SP-10 mkII, the next project

[Rush]

I spent a few weeks reading the Audio Qualia site. And ultimately built my own version of the cast plinth using isophthalic polyester resin and bentonite (dry cat litter) for filler. I did not use fiber glass of any sort. I saw no advantage to adding glass product into a proven highly damped combination and I do a bit of boat work on the side, so I have it available.
new damping factors - Audio qualia
The damping factor of this mixture is .618 where as Panzerholz is .599 and the frequency of the mixture is very low at 48HZ. From the reading on Audio Qualia site the lower frequency is better. Considering how hard and expensive Panzerholz is to get, it is a no brainer to go with isophthalic polyester resin and bentonite.
The mold is another story, it is like building the plinth inside out before you can start the actual plinth. I poured mine all at once and once it was not tacky to touch, ran water over it to keep it cool for the next 1/2 hour. No issues with fire or cracking. I did destroy the mold getting the plinth off of it, so it was a one time use.
I put wood sides on my plinth as part of the mold so the outside finish would be nice. We had just put down new hardwood floors and I used some spare pieces left over. See the attached picture. So they are glued on while casting the plinth. There are some thickness ratios to remember with any material and I tried to keep the wall and top thickness to about 30 mm.
I would do the mold differently next time (so it could be reused), but it came out pretty well and I hope there isn't a next time.

Rush

[user510]

Originally Posted by Rush

I spent a few weeks reading the Audio Qualia site. And ultimately built my own version of the cast plinth using isophthalic polyester resin and bentonite (dry cat litter) for filler. I did not use fiber glass of any sort. I saw no advantage to adding glass product into a proven highly damped combination and I do a bit of boat work on the side, so I have it available.
new damping factors - Audio qualia
The damping factor of this mixture is .618 where as Panzerholz is .599 and the frequency of the mixture is very low at 48HZ. From the reading on Audio Qualia site the lower frequency is better. Considering how hard and expensive Panzerholz is to get, it is a no brainer to go with isophthalic polyester resin and bentonite.
The mold is another story, it is like building the plinth inside out before you can start the actual plinth. I poured mine all at once and once it was not tacky to touch, ran water over it to keep it cool for the next 1/2 hour. No issues with fire or cracking. I did destroy the mold getting the plinth off of it, so it was a one time use.
I put wood sides on my plinth as part of the mold so the outside finish would be nice. We had just put down new hardwood floors and I used some spare pieces left over. See the attached picture. So they are glued on while casting the plinth. There are some thickness ratios to remember with any material and I tried to keep the wall and top thickness to about 30 mm.
I would do the mold differently next time (so it could be reused), but it came out pretty well and I hope there isn't a next time.

Rush

That reads like excellent damping ability. Kitty litter in with isophthalic polyester resin. I see at Cat's site he discusses the material further.

Of course damping ability is one of the characteristics needed in producing a suitable plinth. Others might include rigidity and tensile strength. Of course if one is planning to use the original cast aluminum chassis, that chassis will provide much of the rigidity needed within the platform. But if one is to remove the motor from the original chassis and place it into another, then the new chassis now needs to combine not only damping ability but also meet the needs of rigidity.

So I'm thinking that while the 'kitty-litter-in-resin' material might be part of a solution, and is very interesting.... but if it is to be the entire solution it needs to harden into a material capable of becoming the entire supporting structure.

How hard and rigid does that stuff become after full cure?

-Steve

[Rush]

Originally Posted by user510

That reads like excellent damping ability. Kitty litter in with isophthalic polyester resin. I see at Cat's site he discusses the material further.

Of course damping ability is one of the characteristics needed in producing a suitable plinth. Others might include rigidity and tensile strength. Of course if one is planning to use the original cast aluminum chassis, that chassis will provide much of the rigidity needed within the platform. But if one is to remove the motor from the original chassis and place it into another, then the new chassis now needs to combine not only damping ability but also meet the needs of rigidity.

So I'm thinking that while the 'kitty-litter-in-resin' material might be part of a solution, and is very interesting.... but if it is to be the entire solution it needs to harden into a material capable of becoming the entire supporting structure.

