Direct drive/rim drive/idler drive vs. belt drive?

Axelwahl,

On your first point: That one's easy. I take it as a truth universally acknowledged that if the motor were not running, frictional forces (of whatever source) would cause the system to slow down and in doing so it would lose kinetic energy.

It therefore must be the case that energy is being put into the system for it to remain at constant speed. The only possible source of such energy is the motor, therefore the motor must have the primary role in maintaining constant speed. All that platter inertia does is to reduce the slope of the decelerations (and equally the accelerations).

Your second point falls foul of a fundamental property of feedback loops, that the speed of the feedback loop must take account of the speed of the "forward" portion of the loop. If you interpose a low pass filter such as the belt and platter in the loop then you must slow the loop down to prevent oscillation. If you leave the belt and platter outside the loop then the loop cannot possibly compensate for losses in the belt / platter system such as torque dependent belt creep.

No free lunch.

Mark Kelly

I want to ask opinions on belt approach that Hiho has mentioned a few times. I was also of the opinion that any kind of compliant belt was not going to be the best solution. It seems in my brief experiences with speed controllers of late that with a DC approach the system does benefit from tight coupling, hence mylar can work very well.

The difference in DC controller operation can make or break this theory of mine. I would characterize one controller/drive system as being similar to the approach Dertonarm mentioned, that of counting on a certain amount of slippage. Another controller, (same motor, different controller) just happens to be one of Mark's old DC controllers, seems to improve with increased tension => no, or almost no, belt slippage. This does require a pretty hefty motor pod to help keep things taught. The presentation of this second DC approach is very much like what I hear with rim drives and maybe some lower end DDs. How much like rim drive I can't say as I don't have a good candidate for A/B.

So I thought I was all set with my drive choice. And then a friend brought over an AC motor and a Kelly AC controller. As you might expect, the cogging of the motor is very apparent with the mylar belt, even with less tension. The best setup was achieved with very little tension and a fair amount of slippage. Even more than what was used with the first DC controller. So it would seem that a more compliant belt would be called for with the AC approach. The amazing thing to me was that even with the huge amount of slippage the AC motor/controller was really kicking the DC arse in many ways.

After learning some these new things for myself I am no longer so convinced of what I thought before about belt compliance. Surely we would all love to have the perfect motor. But since that probably isn't going to happen, it seems to me that the controller becomes more important. At the same time, the selection of the motor and controller would seem to dictate what belt candidates should be considered.

Mark, I am still somewhat surprised that the particular platter/bearing does not seem to influence your designs. Or do you just make it look too damn easy? :-)

Dan

For the kit designs I had two overriding criteria: they had to be as cheap as possible whilst still providing acceptable performance and they had to be almost universally applicable (hence your comment about platters / bearings). Unfortunately this means that they are a long way from optimised for any specific application.

For the bespoke designs I gather as much information as possible, to the extent of getting specific numbers for the rotational moments of inertia of the motors used from the motor manufacturers or in the case of one drive having to measure the numbers myself. I then build a model in a Spice program using some translational analogies and spend a lot of hours doing dynamic modelling.

Depending on the sophistication of the drive, the specific platter and bearing numbers can have some influence on dynamic performance but the most important parameters are the motor and its electrical control. When I am happy that I understand what's going on, we go to prototype.

The results? Well, I think you'll be surprised at what can be achieved with standard AC motors and belts even less compliant than your Mylar when the drive mechanics are understood. Similarly if a manufacturer sends me several samples of a high cost three phase motor and says "do your best then bill me" the results can be pretty good.

The downside is that the controllers end up being quite expensive. I don't know if there's a viable model for producing an aftermarket controller using any of these techniques.

Mark Kelly

Axelwahl says, "One expert put two Micro-Seiki on top of each other --- and then waits 5 minutes for the darn thing to stabilize the speed."

I think the flagship Nottingham Deco took such approach, with a massive 64 pound that is as thick as a microwave and driven by an extremely low torque motor that you have to finger spin it to start. Definitely doesn't seem very user friendly to me. I had a Spacedeck in the house at one point, very quiet table but the sonic was so mellow that it put me to sleep.

I use direct-drive turntables. Sometimes I use them to (VHS)tape-drive a "passive platter". So the motor is any one of my dozen direct-drive turntables. I no longer see these DD tables as record players, they are motors with a 12" pulley, "active platter", along with a controller. The combination of two platters takes up a lot of table space for sure. But they sound good. The only time when I can't hear any improvement from this tape-drive approach is when I use a dd table with a coreless motor. The coreless motor DD table sound just as smooth as the tape-drive set up. I have yet to try it with my Technics SP10mk2 table. It will be fun. I don't have any Denon turntables right now but I would like to try them as they are the only company I know who use an AC motor for their DD turntables.

I have to jump in here, perhaps a little late, but I have definitely enjoyed the banter. Especially the literary references; and I thought I was the only one who kept an unabridged Websters first edition at my reach. And what does my wife mean when she walks in the room and says "if you spent a third of the time you spend on this stereo stuff on our rlationship things would be better". This reminds me of a line from a poem I like "If you can keep your head when all about you are losing thiers and blaming it on you"
Back to the topic:
Question #1- Is the torque(twisting force)acheived by the platter dependent solely(sp) on it's driving force, or does it at some point become a product of it's rotating mass?
#2 If the bearing matches the mass of the platter, no matter how great (e.g. the fan blade on a jet engine), are the problems brought up about bearings solved?
#3 I believe the Feikert Twin TT uses a kevlar belt. Not much stretch there. If the belt is for all practical puposes stretchless, does it become a direct drive?

Thank you all for your responses, I appreciate the time you put in this, even if my wife doesn't.

Matt