Yes, this is an advantage of a DD drive that is often overlooked. There is no horizontal force induced by a belt, thread, tape or idler wheel.
There is a toppling force if the thrust point is at the bottom of the bearing but this should be easily absorbed by a properly designed bearing.
Invert the bearing with the rotating mass centre of gravity at the pivot and this force goes away as well.
There is then the obsession with micro polished shafts. It might look great but technically it is sub optimal in an oil lubricated bearing. We want to maintain a continuous thin oil film. This is best achieved by a microscopically dull shaft surface finish which captures the oil film. Built properly a bearing like this has only the thrust pad as a contact point. Since this point has an extremely low radial velocity due to the small contact area, it can be made to be very quiet. Any noise in the bearing shaft/sleeve area is the shearing of the oil itself.
I was merely (re)stating an inherent advantage of a DD design with respect to its bearing, after all the OP has asked this.....
"I'd like to know your thoughts on the strengths and weaknesses of each drive system"
I did not say that overall DD is superior.
An inverted bearing is inherently more stable, on that I hope we agree?
My comment about the shaft finish is simply engineering 101. In the outside world it is common to not polish a sleeved oil lubricated bearing. This for the reason I raised. I know of one, possibly 2 TTs that use the non polished technique, both have well designed and engineered bearings. . These are the Final Audio Research and the big idler EMT. These TTs have very quiet bearings thru good engineering not bias
Sure you can remove horizontal pull on a BD bearing by having 180 degree opposed motors or an idler. That said, one would need to be very careful not to introduce noise into the platter from the second (or third) rotating element.
All TT drives have strengths and weaknesses, there is more than one path to enlightenment. On that we agree. I prefer DD, that is my opinion, others do not. I'm fine with that.
My experience is that it is by far the hardest to achieve good performance from a DD design. This because there is no filter between the drive and the platter. Any aberrations in the drive are exposed warts and all. This is particularly difficult if the designer chooses a high motor torque to platter inertia ratio. Difficult but not impossible.
In a synchronous motor the rotor lags behind the rotating field. This angle changes with changes in load There are subtle differences between motors, even with apparently the exact same build. This results in slight differences in this lag angle for a given load between (identical) motors
In a multi motor design, this means that one motor will be the master and the other(s) will slave this.
The master providing the bulk of the drive torque.
Consider the implications of this characteristic when 2 or more motors are used to drive a platter
mijostin I would offer the counter view. It is far harder to build an acceptable DD than BD for the reasons I noted earlier in this thread
Further with respect to the distance from the source, as you say the inverse square law applies. Double the distance and the field strength falls to 1/4 of the original. The 10 times per cm factor you sight applies only if the reference field source is originally 3.16mm away from the cartridge, a physical impossibility.
Also one would think that a ID would experience this interference issue since the motor is similarly close to the platter.
The problem is not the motor it self making noise.
This assuming the designer has used a good one. The problem is the interaction of the controller/ motor/ platter feed back loop.
IMO the designer should use what I call a very tight motor. I mentioned earlier the phase lag between the rotating filed and the rotor......
Imagine a clock with 2 second hands. One is driven by the motor the other is pulled along by the driven hand via a rubber band ( now where have we seen this before?) As load increases on the hand being pulled, it stretches the rubber band a little but still takes 60 seconds to complete a circle. Reduce the load and the gap between the two decreases, but again it still takes 60 seconds to complete a circle.
In a tight motor the rubber band is very stiff and it takes a lot of extra load to make it stretch further. In a loose motor the opposite is true, the hand can easily move about, increasing or decreasing the gap.
If we now apply feed back around these two types of motors we can see that we can be less precise with the loose motor as it will have a softer response to and input command. Whereas a tight motor will respond quickly feeding into the platter. This does not mean that the loose motor is a better choice, since,it will not control the platters speed as well. There is an unavoidable lag between command and response. What is does mean is we need to be very careful to finesse the feedback to a tight motor to give us good dynamic speed stability. If we don't we will induce noise into the platter as the motor rapidly responds to an excessively aggressive command.
