Skeletal vs Plinth style turntables


I am pondering a new plinth design and am considering the virtues of making a skeletal or closed plinth design. The motor unit is direct drive. I know that as a direct drive it inherently has very low vibration as opposed to an idler deck (please do not outcry Garrard and Lenco onwners coz I have one of those too) but simple facts are facts belt drive motors spin at 250rpm, Lencos around 1500 rpm, DD 33 or 45 rpm. That being the case that must surely be a factor in this issue. What are your thoughts. BTW I like closed designs as they prevent the gathering of dust.
parrotbee

Showing 3 responses by atmasphere

Here is the issue in mechanical engineering terms. It is not a lot different from the steering and suspension of a car.

In a nut shell:

the plane of the platter must be consistent with the plane of the cartridge. As the arm moves the cartridge must remain in this plane. Since the platter must revolve, there can be no slop in the bearing such that the platter can deviate from said plane.

As the arm must be set at a fixed point, it can then be seen that if there is any difference that occurs between the point of the arm and the surface of the platter that is will manifest as a coloration of some sort.

For this reason, the coupling between the surface of the platter and the locus of the cartridge cannot have any slop of any sort. To this end, the coupling between the bearing and the base of the arm must be as precise and tight as possible; IOW of a singe piece which will not respond to vibration, as if any differences can occur they will be interpreted by the pickup as coloration.

What this means is the more dead and the more rigid the plinth is, which also holds both the arm and the platter, the better the 'table will sound.
Halcro, I can't make out from the photo how that cutting lathe is set up. Our lathe incorporates the platter bearings and the bearing track for the cutter head itself as a single piece of metal (which is quite massive).

With regards to the photo of your 'table, if it were me I would have not employed a tower and armboard to mount the arm, instead I would have affixed the arm directly to the plinth, which, in order to accommodate the platter as shown, would have been rather massive. The reason for this is that the tower and armboard both represent moment arms which can vibrate interdependently of the plinth itself. A simple test for this would be to tap the armboard with a metal instrument and then compare to the tone thus created when tapping the plinth. If different, a coloration is imparted.
I tapped the armpod and shelf (plinth) with a metal object whilst the stylus was on a stationary record...and the tone of both was near identical.

I was referring to the sound you get not through the speakers but just the sound of the metal itself reacting to being rung by tapping it with a metal instrument.