Decouple motor/bearing or both?

[Rexton]

I'm contemplating a new plinth build in bamboo and has decided that I'd like to decouple the motor from the plinth. I'm thinking about mounting motor and bearing on some threaded rod so that I can adjust height of both easily. I'm also thinking about decoupling the bearing as well. Would this give me any additional benefit, or is decoupling the motor on it's own better? I'm toying around with the idea bracing the bearing to the base of the plinth just like Mike's PTP and Jolly's bearing design. I'm perplexed and really would like some help!

 

[Avon]

The theory goes that you must not decouple the platter bearing from the arm mount. If they can move independently of each other, then cartridge alignment will be unpredictable during playback. Look at the Rega decks, which put great emphasis on restricting movement between the two. If bearing and arm are connected together on a sub-chassis however, then yes, they could be isolated from the plinth. But then the arm mounting needs to absorb (dump) energy from the cartridge. Not that I’ve ever designed a TT myself mind, so I’m not speaking from experience.

 

[Darren L]

That's one theory ^^^^^, there is others, that's not to say that Regas engineering solutions for record players don't work but there are many others, I've seen many other turntables which use many different solutions to achieve the same goals, even some that use a totally separate 'pod' onto which the tonearm mounts, so totally decoupled (or is it uncoupling? ) either way as long as your geometry 100% correct for the tonearm in use and the platter is turning at the correct speed without mechanical or electronic noise there's umpteen different ways to build a record player, .

 

[Tony L]

It is an interesting area of turntable design as there are conflicting requirements. It is clear the main bearing and arm-mount must be fixed relative to one another as the modulation the stylus is reading is so small. That is however a different thing to being rigidly coupled, though that seems to be the cheapest and easiest way to achieve something at least theoretically close to that ideal. The risk as I see it is trapping energy in that loop that will find its way back to the cartridge sometime later, e.g. sink energy back into the armbase and plinth and chances are it is going to come back at some point. It is an engineering question that has never been satisfactorily answered to my mind. Mass is often used, and I’m far from convinced that is the best approach, at least not without clear logic behind it, though it certainly seems to dominate current high-end thinking. Also many things are claimed to be “rigid” which clearly aren’t, especially at audio frequencies.

I find it fascinating, but I’m not sure I’m convinced by any argument. I can find turntables and arms I like in most design schools so I suspect implementation is really the key.

PS I assume it is a Lenco that is being considered here? In which case mass does seem to be the dominant theory by far. I can see a strong argument for mounting the motor on the bottom part of a split plinth though. JC Verdier did this with TD-124s and I think Martin Bastin did the same with Garrards too, i.e. remove the mechanical link between the motor and metal top-plate even more so than the existing suspension. Makes sense.