My VPI Ares3 turntable which is made with a laminated acrylic/aluminum/acrylic plinth, about 2.5 inches thick in total, has a 5/8 inch circular cut through the top layer of acrylic which exposes the center laminate of aluminum. Set into this circular cut out is the 'puck' arm board mounting disc which is about 7/8 inches thick acrylic. This bolts to the aluminum via three machine bolts. My SME Series IV magnesium tone arm is bolted to the acrylic tone arm puck.
Magnesium is known for its superior vibration and sound deadening qualities, so I was wondering if anyone has any thoughts on making the tone arm board 'puck' out of solid magnesium vs. the factory supplied acrylic?? I can see why they probably don't use magnesium as it's super expensive. I just got a quote on a small piece of magnesium to machine a new tone arm board and its $125. But if it were to make an improvement in performance, its cheap money.
The stuff I used was tan colored “plastic damping sheet” I bought from small parts years ago, which I think is no longer in business. The material was meant for damping of electronic chassis etc.
@travbrow, is the damping material bright blue in color? If so, it may be EAR C-1002 Isodamp, considered the constrained layer damping standard in the industry (though Geoff Kait disapproves of it). Michael Percy Audio sells C-1002 in varying thicknesses. Another good cld material is Wall Damp by ASC (Acoustic Sciences Corp., makers of Tube Traps), sold in 4" X 4" pieces for a pittance.
I’ve read that a plain old aluminum/MDF/aluminum “sandwich” has very good damping properties. I’ve made a Lenco based turnable plinth a few years ago from aluminum. It’s two layers of aluminum with a thin layer of damping material in between, I can’t remember the name of the damping material I used though it worked okay. I haven’t been using that Lenco build, but plan on redoing it with MDF as the middle layer. Also scrounging up cheap parts for a DIY tonearm.
While I don't claim to be an expert, I nonetheless agree with Ralph. I am not new to plinths and armboards. If possible, mount the arm directly to the base of the plinth. This is not possible with a Thorens TD124 unless the armboard chassis is cut off, but it is easily done with a Garrard 301/401 or an SP10. It surprises me that so-called state of the art plinths by some vendors have pivoting leaf armboards. Clever and versatile yes. Ideal, no.
Thank you Atmasphere for the thoughts on materials used. My Ares3 is Acrylic sandwiched with a center layer of aluminum. The top layer of acrylic was machined out down to the aluminum layer so that the same acrylic material used for the arm board is bolted directly to the aluminum. In other words, the arm board is the exact same construction and materials as the plinth which mirrors what you stated.
I will stay with this for sure (after the turntable does sound fabulous) and why fix what is not broken.
sleepwalker, when I began researching plinth building I found a few suggestions for Corian. However I then read that Corian can in fact be subject to vibration transmission. Otherwise it might be more commonly used I guess. One comment stated that transmission will vary depending on the amount and size of the aluminum particles in a given Corian sample.
It may look nice and be easy to work with, but I ask myself if it is so good why is it not more commonly used?
Regardless, I think Ralph offered very good basic advice.
The plinth of the turntable should be as dead and rigid as possible.
The thing that most people don't think about is that the arm board is part of the plinth. The base of the arm should couple as rigidly to the bearing mount of the platter as it can. If the arm board is made of a different material than what supports the platter bearing, its possible for vibration to move the platter on one plane while the base of the arm is moving in another or maybe isn't moving at all.
You want the base of the arm and the base of the platter bearing to move in the same plane at the same time or else the difference is interpreted by the pickup as a coloration.
IME for this reason the arm board should be the same material as the structure that supports the base of the platter bearing and hopefully that is the same structure that supports the arm board too.
another vote for Delrin, if and when you choose to make another armboard. I fabricated an armboard for my Aries 1 using Delrin at the suggestion Harry W. himself. It wasn't hard to do and it has worked well, although I haven't compared it to other materials. the next one I do will probably be in ebony.
The service dept. at SME sent me a note this morning, they say magnesium would not be beneficial for the arm board, they like to use either aluminum or brass. Delrin is good too as yes its part of the super platter, which by the way I have on my Ares3.
But since the stock acrylic material appears to be a desirable material and its already made, I’ll stick with it. My plan was to make a one piece new arm board with a bump out on one side to accept the Tru-lift arm lift for end of record pick up and another bump out to accept my new micrometer head VTA adjuster. However last night I machined a stainless steel part which will bolt to the acrylic arm board and it will support the Tru-lift and offer adjustment so that the lift can be perfectly dialed in for where I want it to activate. I’ll make up other parts to mount on the other side for the VTA on the fly using the micrometer head. This is actually easier for me as all I have to do is drill and tap the arm board to accept the new parts.
One other material to consider is a close cousin to acrylic---Delrin. That's a brand name I believe, not the material's generic name. Delrin is the material VPI used for the platters of the HW-19 MK.2-4, Aries 1 (top layer), and TNT (pre-Mk.6) tables. It is a little denser and harder than acrylic, and has a matte sheen in contrast to the high gloss of acrylic. I had an acrylic arm board for my Aries 1 made at a good plastics supply house, and the price for a Delrin version was only slightly higher than for the acrylic.
Another option to consider is to have two 1/2" thick boards made, with a thin piece of constrained layer damping placed between them, for damping of any ringing in the material.
May I suggest Corian from DuPont? It is easily machined, very stable and very inert to vibration. You can probably get a scrap from a counter fabricator for free. I’ve also made shelves for my equipment racks from Corian and wouldn’t use anything else for that.
If all else fails, shoot for the moon and go for unobtainium!
OK, this is interesting; I spoke with the chief designer at Oracle Audio in Canada on this subject. He said that my stock arm board puck on the VPI Ares3 is 1 inch thick acrylic which is a very good material. He said Acrylic is a surprisingly good acoustic dampening medium and he suggested I stay with it. If I go to another material, there is a risk of 'ringing'. That will simplify my project, I can fabricate the parts I want for the VTA on the fly adjuster and bolt them to the stock arm board.
I do understand the strength to weight issue with magnesium, however I have also been told that it is very good at dampening sound vibrations which is why I was considering this material. It would be nice to find a table of dampening values for various materials.
Magnesium is more known for its strength to weight characteristics and that is why it is used for tonearm. I would experiment first with other tonearm board materials like ebony and purple heart woods. Possibly brass etc.
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