Turntable power supply/motor’s role in SQ for belt drive TT


I’m trying to understand the ways that an upgraded motor/power supply can improve the sound quality of a belt driven turntable. 

OK - speed control that is more precise, I get that. But is there some inherent electrical noise from the motor that makes it way into the analog signal thus producing an audible distortion through the cartridge/phono stage?

I’m also curious about why an AC motor would be less desirable than a DC motor (for a belt driven turntable. 

Some back story: I am preparing to send my 35 year old Linn Sondek LP12 turntable out for servicing and upgrades. Linn has the promotion going on where you get the new Karousel bearing for “free” when you spend at least $4,000 USD. Kind of a gimmick, I know. But I would like to get at least the Lingo 4 power supply and am considering “Akurate” level “Radikal” power supply for more $$. I am looking forward to spinning my old vinyl again soon and hopefully with upgraded SQ as well. 

Thanks for any replies.
masi61
In two separate experiences, I have found that a motor controller, with or without a feedback mechanism, dramatically improved the performance of a belt-drive and an idler-drive turntable.  First was with a Nottingham Analog Hyperspace for which I bought a Walker Audio Precision Motor Controller, sort of with tongue in cheek, because I did not expect much if any difference.  Holy Cow, it was major.  The most significant improvement was in the ability to hear separate instrumental lines in the context of complex musical passages.  Next level improvement was bass definition. This sounds like audiophile BS even to me as I write it, but it's a true description of what I heard.  I then replaced the Walker with the Phoenix Engineering Eagle PS and Roadrunner tachometer, which does provide feedback to the Eagle.  That is even noticeably better, then on my Notts, and now on my Lenco idler-drive.  You're better off with an AC motor and a drive system.  The Phoenix stuff is now marketed by SOTA, as Bill Carlin just told you.  His stuff works.
LOL, same old naysaying non Linn owning members are at it again I see! 

OP, ignore those folk and concentrate on your Linn upgrade.

I think IF funds allow, definitely go for the Radikal and the Karousel.
The Radikal is a excellent speed control and the DC motor is going to get you excited about your records again. The new Karousel ( I just fitted mine) is a superb upgrade...and if you do nothing else, this is a no-brainer. Not sure who told you AC motor is superior to DC motor, not the case with Radikal vs Lingo. Lingo 4 is good, Radikal is a LOT better, IME.
Just another reason to get a Sota. Better motor, better suspension, better bearing (magnetic thrust control), Better tonearm mount and you can throw the wall shelf away (which does not work in all cases.) 
The LP 12 has more "upgrades" than any other audio device on the planet. What a racket and LP12 people drink the Kool Aid. The LP 12's two basic problems are it's suspension design and cheap construction.