Direct drive vs belt vs rim vs idler arm

Richardkrebs

The jitter artefact, as Mosin quite correctly points out, is not limited to the Technics line, he hears it in DDs in general. Actually my testing and listening has shown it to be present in any motor that uses feedback be it local feedback ( self correction) as in free running motors like those in most BDs / idler drives or global feedback as in DDs. That is it occurs in all motors. Since all TTs have motors ......

This statement is an oversimplification and a little disingenuous. You are conflating apples and oranges. There are 2 areas of instability
1. Any slip in the motor.
2. Any instability induced by speed correction and servo action.
You are grouping 2 issues together, which have quite different outcomes in terms of how they impact a turntables speed stability and sound. “Jitter” is associated more with instability due to speed correction and servo action. Motor slip and its self-correction is much softer and more benign than jitter induced by speed correction and servo action.

1. Motor Slip

Local feedback is softer and slower to respond to demand, it creates a slurring effect. The music is slightly blurred like a soft focus lens. Drive and dynamics are diminished.

I would agree with this statement, but it should be qualified in that the amount of slip and its impact on the sound will depend on the quality and type of motor, it’s power supply and the inertia of the platter.
One of the fundamental differences between AC synchronous motors and Brushless DC motors as used in the Technics SP10 is how they lock into speed.
AC synchronous motors are dependent on the frequency of the drive current. In the Final Audio TT sine and cosine waves are generated from a motor controller and are not dependent on the stability of the mains supply.
In Brushless DC motors, the stator windings are energized in a sequence. The sequence and timing is determined by the rotor position. Therefore accurate measurement of the rotor position is required for speed accuracy.
In a perfect world these would sound the same, but the world is not perfect and they don’t.

My research into motors has shown that this can be reduced only slightly. .

This is a generalization - how the motor self corrects will vary considerably depending on the motor design and how it is used.

For example –
AC Synchronous motors and DC Brushless motors can behave quite differently.

AC synchronous motors tend to be sinusoidal; DC motors tend to be trapezoidal.
Trapezoidal motors give a back EMF in trapezoidal fashion and the sinusoidal motor‟s back EMF is sinusoidal. In addition to the back EMF, the phase current also has trapezoidal and sinusoidal variations in the respective types of motor.

This makes the torque output by a sinusoidal motor smoother than that of a trapezoidal motor.

Similarly the self-correction in a sinusoidal motor will be smoother than a trapezoidal motor.

The method of coupling the motor to the platter is immaterial, as this effect is built into the motor itself.

You are wrong. If a platter is direct coupled to a motor then the platter will act as a flywheel, with inertia. This will increase the resistance to drag induced motor slip and the behavior of the self-correction by the motor to recover will also be variable, for example higher inertia slower recovery.

Factors that will influence the motor slip will be the design and implementation quality of the motor design, power supply, inertia of the system etc.
You have not provided any comment on the magnitude of any instability due to motor slip you measured.
How many motors did you measure?
What types of motor did you measure?
What parameters did you measure?
How did you measure them?
Did you use your thumb and pencil method you recommended in earlier posts?

2. Any instability due to speed correction and servo action.

This is what I hear.
Global feedback when carefully designed, but not properly put together, creates a tension in the music, a greyness.

I agree with this, it is, I assume, what I have heard from most DD’s I have heard, including the SP10. It is less apparent in the L07D’s that I have heard.

The music does not flow and does not properly connect with the listener. It is not servo overshoot, hunting or cogging. This sound is what the BD people talk about. This problem, in my opinion, can be largely removed.

I agree with the hunting and cogging – these issues have quite a different impact on the sound, musical timing and instability, particularly noticeable in the bass range, for example the Goldmund Studio.

With regard to loop control feedback systems as used in most DD’s you seem to be in denial of system overshoot. As far as I am aware most feedback system will have input parameters based on some sort of modeling. Usually algorithms are used requiring control inputs based on measuring past errors, the present error and a prediction of future error based on the current rate of change.
Past errors (Integral) are included to accelerate the process and remove any steady state errors in the present error (or proportional if you want to get technical).
Since the integral term responds to an accumulation of errors from the past, it can cause the present value to overshoot the set point value (where you want to get to).