And no, a high inertia platter won't save you, it just extends the time constant, meaning that it takes longer still to correct. The motor needs to be able to dominate the platter.
An iron fist in a velvet glove.
An update to my last post. A point I didn't make clear.
The change in angle between the rotating field and the rotor due to dynamic changes in load is a momentary change in platter speed. It is measurable and audible. Over time the average speed does not change, so we would not see this effect with say the Timeline laser. This, because it only tells us if the average speed is correct. These multiple subtle speed changes go unnoticed when this type of measurement is used. We need to use much higher levels of granularity to see them.
Further we do not perceive them as actual speed changes. ("that piano decay is wavering") isn't a descriptor one would use for this effect. It is more along the lines of solidity. Can we imagine walking up to the sound and actually holding it. Does it have mass and texture. These features are negatively impacted by these micro speed deviations.
An aside, the cartridge doesn't differentiate between a change in platter speed or a momentary change in platter/ arm position. Both actions will be interpreted as a speed changes. So we need to have brilliant bearings in the arm, we need to pay close attention to the paltter bearing, the mounting of the arm with respect to the platter and of course we need to carefully control any tendency for things to resonate. I think that the key imporvements we will see in future TT designs will be the holstic attention to this time domain parameter.
Many thanks for the invite. There is a little problem of distance however, since I live in New Zealand.
I do not disagree with you at all re the signature of "most" DDs. I spent 15 years whittling away at my mk3, little by little mitigating the very signature you describe. This journey started back in the mid 80's when this trait in my Sp10 mk3 was, to me, obvious Note I did not say that I totally eliminated it. Still, I really liked the good things it did, does.
More recently, I was approached to build a ground up TT. The options were ID or DD, I chose DD. I was given total license to do what ever I wanted. This was an opportunity to apply all that I have learnt in my professional life and in this hobby and someone else was offering to pay for it. I said to myself " How hard can it be?'.....Well that turned out to be a very naïve question.
The project took five and a half years, with 1000 hours of that time ( six months, 8 hours a day, equivalent) devoted to programming the controller. There are literally thousands of setting options and many of them interact with others. We were making changes to the speed stability in the order of 0.001% and we could hear them. We were actually changing the 'shape' of the W&F. As I have said in this thread, it is really hard to get a DD to sound right. But I do not consider it to be impossible. Time will tell if others agree with me, I'm fine with their opinions either way. This is inherently a subjective hobby
Servo controllers get a bad wrap in these and other parts. Yet most of us are listening to systems where amplifiers are using feed back and we do not think twice about this. A servo controller is a form of (electro-mechanical) feedback. Its application is a bit like Goldilocks and the three bears. Too hot, too cold or just right.
Another thing which many DD owners like to demonstrate is the long term speed accuracy. Watching a stationary laser dot on a distant wall. This is touted as a virtue and proves the superiority of DD over all other drives. I do not agree, and do not claim to be able to hear the difference between a constant 33.334 and 33.332 rpm. My design doesn't concentrate on this metric. What, to me, is important is the speed accuracy at a micro level. How speed stable is the drive under dynamic load, between a few arc seconds of rotation and the next set. Firing of a shot of red light once every 1.8 seconds does not tell you jot about what is happening at this microscopic level.
Yes it goes without saying that the drive can manage an apparently stationary dot over 1.8 seconds. I was referring to a longer time frame, say a complete side of an LP. To me it doesn't really matter if the dot has drifted a little after 20 or so minutes.
We are referring to speed drift. Quite a different metric than [email protected] and short term speed stability.
In a DD, this drift is related to temperature and the accuracy of the speed reference. A quartz crystal or another form of precision oscillator.
Speed drift on my design is < +- 0.0025%