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Plinthing for Idiots...

Posted by melomane (A) on June 24, 2005 at 17:48:55

In Reply to: Dead horses and plinths ........ What is really going on? posted by Elmo on June 24, 2005 at 05:53:51:

Here's a summary devoid of all the scientific jargon and gobbledegook which is often impressively spouted out by inmates, which is only worth as much as results, as pointed out by the chief research engineer for the famed Scott equipment: "If it measures good and sounds bad, it is bad; if it measures bad and sounds good, you have measured the wrong thing." Despite all their measuring equipment and thoeries, they still don't know why great violins sound great.

This is more applicable to idler-wheel drives than belt-drives, but gives a rough general idea anyway. You want the 'table to be stable, so the plinth should hold the 'table in place, and I mean hold the 'table in place, so vibration - which is a form of noise - doesn't contaminate the 'table/tonearm/cartridge and cause undue relative motion betweent the different parts. Mass is the easiest way to achieve this, though one can be clever in the way to get there (large massy bottom plinth on which is firmly sited a lighter upper plinth; single big huge plinth, small but heavy solid plinth, etc.). You don't want the plinth to vibrate and thus communicate noise to the tonearm/cartridge. With idler-wheel 'tables this is a much larger problem than with belt-drives, due to their big manly Mighty Motors, which have much more power than the cute little motors which come on belt-drives. Often, however, these Mighty Motors are hung on sprung suspensions, which mitigates their Mighty rambunctiousness to a certain degree, but should still be taken into account (as the flimsy sprung hollow boxes they originally came in prove: noise noise noise, elimnated by secure bolting to strong plinths). Though you don't want your plinth to vibrate and thus contribute noise, you also don't want it to be so effective at eliminating noise you also eliminate the music. Think of a plinth made of pure Sorbothane: sure it'll be quiet, but your music will have all the impact of wet overcooked spaghetti. You want your plinth to be as neutral as possible, not neutral at this frequency but adding or subtracting at another frequency. You want it dead, inert, inactive, neither adding nor subtracting, as seen from the cartridge/tonearm turntable: an immovable object (at least so far as idler-wheel drives are concerned). Seems to me many fashionable materials out there are comb filters: emphasizing certain frequencies and destroying others. Sure they sound flashy (old stereo store trick), but are they satisfying in the long term? Not if you're familiar with the sound of real instruments, or get headaches easily.

Some suspended belt-drives do benefit from heavier inert plinths, depending on their design, as is attested by the many Thorens tweakers out there. I would think that the better the plinth the better the sound for an unsuspended belt-drive, same as for idler-wheel drives, as attested by the many materials they come in: MDF/damping material, or Corian, or acrylic, or some form of CLD (contrained-layer damping). The whole point behind CLD is to achieve what I've outlined above, a neutral plinth. I favour CLD plinths for my Lencos, but I suppose any mass will improve a naked idler-wheel drive, which are fantastic due more to their drive systems than to the materials in their plinths.

What does this mean in practice? For my Lencos, a Lenco on it's sprung suspension and a good tonearm sounds stunningly good, as long as care is taken in restoration. The same Lenco off its springs and bolted to a big, heavy non-resonant plinth increases in focus and clarity, and the bass drops a couple of octaves (this is a CLD plinth anyway). The same Lenco not only bolted to a big heavy plinth but actually married to it so a large portion of its metal top-plate ends up being constrained layer damped by the mass itself (plinth contacting flat bottom of metal top-plate directly and bolted to it) results and a HUGE diminution of noise, to the point where fellows with very high-fallutin' belt-drives said to be extremely quiet are shocked (I only recently tried out this last formula). Surface noise disappears, there is no hint of rumble or low-frequency noise, notes leap out at you and everything is in sharper focus, imaging knocks out your side and back walls. This is the effect of a CLD plinth firmly married to a Lenco.

Jonathan Swift in Gulliver's Travels has a war break out over the correct end from which to eat an egg: there's the fat enders and the narow enders. People are like this: get them to agree on something (like "We Love Idler Wheels!"), then they will disagree on the implementation (all the people in Lilliput loved eggs).

So, heed the words of Daniel R. von Recklinghausen, Scott's chief research engineer, and trust your ears and take theories and measurements with a grain of salt.

Flame suit: On!

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