DIY Ideas for TT Magnetic Shield Needed

5

5th beatle

Active Member
Been told the motor of my ROK L-34 may be the culprit causing turntable or rather the cart to pick up hum when it comes into contact with vinyl. Does the motor need covering and what material(s) are needed? Thanks for all suggestions.
 
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In trying to reduce hum of guitar pickups a common practice is to line all of the cavities with copper shielding tape or brush on copper shielding paint. I wonder if treating the underside of the platter would be beneficial?
 
As far as I remember high school and college physics, in order to shield something from electromagnetic field one needs to encapsulate the source of the field or the object that needed to be shielded into some kind of conductor. That's why transformers and relays in turntables and some amplifiers are covered by a grounded metal cover.

So in your case you probably need to wrap the motor in thick foil as much as possible and ground it. Or perhaps use the mesh made from metal. Again, this is IIRC the physics...
 
What cartridge are you worried about?

Not much outside of Grados have any hum pickup to worry about...
 
MuMetal

Do a google search for MuMetal, It's an adhesive backed foil like tape made just for grounding things like turntables. I used it to ground the motor on my ARXa so that I could use the Grado Green, works great. Mark
 
kotofei said:
As far as I remember high school and college physics, in order to shield something from electromagnetic field one needs to encapsulate the source of the field or the object that needed to be shielded into some kind of conductor.
Click to expand...

Just my 2 cents on this:

This is strictly true for the electric field ( Faraday's cage ) and it is not so difficult to implement. Shielding a magnetic field is much more difficult (there is no Faraday's cage for it). If there is a motor involved, many effects could be blamed for the noise but certainly, the magnetic field should be counted among them. The problem with a time-varying magnetic field is that it induces electric currents in conductors due to Faraday's law. However, a metallic shielding could work for the magnetic field in that, although it cannot block it, it makes it weaker given that some electric (eddy) currents are induced in the shield, and the latter dissipate energy that is taken from the magnetic field. So, a metallic shield could work but for different reasons. It would be best to make it thick, of a material with the largest available resistivity, and allow for some cooling in its design (air convection).
 
Another idea would be to place a thick aluminum layer around the motor (having in mind to leave some necessary cooling for the motor ). Aluminum is a diamagnetic material, for which its atoms generate a local magnetic field that is opposite to an external one. This effect is, however, small but might help. I've seen some "magnetically shielded" speakers that had a layer of aluminum around them.
 
To shield the motor you could make a cover for it out of "Fish Paper". Non corrugated cardboard similar to that used in a shoe box. Then completely cover with copper tape affixed to the chassis and it's frame ground.
 
MuMetal.jpg


It's much easier to shield for EMF on the platter, than the motor. I used two MuMetal foils on this Lenco platter. Expensive, but nothing gets through.
The catch is that the platter needs to be big enough - with no record overhang, because that EMF comes blasting around the edges greatly intensified.
- Mario
 
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