Thorens TD124 “Mk1”/Mk2 plinth

The after-market Schoppers subplatter IS made of a type of cast iron similar in acoustic properties to the original subplatter.  However, the cast iron is impregnated with a material that greatly decreases magnetic attraction; it was material supposedly invented for use in submarine construction where not attracting magnetic mines was a big help.

Schopper call it “bronze”.

Which is an alloy of mostly copper and some tin. No iron content.

It has been been a while since I was on their site.  It was either Schoppers in the past, or some other company that offered the non magnetically attracted cast iron subplatter.  Schopper's current offering is bronze.  I have no idea if it is as non-resonant as cast iron.  Cast iron rings for only a brief moment when struck, I saw this demonstrated in a Fern and Roby turntable that was made of cast iron for that very reason (their first audio product).  

I have not heard a Thorens 124 with the aluminum subplatter so I don't know from my own experience if it doesn't sound as good.  I have only heard tables with the original cast iron subplatter.  I have helped with setup of cartridges on the tables with cast iron subplatters and did not notice any issue with magnetic attraction.  In any event, most of the tables reconditioned by this dealer needed nothing more than a good cleaning, a replacement of the rubber mushroom suspension parts and perhaps a gentle resurfacing of the idler wheel.  The braking mechanism almost always needs readjustment too.  

My local dealer likes this table so much that he recommends it for systems well into six figures where the customer would be willing to spring for something much more expensive.  He puts modern arms on the table and uses plinths from various commercial sources or plinths made by the person who makes his custom speaker cabinets.  These plinths are simple, somewhat light weight boxes; he prefers this to the heavier plinths that use much more wood to make them extra rigid.  

I can believe that cast iron is relatively nonresonant, if only because Albert Porter uses a large cube-shaped cast iron block to dampen the bearing inside his Panzerholz plinths for Technics SP10 mk3s and Mk2s. When I was creating my own plinth for my own Mk3, Albert advised me to use iron for that function and for the reason that he felt it was nonresonant, but I was fearful that such a huge chunk of iron would affect the massive magnet that is fastened to the Mk3 platter, which functions as the rotor of the massive motor, thereby possibly reducing motor torque.  So I went with brass instead, inside my Mk3 plinth which is made from a huge piece of slate cradling the chassis. Then the slate cum chassis sits on and is bolted to a cherrywood base that harbors the large chunk of brass. In both Albert’s plinths and my one-off plinth, there is a threaded rod that passes through the chunk of metal (iron or brass in my case) and snugs up against the base of the mk3 bearing housing. Interestingly, you can tune the result by tightening or loosening the contact between the rod and the base of the bearing housing. Not too tight sounds best.

I think but am not certain that there is a layer of bronze in the 21-lb Mk3 platter.

To the OP: Woodsong Audio (Chris Harban) makes some beautiful plinths for the classic idlers, including the TD 124.  I have two of his creations, a Garrard 401 and a TD124.  The latter is in a figured mahogany - check my systems page for pictures.