DIY composite platter

[marekst]

My DIY turntable had a VPI acrylic platter and Teres motor. The combo had small speed stability problem caused by not enough inertia for Teres speed controller. I managed to eliminate static buildup by covering acrylic with copper foil. The sound was good, but it wouldn’t be a true DIY turntable without home made platter. I came up with the following:
http://gallery.AudioAsylum.com/cgi/g...f=platter4.jpg
The platter was cast in one operation and consists of 3 layers:
3.5mm solid graphite
3.5mm carbon fiber/metal powder/epoxy composite with gradually increased density
54mm high density fiberglass/metal powder/epoxy composite.

I used fine, heavy powder, about 55%W, 40%Cu and 5% Pb. Lathing was a pain - my universal milling machine is too small for the job. The platter (without center weight and peripheral ring clamp) weighs 11 kg, most of it is supported by 2 neodymium ring magnets. The bearing had to be extended for the new thicker platter; the center of gravity is deliberately raised to bring it closer to bearing support point. The top magnet is set in resin ring which is set in steel ring for shielding (see pictures). The position of steel ring is adjustable to compensate for possible future weakening of magnets. The Teres speed controller works fine now. The shielding is not quite what I’ve expected. The needle of the compass placed on top of the platter bends downward. I need more listening time to comment on the sound.
Fill free to criticize and ask any questions.

Warning!!! Epoxy resin is toxic and allergenic; air born metal powder is toxic. Hardening process of thick composite may run out of control, do not do it indoors.

http://gallery.audioasylum.com/cgi/v...21150&session=

Marek

The system
DIY turntable with Teres motor, DIY tone arms, Shure and Shelter carts
Cary PH302, Moth S2A3; DIY composite Lowther horns
DIY dedicated room; DIY balanced power supply etc.

[livemusic]

Hi Marek,
Nice job indeed! I like your idea of one-piece composite platter, it shall be more inert than multi-layred one, since you are getting rid of inter-layer deflections. Must be great deal of preparation work messing with all this stuff. I just wander how did you manage to distribute the platter mass evenly, especially within the variable density layer. And did you leave bare graphite surface or apply some protective coat on it?
I was also worried with the compass niedle behaviour placed on top of my magnetically levitating platter. It's been working hard for three months, no problem with the cart or unshielded tonearm wires so far...
Looking forward for your listening report. Must beat your previous acrylic platter hands down..
Cheers,
Michael

[marekst]

Thanks Michael,
As far as preparation, more time went into research and planning. All materials were ordered on Internet, and the form was made from MDF. I tested the epoxy for hardening and took the risk of throwing a few hundred dollars worth of material . I was very lucky: the metal powder in the mixture absorbed some heat during the hardening process and stopped it from overheating.
I made a progressive density layer to provide smooth transition from graphite to high density layer without sound waves reflections. The first thick carbon fiber blanket was layered on graphite plate with epoxy, the second with epoxy and powder. Some powder sedimented into the fibers. A thick, heavy mix of powder/epoxy/chopped fiberglass was poured on top of the second blanket. Again, some powder sedimented into the carbon fiber. The form was level so density should be fairly uniform. Because of sedimentation the top layer of cast (bottom of the platter) had less powder - I planned for that and milled it off. After lathing, I statically balanced the platter. Actually, it didn’t take much, it was not bad to start with.
The graphite is not coated, I washed it with acetone and it does not blacken napkins any more. Coating may change the sound and I do not know if it would be good or bad.
What cart do you have on your table MM or MC ? I’m afraid that cart may get magnetized or demagnetized or whatever.
Did you notice any weakening of platter’s magnets ?
Marek

[livemusic]

Magnets wekening? To some degree, I would say; I feel it trying to lift my plate with fingers. So what? Initially about 3 kilos out of 33 total platter weight was left for the bearing ball to support. Now the load increased say to 4 kilos; still pretty low to bother, and I believe it's pretty much set. This is one of the hybrid bearing advantages above pure magnet levitation: you do not have to worry about constant platter heigth.
I have two MC carts on my TT: D102 and Ortofon SPU RN; no problem observed so far.
If I were you, I would probably consider to add some more mass underneath your platter, like aluminum ring with cavities filled with leadshot (?). Shouldn't cost a fortune. I believe additional rotational mass will not hurt.
Good luck with your project!
Michael

[marekst]

Thanks Michael,
Lead shots are a good idea but almost halve of my platter mass is distributed on the periphery increasing rotational mass adequately:
http://gallery.AudioAsylum.com/cgi/g...f=platter1.jpg
Also, I use heavy 13.5” stainless steel periphery ring.
Marek

[livemusic]

What is your experience with outer ring?
I was playing with this idea, but in the end I left it because of fear of damaging needle accidently. But I still have lot of records I cannot fix firmly to the platter surface with central reflex clamp.
Can you provide some details of your ring?

