Plinthology1: high mass and solid plinth

An innovative recipe for a high mass plinth

[Lenco plinth glass finished]
[Italian version here]

Product: Plinthology1 - high mass solid turntable plinth
Manufacturer: not for sale, TNT-Audio DIY design
Author: Piero Canova - TNT Italy
Published: September, 2021


Over the last 15 years I have had plenty of fun in experimenting several different solutions on how to build a good sounding plinth for a turntable. Obviously some did work fine, others were very average and many very sub par. I thought it could be interesting to share some of the best solutions I found, both sounding well and having a reasonable cost. Of course the projects must be adapted to the motor unit you are planning to use, but I hope to give you enough information on how to build and adapt the design to your project. There isn't a single path I have followed in the different projects with the exception of all being large in size and being quite heavy. In my experience, a turntable performs well, specially in the lower octaves, if it has a sizeable mass. For sure this works very well with Direct Drive (DD) turntables and with Idler Wheel turntables; I haven't made so many tests on belt driven turntables because it is difficult to find good motor units at reasonable prices, but if you look at the best belt units on the market, the high mass concept is often seen. Having said the above, let's look at the first plinth I would like to suggest you.

High mass and solid

The plinth I am proposing you today is characterized by both a high mass and not having any flexible elements in its construction. The dampening of the vibrations both external and generated by the motor happens through the properties of the materials used. Our ingredients today will be laminated glass and sand. The result you can see in the picture below the title, using a Lenco L75 turntable as a motor unit, giving a turntable weighing roughly 50Kg. Laminated glass is a sandwich of materials, glass - Polyvinyl butyral-glass that shows some very interesting performance. It is quite resistant mechanically, very "dead" sounding and finally quite heavy.

The project begins with the construction of a first lower box of the outer sizes 433 x 377 x 110mm; on the bottom will be glued the feet and it will be partly filled with sand. There are several thicknesses of laminated glass on the market; I have used the 5+5mm but you can also find 6+6mm or even 8+8mm. I made some tests and above 5+5mm there aren't improvements acoustically, but the cost is much higher. There is no need to use tempered glass which again greatly increases the cost. You can also decide to use coloured or etched glass, but these are aesthetical considerations, which I leave to your personal taste. Summarizing you will need:

You can buy the glasses in Glassware shops or in some large DIY stores; if possible have the pieces glued together at the shop. They have some special UV cured glues that offer outstanding mechanical characteristics and durability. If you have to glue the pieces yourself, remember to ask to have the edges of the pieces ground otherwise they cut like razor blades with evident handling problems. Remember to ask the supplier to drill a hole on one of the sides of the box for the power supply cable. It is possible with the proper tools, and a bit of patience, to drill a hole on glass, but you must be sure it isn't tempered glass and that you know how to do it. The best is to have it drilled in the Glassware store: drilling holes in glass is expensive, but if anything goes wrong it won't be your fault. I have made a hole large enough for the cable so sand doesn't comes out of the plinth just by interference fit, but if you ask for a larger hole you can use a chromed cable gland which is much better looking.

Now you can glue on the feet. I have chosen three spikes that can be altered in height to level the turntable because I didn't want any elastic component in the system. Nothing stops you from using some elastic feet or springs; just be sure the load on each feet doesn't surpass the maximum load allowed or you will have a permanent deformation. Now you can begin to fill the lower box with sand. To fill it uniformly I suggest you should use some tape so you can have a visual reference on the four sides of the level of the sand. Don't fill it completely ot it will begin to escape from the hole for the cable. Final thickness of sand should be no less than 30mm and no more than 50mm. The type of sand isn't critical; I normally use bags of sand sold in most DIY for plaster or grout; it is of fine size and clean so it is also good looking since you will see the inside of the turntable. (This sand is known as sliver sand in UK). Leave aside 25% of the sand for later use. The result at this stage should look like the picture below.

[Lenco glass plinth bottombox]

After passing the power cable through the hole you can fill the box with sand to the edge of the tape.You can use a trowel or any tool you like to flatten it. Try to be precise on laying the tape and on flattening the sand. Later on you will compensate some small height differences, but if there are too large differences it will be tough to flatten the whole.

Now we can start with the second structural piece which is made with 4 sides and two bottom bars having the function of stiffening the upper box and as a support to avoid the upper box sinking into the sand. It is a parallel sided rectangle with the upper side made by the turntable and the lower side having the center part missing. The sizes allow a gap of 10mm between lower box and upper box so there is no physical contact between lower and upper box apart through the sand. Construction is the same as for the first box so use the same glass and the same gluing method.  The sizes I have used here are those right for a Lenco L75, but you can adapt them to any other motor base. You will need then, the following pieces:

The two 70 mm pieces must be glued to the ends of the wide side of the upper box on the bottom. Their function is to stiffen the four sides and to avoid them sinking in the sand. This picture I believe clarifies how it should look.

[Lenco plinth glass twoboxes]

Concerning the arm and its cables, I have chosen to install it in a separate column to completely insulate it from the vibrations generated by the motor, but if you want to use the standard Lenco arm or another 9" arm, you must have a second hole in the upper box for these cables. Just carefully measure the size of the hole you need to pass through the RCA connectors you are using or alternatively a 5 pin DIN connector. You can eventually also decide to cut a rectangular slot and then isntall two female RCA sockets so you can use any interconnects you prefer. I personally prefer to minimize any soldered joints in my cables so I use single strand cables from cartridge clips to RCA, but if you prefer differently the plinth allows you to do it.

Now you can lay the upper box on the sand contained in the lower box. Try to lay it flat and not on one side to avoid holes; now you can cover with the remaining sand the flat parts of the second box in order to have a bottom completely sound adsorbing.The last step is to assembly your turntable on the upper box. Gently pull the power cable in order to have a gentle curve inside the second box. To fix the turntable to the second box leaving the possibility to remove it in case of repairs I use a structural double tape like the one used to hang mirrors inside furniture. In case of removal it is enough to cut along the edge with a cutter and clean the glasses before reassembly. To complete the job pour a little of sand in the slot between the two boxes and level it with a finger or any tool you like. Fill it uniformly just leaving a small gap between sand and the edge of glass so it won't spill around.

Now you can transfer the turntable to your shelf. Just have some help available because it will weight 50 Kg and I do not accept any liability for any damage to your back. Please check also that your shelf is strong enough to carry the weight of the turntable. Finally level the turntable using the adjustable feet and you are ready to listen to some music.

Now let's talk about costs: this can vary depending on the glass you decide to use and the price policies of your glassware but my estimates are between 250 and 500€ for the two boxes; sand and the other materials are insignificant.

How does it sound?

Listening test shows that this plinth is both powerful and very accurate. It is like installing in your car a six cylinder engine of the latest generation, solid and powerful with plenty of torque at low revolutions, but fast in picking up speed. Pick a record with very low frequencies and you will listen to a a powerful low range, but always precise and controlled. The combination of idler wheel + high mass plinth + a church organ goes dawn to the first octave without any effort or distortion; it seems to say to you “no problem, the signal is recorded and I am playing it back”. There's no special effort required to play low frequencies as it does the same on the rest of frequencies. It doesn't sound dull or slow like some other plinths I have heard and every change in setup of the arm or cartridge is immediately evident. It doesn't subtract energy, but simply isolates the record from any disturbances allowing you to listen to just music. You can make the test of knocking on the base when you are playing a record and the result will be just some muffled "thud". It works also perfectly well with DD turntables: I have tested it using my Denon DP 6000 with excellent results.


After all this work we have achieved:

It performs very well and, to me, looks very nice and for sure it won't go unnoticed.

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