Hi everyone,
I know I will regret posting in response to some of the above, but before I comment on some of what Dertonarm has postulated above repeatedly, allow me to address the original question:
For any given motor/powersupply combination there is a specific load against which it will work producing the least vibration/smoothest rotation. This is, strictly speaking, only true for a motor which has perfect spindle bearings. They are another source of irregular behaviour, particularly since the load on a belt drive motor pulley when used with a thread is rather high(compared to the tension required if a rubber, neoprene, silicone... belt is used). It is possible to address this problem, at the price of complexity and noise(i.e. counter-pulley for the motor spindle...other options exist).
Off course stylus drag isn't capable of slowing down a platter AND at the same time modulating the deceleration(or the acceleration!) at the exact frequencies that are being played back at that moment. The platter acts as a low pass. Nevertheless, stylus drag is NOT constant and therefore requires lost energy to be fed back into the system at an ever changing rate to maintain absolute speed, requiring constant acceleration/deceleration. If the platter is loosing energy due to additional "drag", be it generated by an eddy current brake(no noise added), a felt brake, a paddle running through a silicone bath or a particularly lossy bearing(i.e. the DPS turntable), the influence/impact of the losses generated by the stylus drag become smaller.
Increase the drag tremendeously and you will get the influence of stylus drag below the threshold of audibility(let's just say for a moment that there is such a thing...)*.
You need to increase motor torque(meaning, all other things being equal, increase motor noise) once the losses become too high, both to be able to maintain target speed and to bring the platter up to speed within a reasonable amount of time.
*The same can be achieved by increasing platter mass/inertia, BUT the remaining speed variations will be lower in frequency and higher in amplitude if no means of damping(same as aditional drag, sorry)is employed. Low pitch variations are more obvious/bothersome to some than they are to others.
Since amplitude and frequency of the platter speed variations can't be reduced to Zero, any turntable will be compromised, unless it's platter would feature infinite inertia, in which case all other factors(motor, idler, belt etc.) become obsolete, since nothing can move an infinitely inert structure.
The insistence on the superiority of Dertonarm's super heavy platter driven by a lossy drive/"slipping" thread is hard to understand, particularly if we apply the laws of physics, as continously demand by the master(couldn't resist that one ;-) himself.
If the thread is allowed to slip while it is driving the circumference of the platter, slippage will (likely)occur at the pulley too. Even if the slippage was constant, and it is NOT(according to physics, keyword:"stiction") , slippage on both the driven and the driving surface will result in chaotic behaviour, COMPROMISING the evenness of energy transfer.
If, as some turntable manufacturers have done, the platter speed is monitored and used to control the motor, hunting and pecking is inescapable and only the chosen feedback time constant, the platter mass, the motor torque and the little bit of stretch left even in Aramid or Dyneema threads will govern the speed variation amplitude and frequency around the nominal target speed. And if there is belt/thread slippage or an out of round platter, the "error correction" will have to work very hard.
Dertonarm stated that neither direct drive nor idler drive was usable to build a turntable with high mass/inertia platter. Oh boy... we are mostly listening to shitty records cut on Neumann lathes, some of which use a Technics SP10 MkIII motor(albeit driving a 40kg, large diameter platter)
The Onkyo PX-100M, an eddy current direct drive tt features a 24lbs platter(without the mat) and is an excellent deck.
Other examples in conflict to the above statement were mentioned by Chris Brady.
What does a super heavy platter buy the designer other than inherent higher frequency speed stability? Problems!
You'd be surprised how much wobble/tumble can be detected on platters even if the bearing tolerances are super tight(the Continuum Audio site once had some indepth analysis graphs and animations)
Dertonarm will likely reply that an airbearing is the solution, but it isn't(it's a compromise too). It minimises bearing noise and friction(a major problem with super heavy platters) but can't restrain platter tumble as well as a pressurised oil or grease bearing. For what it's worth, essentially all industrial axial airbearings for heavy loads are made for much higher rpms.
Dertonarm suggests/requests the side load on the platter to be zero. Very good(unless a spindle bearing à la Bill Firebaugh is used)! But if you do this by putting a counterpulley opposite of your driving pulley and maintain not only the distance but also the diameter of the counterpulley, there is a strong risk of all 4 "free" thread sections acting as strings, resonating at the same frequency. Any such behaviour will wreak havoc on the smooth operation of the motor as it changes the load it sees rapidly. Put a break on the counterpulley or change the pulley diameter and the distance accordingly and the problem is solved.
Off course, once the platter is made heavy enough, that becomes neglectable too... depending upon your neglectability threshold.
My conclusion: Dertonarm's way is one way, but not the only way. It is, like all ways, compromised(I wouldn't dare calling it flawed).
A few last words on how "commercial" designers/engineers are often described here and in other forums.
Yes, there are some(too many) that do not have a deep understanding of physics or electronics(not to mention the growing number of copycats). But just as many do and they are into it because they happen to share other people's(be it customers or colleagues) enthusiasm for music and all the gadgets that allow us to enjoy it in it's preserved form. It is simply not true, that all of us think with a target price tag in front of our eyes first. It is simply not true, that all of us think: "yeah, I guess that'll do..." And that is a FACT.
Finally, it is downright ARROGANT to belittle designers who are capable of coming up with a component that delivers 95% of what is currently possible at 10% of the price. Yes, it is all about compromises and ESPECIALLY an all out assault on the state of the art will eventually run into facing this as well.
Happy Easter,
Frank Schröder
