Plinth Design

[dude007]

I have a 3 inch thick acrylic Teres platter and bearing.

I was wondering how to design a plinth.

What goals does one strive for? Is the goal to simply find a material that is non-resonant with my arm and platter.

Is the amount of mass important? Is the location of mass important - close to spindle or sprawling mass?

What materials impart a good sound? bad sound etc etc


Can someone communicate some guidelines and principles of plinth design?

[farsch]

Hi Dude,

the guideline is:

to deal with reasonances.....sounds easy but its a completly different piece of cake on every kind of player.

If it is a Belt Drive, Direct Drive, etc.....

Purely damping everything is the wrong way.

The guideline is to deal with reasonances -

1. Best way is to have a drive with less of them.

2. If there are reasonances handle them, don't try kill them directly they kill you.

Best

René

[Conrad Hoffman]

The question is almost too general to answer. I'd start by scouting used bookstores (online or local) for a textbook on vibration. The usual title will be "Vibration". The older texts from the '40s and '50s are actually easier to understand than the math nightmares published today. Study spring-mass situations and beam vibrations, and you'll have a good place to start.

[kevinkr]

Indeed a very general question, but you need to decide before you go any further whether you are using a suspended sub chassis design with the motor mounted on the plinth or a solid plinth with bearing and motor mounted on the plinth.

An example of a table with a solid plinth would be the Garrard 401, most Thorens models will do as a model of a suspended sub chassis table.

Once you know this people who have used one approach or the other should be able to provide you with useful information.

I have heard a number of tables which I considered sucked the very life out of the sound because they were very over damped by design. One very heavily hyped high end table from a nearby city was a major disappointment to me for this reason. (Not that I could have or would have wanted to afford one at the time.)

I used red oak in the latest plinth for my ancient Thorens TD-125 (no toy boy audiophile cachet, but technically solid, and the best I've owned to date including an early PT) which I thought was a huge improvement over the stock plinth.

You might want to try a number of different approaches and materials quickly bodged together to help determine what sounds right with the Teres platter and bearing. What motor are you going to use? Once you know the materials you want to use you can build the final table.

People have used marine plywood w/wo lead shot, corian, cement, and all sorts of wood.

[ashok]

We need to absorb vibrations and keep stored energy down .
So how about a thin MDF cabinet stuffed with fine sand . Can be made up of layers of thin MDF with cut outs for areas to be sand filled.

The thin MDF should have better vibration transmission characteristics than plywood and sand should probably dissipate the vibrational energy as heat (?). It would also be heavy and reduce the LF resonance of the system. Most likely with a very low Q .

The feet on the plinth and the interface with the turntable itself will also matter. Would 2 inch rubber balls fitted in a cup be a good solution for the plinth to ground interface ?

I need to build a new plinth for my TD124 MkII.
Cheers.

[OzMikeH]

Squash balls?

[YNWOAN]

The questions you ask at the start of this thread are all good ones but unfortunately the answer to all of them is yes, no or possibly, depending on who you ask.

With regard to the chassis some designers believe in maximum damping (but all have different ideas on how best to achieve it) whilst others promote ultra stiff, low mass designs intended to store less energy (personally I favour this route). When it comes to isolation things are even less clear. Many decks now use absolutely zero isolation except for cones or spikes. Some use moderate isolation like squash balls, and others use attempt full isolation through springs or magnetic levitation (or a mixture of both). Anybody who tells you that there is any consensus of opinion on turntable design is wrong. It's not even a question of price; I can think of mega bucks decks that use any number of clashing philosophies. All I can recommend is to pick a path and experiment (as I did), you will have to form your own conclusions I'm afraid - good luck.

With regard to the rubber or squash balls; I recently experimented with both of these. The mass they are supporting makes a difference but in general they offer higher frequency isolation and are less good (to no help at all) at low frequency isolation.

[Conrad Hoffman]

Concerning setting something (a turntable for instance) on springs (or tennis balls)- If you reduce the equations, it's interesting that deflection is the only thing that determines the resonant frequency (assuming you remain on earth). If you want a low resonant frequency suspension, it has to deflect a substantial amount between the loaded and unloaded positions. IMO, solid coupling (cones) on a turntable is misguided unless you happen to own a small boulder, or know exactly what it will be sitting on. Why would you want to acoustically couple a turntable to an unknown source of vibration? A very interesting material I've worked with is Sintra. It's normally used in thin sheets for signage, but is available in heavy plates that are very non-resonant. It's an expanded plastic that's easy to work with hand tools and looks good.

[YNWOAN]

I completely agree with Conrad on the suspension question. The Sintra sounds interesting too.

[dude007]

Thanks for all the pointers so far. I'm starting to understand a little about plinth design - just a little.

My eductation is electronics and computers so I was hoping to find some parallels in approach. Desiging and building amplifiers for instance has many texts, equations, rules of thumb and best practices one can rely on. Not so with plinth design as I see.

I think the trial and error approach with attempting to understand whats going on with each trial maybe a little more the style here.

So far I've built one plinth. A massive 3 inch thick birch ply affair filled with holes and cavities which are filled with lead shot. Probably weighs 60 pounds. The arm is a VPI 12.5. Tapping on the plinth with needle down produces a mid to high frequency ring from the speakers. Sitting the plinth on squash balls produces the same ringing but a little louder when tapped.

Interesting thing that happens when the air conditioner is running is that the same mid-high frequency ring can be heard but not so loud. The funny thing is that the air conditioner is on the ground outside and 60 feet away. The TT stand and plinth are on the basement floor inside the house! Moving the plinth onto squash balls eliminates the mid-high frequency ring from the air conditioner but also kills the bass from the music.

The TT stand is maple wood and heavy. I'm thinking maple at least in my case actually makes the highs more pronouced and I plan to get a real metal TT stand soon. Oh by the way no air conditioner ringing if the table goes directly on the floor. Canadian companies make some great TT stands for low $$$. BTW Canadian audio is alive and well with many little Canadian companies producing product well appreciated and as they say your always famous a thousand miles from home but never in your own back yard.

So back to the plinth. My original approach was to dampen everything with weight and yah thats bad as I've found out as there is incredible details but little life. One last experiment with this plinth is to add a piece of 1/8 inch copper between the inverted bearing and the plinth. Pictuce a big big washer maybe 5 inches in diameter and 1/8 inch thick on the plinth and under the platter with a hole in the copper for the bearing to pass down through. Maybe this will bring back some life. Why would I try this - well because I can - and to learn more through experimentation - I have a piece of copper - and I think I remember VPI putting a piece of stainless steel on one of their models and I've heard that copper is even better than steel for this purpose.

So you guys that understand the mechanics of vibrations in materials - Does anything I've observed and written actually make sense from a engineering perspective - can any of this be explained just a tiny bit?

Thanks for any reply

I think the next plinth might be a stiffer less massive approach, perhaps 3/4 inch cocobola wood or maybe 1 inch acrylic, corian?

/.cheers