dazzdax wrote:A turntable is a mechanical device and a motor unit is mechanically decoupled from the rest of the plinth and should in principle not alter audio signal itself, which is an electrical phenomenon. I hope it does not sound to ignorant to you.
No, not ignorant at all, but I think the answer is right there. In theory
the motor is mechanically decoupled from the plinth and in top notch tts in practice it is. If a clean sine wave results in less motor noise and any motor noise finds its way back into the plinth directly or into the platter via its belt (or idler wheel I guess) as a result of a dirty sine wave, then that's definitely going to impact background noise.
I'm currently using a Heed Orbit on a NAS Spacedeck ... and the impact on the sound quality is either negligable or non existent ie. I can't really hear it (I use it purely for the convenience of push button speed change).
But the NAS has superb speed stability to start with due to a high mass platter and has an exceptionally quiet motor, as its a very low torque design which doesn't need stepping down when up to speed as it won't get to speed without manually pushing it. Its also completely separate from the plinth. So there's very little background noise to start with.
On a deck with a much lighter platter and the usual motor on the plinth design running the same voltage at speed as used to get it there, a bigger difference will theoretically be more apparent. I've experimented with this on a Rega P2 (with the Heed) and a Pro-Ject Debut (with their budget speedbox) and it would seem to be the case in practice too. BUT its a question of where the money is best spent.