Listening To Plinths
" It has been said that idler wheel turntables easily cause rumble, but the problem lies in the plinth rather than the motor itself. Our plinth is made of solid laminated cherry wood designed to deaden outer vibrations. Unlike conventional plinths, this plinth allows only minimum space for the motor and arm base. The construction of the plinth virtually demolishes any vibrations caused by the motor but not by deadening the overall sound, which often happens with other plinths that are also designed to eliminate vibrations caused by the motor vibration. Once I designed a plinth that killed vibrations without killing the life of the table, I stopped experimenting and have been making this plinth ever since.
The plinth is the key." - Shindo
Design Phase, Build Phase
It's been over a year now since starting work on my 301 project, and more than two since setting out to find one. In the meantime, small parts have been bought, sold & flown across both Atlantic and Pacific; a second greasebearing 301 has been purchased; a search for the right tonearm has turned up a period-correct SME series-one 3012; and three plinths have been built.
This is titled a 'Listening' project, because the turntable plinth has an enormous effect on the sound, and the design of plinths is as much about the tinkering and evaluating as it is about the build.
However, it must be said that, well, I'm listening, but I still can't hear any records playing. Yet.
The secondary 301, several critical years newer, is to allow the original No.6435 to spin with optimal parts, fittings, and motor, as close to the Garrard level of spec as possible. When it's all together this will be a fully-optimized 'ultra' 301, and the secondary one will still be suitable as a mono deck. And thereafter lots of record playing will ensue ...
The three plinths are : first, what has become the servicing jig plinth, which provides any-angle access or positioning of the deck for adjustment. Sidepanels are tall to protect switch-levers when the deck is inverted on jig.
Second, a more high-mass plinth, a shot-in-the-dark idea prompted by the discovery of a massive, free-for-the-taking segment of roof-beam in my construction-obsessed neighborhood. For novelty's sake, this was fashioned by manually chiselling out the closely-conforming shape of the 301's under-carriage from solid, ala Shindo. With a sliding armboard block. Considerably quieter and much more resolving than the open-frame jig, this high-center-of-gravity design pointed the way, by default, to the resvised shape.
A very much lower drive-motor position and much wider base ..... The third plinth, a slight rethink of the classic stacked baltic birch-ply plinth ---this one is much lower, leaner, and yet still very high-mass.
And laminated, very tightly indeed, with the Tomb-Of-The-Pharaohs adhesive compound, known in Egyptian times as The Only Glue There Is. Today known as "hot hide-glue", it is a really ancient means to bond wood for furniture, comprised of boiled-down animal hides, as gelatin. Luthiers and pianomakers still use it to construct musical instruments, due to it's ability to shrink in volume as it cures, and to draw the adjoining layers together while doing so. An internet source describes what happens thus :
As the moisture evaporates from curing hide-glue, the polymer chain bonding the wood shrinks to less than a third of it's original size.
If the objective is to closely bond the layers of birch-ply, so as to effectively transfer a wide band of resonance artifacts through the plinth, hot hide-glue's "death-grip and disappearance" characteristic is absolutely perfect.
Because the hide-glue has a very fast set-up time--- really little more than a minute given wide areas to cover--- the clamping procedure must be really quick, and critically accurate.
Designing the clamping press consumed the lion's share of the effort with this plinth. After the overall scheme was sorted out, the whole ordeal was reduced to nine glue-ups, over ten days, to bond ten layers of birch.
The press itself is a little like a massive Easel--- one that kneels like a praying mantis. The way it works is that the ply layers are loaded--- one layer is glued at a time-- into the folded-down table with legs on the front edge in the kneeling position. The front edges of the ply layers rest against the spines of the bottom-most furniture clamps, which are firmly seated in Jorgensen Bar-Clamp Saddles, to maintain a straight & square line-up for the successive layers, courtesy Gravity.
A heavy press-cover is placed over the birch layers, and the edges are double-checked, all around. Then the clamps are tightened, in order front-to-back, slowly so as to allow the glue to spread, cover, and if necessary bleed out. But by tightening in order, the bleed will go harmlessly out the back of the ply-stack.
Then the easel is up-righted, legs locked out, and the plinth is now level for final cure. At this point, bottom clamps can be detached from their 'saddle' fittings (by unfastening small thumbscrews) and, with all clamps still in place, the layer-stack can actually now slide out and along the table-surface for all-sides glue mop-up.
Which will be necessary, given the chaotic fury of aligning, hot-glue spreading, fixing, and clamping down each layer, since the glue is always cooling ... while the clock ticks.
Even in this phase--- Time. It is always Time that sets the rules of the game.
James Donahue, 2006.
Photos can be found at The Analog Department site :
Notes On The Glue
Use of hot hideglue in a stacked plinth requires a little research on methodology, as well as several practice runs with the actual materials. This isn't overly difficult, but it is very easy to get wrong if not well acquainted with the process; trial runs with wide areas of sample material are highly recommended. I made various platforms and shelves to get up to speed before attempting a plinth.
To condense the information, the following will tend to slow down the quick set of the hideglue : High room temperature, high object temperature, high room humidity, low room ventilation all keep the bond from setting too quickly. It's an organic, non-toxic glue, so those conditions aren't impossible to work with. Also, more water in the mixture, a less absorbent surface (ie fine-sanded and / or sized), and a thicker overall layer will promote a slower set.
The best single tip for hideglue work is that a common everyday kitchen Coffeemaker, on US voltage at least, will provide a carafe of constantly-maintained 140 degree hot water in which to immerse the jar of hideglue gel. Double check with a thermometer. ....