How hard and rigid does that stuff become after full cure?

-Steve

It does harden and become very rigid. The cat litter adds tensile strength as well. The Isophthalic polyester resin is the best of the polyester resins and you won't find it at Home Depot, you will have to go to a boat building supply company. It is very hard to sand and it can be polished. This stuff is perfect for this application. You can always add some carbon fiber to the mix for added rigidity, but I doubt it would be necessary if you keep a 30 mm thickness. If you needed a thin section, I would then use some fibers to reinforce along with the cat litter. You can even machine this stuff with a milling machine. It will wreck havoc with a bit though. It is very hard!

Rush

[Bon]

Originally Posted by Rush

It does harden and become very rigid. The cat litter adds tensile strength as well. The Isophthalic polyester resin is the best of the polyester resins and you won't find it at Home Depot, you will have to go to a boat building supply company. It is very hard to sand and it can be polished. This stuff is perfect for this application. You can always add some carbon fiber to the mix for added rigidity, but I doubt it would be necessary if you keep a 30 mm thickness. If you needed a thin section, I would then use some fibers to reinforce along with the cat litter. You can even machine this stuff with a milling machine. It will wreck havoc with a bit though. It is very hard!

This seems a great result. I can attest to the hardness of the cured resin. I used the pure resin to mold a loudspeaker baffle with complex angled faces. The interior needed some shaping after curing. It was very difficult with a bastard rasp. Using an angle grinder with a coarse grinding pad produced unpleasant fumes and I don't recommend it. The resin does bind very well to timber, so using the mold as a cosmetic edge is a good idea. I have found though that any overfill runs, will soak deep into the timber and be very difficult to sand off. The timber tends to sand away and leave the resin proud. Your picture appears shows a high percentage of bentonite to resin. Were you happy with the ratio? Was even consistency easy to achieve?

[Rush]

Originally Posted by Bon

This seems a great result. I can attest to the hardness of the cured resin. I used the pure resin to mold a loudspeaker baffle with complex angled faces. The interior needed some shaping after curing. It was very difficult with a bastard rasp. Using an angle grinder with a coarse grinding pad produced unpleasant fumes and I don't recommend it. The resin does bind very well to timber, so using the mold as a cosmetic edge is a good idea. I have found though that any overfill runs, will soak deep into the timber and be very difficult to sand off. The timber tends to sand away and leave the resin proud. Your picture appears shows a high percentage of bentonite to resin. Were you happy with the ratio? Was even consistency easy to achieve?

I used a 50/50 mixture resin/bentonite, without re-researching I think that was the ratio recommended on the Audio Qualia site. I guess I was happy with the ratio, it was thick, almost like peanut butter. I blue taped the finished wood to keep the resin off it and removed the tape before the resin got very hard, a few minutes. You always wonder if you put too little harder in the resin until it kicks off, then you worry you put too much.
You can grind it if you use proper respiratory gear.
I poured mine from the top and the finish you see in the picture is the way it flushed out with a screed on the top of the wood sides. As far as even consistency, you had to keep stirring as you poured the mix to keep it even. Once in the mold it seem to be pretty consistent. The bentonite didn't settle as far as I could see. The top is slightly uneven due to the bentonite "rocks". It is very much like concrete, except it sticks to the screed when trying to smooth the top and leaves a void behind the screed. So you have to work it and push it around and let gravity level it out. I made 3 batches, 2 liters or so at a time as I recall, to fill the mold.
After it was hard, I used a saber saw to cut the correct cutout for the turntable. When I place the SP-10 in the hole the edge sat on the bentonite "rocks" with small voids around the edge. I tried to sand the area and realized how hard this stuff is.
So I taped the bottom of the turntable lip with aluminum tape and used mold release on the on the tape. Then I placed wood matches on the edge of the cutout to hold the turntable lip up off the top of the plinth a couple of mm and mushed more resin with chopped cotton fiber as a filler with a true peanut butter consistency about 13 mm under the lip. Just used a 25 mm spatula with a square edge to push the mush in.
I hope you guys across the pond will appreciate my use of mm instead of inches. I think in inches.
After that mix setup, removed the turntable, removed the matches and tape and have a level even surface for the turntable to rest on.
I used a Forstner bit in a drill press with a stop for the bottom of the plinth so the 5 screws would have a level and smooth surface to bear on. Same thing for the feet.
This project took me a few months from start to finish, but if you devoted a few weekends you could probably knock it out. Wood working is particularly hard for me. The wood edges were cut several times, mitering the corners and keeping the lengths the same. The mold was a lot harder than i first thought and I redid it a couple of times.
One thing about using resin is you can always sand it to roughen it up and add something to it. So if you have a bad cut or drill something in the wrong place, just fill it with more resin/bentonite and start again.
This would be perfect for a speaker enclosure or baffle. A couple of flat molds 30 mm thick and glue the edges together.

