Sandbox Isolation Platform


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Box Construction

NOTE: The sandbox idea has since been abandoned and I have returned to using the TNT turntable (with integrated motor/flywheel) on a simple granite platform. 

The sandbox described below was designed to accommodate the T-base pulley drive system of my original TNT-1.  This was later redesigned to accommodate the single motor/flywheel arrangement (sorry, pic not provided). The pulley support pillars, being obsolete, were removed. The top plinth was split with separate plinths under the motor/flywheel and main turntable. The undersides of both MDF plinths were covered with aluminium angle (screwed and glued) to provide greatly increased stiffness and contact with the sand in the base. The split plinths with finned bottoms were significantly better sounding than without the aluminium angle - though per above note, I ultimately returned to the granite plinth.

I originally made a basic sandbox using an MDF bottom with pine sides nailed and glued to the base. Outside dimensions were 720 x 570 x 140mm (LxWxH), this would allow me to insert a plinth large enough for the TNT without touching the inside edges of the box. The top plinth (painted MDF) had circular cut-outs to provide isolated support for the TNT's separate motor assembly. Unfortunately I underestimated the weight of sand required to fill such a box. It was extremely heavy and I was unsure whether the 18mm MDF base would support the weight. In addition to the weight, the small circular cut-outs (which I proposed to support the TNT motor base with) sank into the sand and couldn't be levelled properly.
I shelved the project for about 12 months.

Eventually I decided to have another crack at the project.
First, to reduce the sand weight and stiffen the base, I nailed and glued a few pieces of scrap pine to the bottom - the base was now ~36mm thick. Then I screwed on stronger steel corner braces and a diagonally placed piece of angle iron for overkill (since it was lying around in my shed).
I was considering what to use for the motor assembly supports, when a friend suggested setting the threaded rod into PVC pipe with cement.

Ironically the hardest thing to source for the project was the sand. Taking Brightstar's advice, I tried to get some blasting sand, which is (of course) dry and free flowing with a graded size. There are several local blasters, but sand has recently been banned (in Australia) for blasting purposes because of the health risk. Eventually I found a blaster with some leftover sand he was willing to sell me.

Note:
For those unfamiliar with the TNT series 1 turntable, it consists of two main pieces:
(1) The T-shaped motor/idle-pulley base, which sits on three rubber feet - see photo down page.
(2) The turntable proper, which is supported by four suspension pods in the corners.
The only connection between the units is by the drive belt and the support the turntable sits on.
With my box the main turntable structure sits on the top MDF platform, which rests on the sand only. The motor/pulley base rests on the three concrete pillars. The pillars are bolted to the base of the sandbox and pass through oversized holes to be level with the top of the MDF plinth.
The box sides and MDF platform are separated by a 15mm strip of loose cell foam. This gives the top a finished look without compromising isolation and also serves to keep the platform centred.

The total project cost was about $50-60, but some material I already had on-hand.

Braced sandbox with cement pillars for motor assembly ready to bolt on.
Plinth for the turntable proper in the background, shows holes for the pillars to pass through.


Cement pillars bolted in - base painted with sealer
 

Pillars painted black - box filled with sand ready to level

Foam edging between MDF plinth and box sides - Suspension pods and motor unit in position:


View of finished box - motor pillar bolts can be seen on bottom of box:

 

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