofon to fix it. So, be carefully with.

Regards and enjoy the music,
R.

lewm
5,047 posts
09-16-2016 12:39pm
Geoff, I will have to chew on that mouthful for a bit, but can we agree that EMI has the property of "frequency"? Assuming you would agree, then in what way would EMI differ from "low frequency magnetic field(s)"? Permanent magnets per se do not exhibit the property of "frequency"; you need a coil AND a permanent magnet and relative movement between the two, to generate a pulsatile field. At least, that’s my story.

the reason I call mu metal low frequency high permeability material is because it is normally used to absorb magnetic fields around transformers which are 60 Hz devices, hence the low frequency attribution. There is also high frequency mu metal for use around speaker magnets. EMI is equivalent to RFI. RF is radio frequency that is usually specified in GHz or MHz but can be lower. Radio Frequencies are part of the ElectroMagnetic (EM) spectrum. Magnetism itself does not have a frequency associated with it. It is normally specified in Gauss which is a measure of magnetic flux density. Another distinction is RF is light speed whereas magnetic fields have no velocity. So, to summarize we actually need to address BOTH EMI/RFI and magnetic fields in the audio system, the strongest magnetic fields being produced by current traveling through wire in transformers and produced by speaker magnets and weaker magnetic fields are produced by current traveling through any wire or power cord or cable.

cheers