In terms of correcting the present error, if the gain is too high you get instability, too low and you get a less responsive controller that may not deal with fluctuations.

The prediction of future errors (called the Derivative) is required to minimize overshoot.

So, summarizing this then these inputs have to be balanced to maximize the correction and minimize instability and overshoot. There are always tradeoffs.
For any given Speed Correction Feedback Loop you should be able to measure or calculate the following parameters:

Rise Time
Overshoot
Settling Time
Steady state error
Stability

Since the Technics was built there have been significant advances in technology. We now have computer simulation software to model motor behavior and generate more accurate algorithms, from which the input parameters are derived.
There are now faster chips and amplifiers for driving the motor – PWM’s are switching at a trillionth of a second today.

The problem is the Technics is full of obsolete chips and trying to optimize the feedback loop would be like trying to tune a car that has a very basic engine management system. You would probably be better to throw the boards away and start afresh.

There have also been newer methodologies to apply global feedback, which are the subject of patents. I think from memory Fujitsu has some.

For the DIY person because the whole feedback loop performance is based on accurately measuring the rotor, eliminating any flex or instability in the physical motor could in theory help to reduce the issues endemic in feedback loops.

You have claimed that you have managed to reduce these issues related to speed correction/servos. It would be helpful to see some hard data measurements to quantify the level of improvement.

I would also be interested to know if there are any turntables available commercially that do not have speed correction/servo induced issues.

The myth of Direct Drives and Lathes

Direct Drive devotees often refer to cutting lathes and make claims about the suitability of DD's for replay based on their use in record cutting lathes.
I would like to point out the following :
The Technics SP02 is considered one of the best DD lathe motors. Yes it is Direct Drive, but in most cutting lathes the platter is 70lbs or so ( Neumann for example ) and designed to act as a flywheel - more weight on the outer. So in other words cutting lathes are usually using very high inertia platters to provide stability in conjunction with whatever drive system is chosen.
Most of the best classical music in my view was recorded before the mid sixties, when multi miking became popular and the engineers took over the asylum. Most music was recorded on shaft driven or belt driven ( some Scully lathes for example ) lathes with very high inertia flywheel/platters.
This to me this is a compelling argument for using high inertia, irrespective of the drive methodology chosen.

I should have made it a bit clearer in the last post, most Technics SP02 cutting lathe motors I have seen are coupled to high mass flywheel/platters in the order of 70lbs/30kg.

I have a pragmatic question. I did some maintenance on my tt last weekend. I checked screw torques and surprisingly, all of the armboard screws turned about a quarter turn. I got it all back together and dialed in speed perfect. All is looking good. I put on Fresh Aire III again. Nice bass, a little tighter than before, I think. So I put on the CD version and give a listen to track 1. On the CD I can hear the skin on the drums; on the record- almost, but just not quite there. My question is: is that the smearing of detail you all are talking about? Is that really due to speed control, or could it be tonearm, preamp, etc?

Dear Dover, There you go again. Trying to "prove" that one drive system is "better" than another (or in this case, "worse") by arguing from first principles. First, I think there are a lot of holes in your arguments, and second, have we, or at least most of us, not agreed that execution is paramount? That each drive system has its failings?

By now I get it; you don't like direct-drive. You do like your Final Audio TT. Live and let live.

On terminology: Motor "slip" is a term I have only read in connection with induction motors. They must slip in order to generate torque. This would apply only to idlers, as the older idlers are the only TT's I know of that used induction type motors. Induction motor combined with eddy current or other type of "braking" can make for excellent speed stability. Also, "jitter" has a specific meaning in terms of digital to analog conversion. Let's not confuse matters further by using it to describe a phenomenon related to what? Cogging or servo correction? These two phenomena are entirely separate and independent of one another, and yet they are being lumped.

FWIW, The top end Denon DD's used 3-phase AC synchronous motors. The top end Technics DD's use what you would call DC motors. But Bill Thalmann told me in conversation that if you look at the actual circuit of the Technics, the distinction between the two is not so great. DC motors are used in some of the most expensive BD turntables, as well, and the manufacturer's tout the fact.