[marekst]

The ring is from VPI. It weighs over 2kg, diameter 343mm outside, 306mm (without a lip) inside, 15mm high. It has an inside lip 0.5mm thick 3.5mm wide which stays on top of the edge of LP, practically on the same level as LP. There are 4 velvet strips inside the ring that help a snug fit. It has two rubber bands on the outside which ads grip when you start the platter.
The ring flattens waives nicely especially those located close to the edge of LP, but it isn’t 100% effective. Even strait LPs benefit from the ring. The ring does not cover starting groove so damaging a needle is not a problem. Some LPs (about 1%) are too small for the ring, but VPI (and my own) platter has a stepped edge which stops the ring from falling down, so you can use it as peripheral mass. You need professional machine shop to make one.
Marek

[marekst]

Originally, I used 114mm OD magnets. When installed 50mm below top of the platter, they created much stronger than acceptable field near the inner grooves. After experimenting with shield, I noticed that field from those neodymium magnets can easily saturate shielding materials and could also saturate the cartridge.
I stated before that magnetic shield was not quite what I expected, in fact, near the end of LP the sound changed enough to make me disassemble the platter and rebuild it with smaller magnets.

Restricted by dimensions of the bearing and platter, I could only use 56/41/6mm magnets. The horizontal distance from inner grooves increased by 29mm. Because I installed new magnets under the bearing, the vertical distance also increased - the upper magnet is 72mm from top of the platter. I made more effective double layer shield using high saturation magnet shielding material and aluminum rings. http://gallery.AudioAsylum.com/cgi/v...=21150&invite=
The new magnets and shield work nicely. However, I noticed that the platter wobbles slightly and has to be corrected on a professional lathe.
Waiting for out of stock magnets I made a major addition to acoustic treatment of my room, which improved the sound considerably, but I will never know the sound difference of the new platter alone.

Regards,
Marek

[livemusic]

Hi Marek,
Can you elaborate about your magnet shield design/materials?
To my limited knowledge, it is quite difficult to isolate magnets:
- exotic materials like Mu-metal are used,
- the shield shall form closed (or about closed) box around magnet.
How did you manage to address these issues?
How did you get the indication, the mag field is weakened?
I guess you are right about magnets screwing the sound when cart approaches inner tracks. This is probably the reason why I sometimes hear distortion on inner tracks, when my linear tracker is getting closer to the platter center (which oterwise does not make any sense). My magnets are a bit bigger than yours, and they are placed 95 mm below surface (which I thought is enough). But, the bearing spindle appeared to be made of ferromagnetic stainless steel, which became saturated very strongly with time, and it probably acts like trans core, driving magnetic flux along it. Looks like I have to replace it with non-magnetic material. But, I would like to add magnets shielding anyway.

[marekst]

Hi Michael,
Yes, it is difficult to shield magnets, especially two repulsing each other; I just weakened the field to the level I think and hope is safe.
For the shield, I bought MagnetShield Plate 36x15x0.015” (Cat#274-15) $39 from www.lessemf.com. I used multiple layers of the material separated by aluminum rings - you can see it in one picture. If you count from the outside, there is 1mm shield, 9mm Al, 2mm shield, 5mm Al and 7.5mm magnet. The outside layer covers side only, not the top. The inner layer forms a box, the side is 2mm the top is 3mm thick. The inner shield is separated from magnet by 5mm of aluminum. I cut a 1” hole for 0.5” spindle. The shield covers both magnets but not from the bottom. There is one additional (0.015”) horizontal shield under the platter, above strobe ring: a 9”OD/ 3.5”ID ring. It is separated by vertical distance of 10mm from the side shield. This ring is located about 4cm from top of the platter - you do not want Mu metal close to the cart.
To determine the strength of the field I placed needle compass on top of the platter - at the outside edge of the platter the needle turns towards the center but does not bend down. About 5cm from the center the needle bends down but not rapidly. On my first design the needle bent down like crazy 8cm from the center.
My bearing spindle is ferromagnetic too, but it ends about 20mm below LP. I have no shield between inside of magnets and the spindle.
I do not post any drawings as my design is compromised by restricted dimensions and parts I used.