Rush

[brianco]

Is there any advantage in breaking the cat litter down to powder form? Also, can the 'mix' be successfully dyed to an acceptable colour?

Many thanks.

[Rush]

Originally Posted by brianco

Is there any advantage in breaking the cat litter down to powder form? Also, can the 'mix' be successfully dyed to an acceptable colour?

Many thanks.

You can use a piece of screen and filter the larger particles out. You will need a lot more cat litter to get enough small particles to do this project or a grinder of some kind. Other than the finish, I don't see a benefit to using powdered Bentonite, but then I didn't try it. Maybe it would level out better.

This resin can be colored before you mix it, every color is available including black. A small amount of color goes a long way. I just didn't think of coloring mine. It can also be painted with any spray paint, although you would now need to sand it smooth. Think about a fiber glass boat.
When I did this I wasn't thinking I would be writing about it or I would have taken pictures and probably thought it through better.

Rush

[simoon]

Originally Posted by brianco

Is there any advantage in breaking the cat litter down to powder form? Also, can the 'mix' be successfully dyed to an acceptable colour?

Many thanks.

There is no advantage to using the cat litter as a powder.

In fact, somewhere on the Qualia site he specifically states that powdered cat litter is not as effective.

[user510]

Rush, Bon.
Great info regarding the properties of this Isophthalic Resin/bentonite.
The ability to make a plinth by casting rather than fabrication from more traditional materials like metals and wood really opens up some new possibilities with regard to shape and style.

Of course there is a fabrication involved. The mold has to be designed and built. But still, the prospect seems very interesting to me. It triggers imagination.

Thanks for this excellent input.

-Steve

[Bon]

Originally Posted by user510

Rush, Bon.
Great info regarding the properties of this Isophthalic Resin/bentonite.
The ability to make a plinth by casting rather than fabrication from more traditional materials like metals and wood really opens up some new possibilities with regard to shape and style.

Of course there is a fabrication involved. The mold has to be designed and built. But still, the prospect seems very interesting to me. It triggers imagination.

Thanks for this excellent input.

-Steve

Hi Steve. Although my main turntable interest is in getting the best I can from the SP10 mkII, I have applied the same ideas to a building a plinth for the Teac TN-400 turntable. This turntable had a brief production life before the introduction of quartz lock. It is a servo control design with a discrete strobe and touch sensitive controls. I have only seen the one in the flesh, for which I made a resin plinth. The magna-float is a partially magnetic levitation of the platter to reduce the load on the thrust bearing for performance and wear reasons. Apparently there were legal reasons for Teac to discontinue the TN-400.

As discussed, expect to destroy the molds even with liberal application of mold release, unless you integrate the mold as part of the plinth timber cladding.
My buddy is ecstatic about the performance of the Teac TN-400 in this plinth and has in fact fitted a $4000 pickup arm.
Audio Origami - PU7
I agree that the potential of molded resin plinth is worth promoting. I have been plinth building for many years, and for high torque, un-suspended turntables, the resin composites is my highest performing construction.

My ultimate vision is removing the motor and bearing from my spare parts SP10 mkII and installing them in a resin composite plinth. The electronics will be left in the original chassis, or possibly installed in a custom metal case. I fantasise about getting close to mkIII performance. Many of the enhanced performance specs of the mkIII are more related to changes in the measurement methodology during the 70-80's spec wars.