Direct drive vs belt vs rim vs idler arm

Spirit of music

Where are the specifications that suggest the Transfi has high torque.
On the contrary, from their website -

"Note1: The 9kg platter has immense inertia & will take a few seconds to respond to the control knob. Only very slight tweaks are needed.

Note2: You will find speed drift may require frequent minor adjustments until the bearing, motor & electronics are burned in.

Typically, starting from cold the system will run slightly fast. As operating temperature is reached it will slow. A suggested method of operation is to let the system warm up then set the speed. Play several records & fine tune until the speed remains constant.....then leave the control alone.
When you switch the system off, the motor does not need to be disengaged from the traction belt. The next time you start the system from cold, it will run fast. If you try to adjust the speed at this stage it will slow as the system warms, so best to let it run fast until it settles to the correct speed.

IMPORTANT

The whole system of speed control is finely tuned, & any variation in consistency may require adjustment, from VTF of the cartridge to the weight of record clamp used. Sophisticated motor speed control methods have been ditched in the case of Salvation as these all proved detrimental to the sound using this direct rim drive method. In practice, speed drift on Salvation is not an issue providing you are not varying the way you play your records from record to record or are a stickler for speed accuracy! Please consider this before ordering."

If you have a temperature controlled room, the angle of your dangle is stable, and you are not a stickler for speed accuracy, then this is the turntable for you - this is their own words.

The Michell Gyrodeck is not a paragon of TT speed accuracy that should be used as a standard either - tiny motor, stretchy flimsy elastic belt, floppy suspension and all.

Dover, your post leaves me with a lot of questions, but I won't go there. However, there is a need to explore torque and inertia. They can be confusing terms. Explained in the very most simplistic way possible, torque is what you need to get the platter spinning, and inertia is what you need to keep it going smoothly.

So, it is possible to get very good results with a very low torque motor because you can still have good inertia. The downside is that the turntable will reach its speed much more slowly, assuming the platter is a heavy one.

You can have both high torque and high inertia, but there is usually a price to pay. That price is motor size and lots of heat generated by it. Ashland was noted for making such motors, and quiet ones at that. The one used in a Fairchild 750 studio machine is the size of a clothes dryer motor, and it runs very, very warm. So does the more common one found on some Rek-O-Kut turntables and the Canadian made McCurdy. I would be happy with either motor in the Saskia, but they are no longer available.

By comparison, we are stuck with lower torque motors, if we want great precision, but we can still have lots of inertia. This is where system design comes into play, and done right a design can greatly benefit speed accuracy.

I suppose the point here is that most turntables, if not all, are compromised to some degree, however small. The trick is in making certain that any of those compromises do not negatively affect the end result. Hopefully, reliability and looks aren't affected, either. It isn't as easy to accomplish as you might think. Such an undertaking requires a lot of thought.

.

...unlike my clunky grammar which required zero thought. :D

Tony,
The issue I see with test record frequency measurements is that on the test record, groove anomalies and the good possibility that the test record center hole is not exactly center can throw off the measurements. Prior to using the laser reader I used a 1000 hz test tone on a record and used a Fluke digital multi-meter to measure the output frequency. It would oscillate + or - 1 hz measuring the frequency at 1000 hz...With the laser, my table is stable at 33.00x rpm, where x fluctuates up to .008 every few rotations. I think that is quite good for a belt drive. I am going to try the platterspeed app for iPod to see how it compares and I am especially interested in the time plot.

Richardkrebs -
Interesting post. Could you please elaborate on a couple of points.

Contrary to some opinions expressed in these forums, the servo does not intervene unless an external load causes a speed change. ( this assumes that the TT has been put together as the designer intended).

the motor does slip back in phase due to stylus drag...This can be seen by scoping its power supply. And watching the current draw. It was actually possible to recognize the music being played at the time on the scope. The speed sensor was measuring these tiny errors and correcting. This could be observed by watching the motor draw current in sync with the music. An amazing observation.

The conclusion is that some form of speed correction is required if we are to seek good dynamic speed stability.

So as I read it
Para 1 - The servo only cuts in when speed changes
Para 2 - Motor slip is occurring as a result of varying stylus drag to a degree that you can follow the music by scoping the current draw
Para 3 - Your conclusion is speed correction is required for dynamic speed stability based on your observations.

These seem contradictory. Are you saying that most direct drives dont speed correct all the time, but that they should be correcting more frequently for variable stylus drag ?
Isn't speed correction for stylus drag shutting the door after the horse has bolted ? If the current draw is up AND down with the music, when do you propose to correct the speed UP and DOWN before, during or after the event ?
Doesn't this just suggest that for whatever drive system/platter/arm/cartridge you have it must have enough energy storage or overkill in terms of motor drive & inertia to ensure the variable stylus drag does not impact speed at a micro level.
Are you not presenting a strong argument that all TT's should have a very high mass platter ? ( unless of course you can scan the grooves and preprogram the required changes to the motor controller to preserve micro dynamics ).
The other question I have for you is torque ripple, or cogging.
As I understand it the torque ripple or cogging torque will vary because a Direct Drive motor is running much slower than a small pulley/platter.
For example
DD - 33.33rpm x 20 poles = 20 poles per revolution of record.
AC/belt - 1800rpm x 4 poles = 216 poles per revolution of record.
Disclaimer here - I know you also have to factor in the number of slots, which could increase or reduce the differential in this example, but doesn't eliminate it.
Cogging torque results in torque as well as speed ripple; however, at high speed the motor moment of inertia filters out the effect of cogging torque.
Presumably a high mass platter, provided it is direct coupled to the pulley with an inelastic belt, can assist in smoothing out any torque ripple if it exists in both instances.
Can you explain why I can hear such a big difference in speed stability between the Technics SP10mk2 and SP10mk3 ? Do you think it is the higher torque, superior motor controller, higher mass platter of the mk3 or all of the above ?

Cheers,

Dover