Vintage DD turntables. Are we living dangerously?

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