Direct Drive turntables

The experiment has been done by anyone who converted his Teres or VPI turntable from belt- to rim-drive using the motors now available from the respective manufacturers. In these instances, all other factors are held constant; only the drive system is changed. As far as I know, almost all end-users have preferred rim drive vs belt drive. One could be skeptical of those claims, because there is a lot of subconscious bias in favor of hearing an improvement, in my opinion, when one spends money for such "upgrades". (The very use of that term suggests a bias.) But I am inclined to believe that the benefits are real, based on my own completely uncontrolled experiments comparing a "good" belt drive table to various idler-and direct-drive ones. (I am a scientist by profession, so this stuff bothers me.)

I don't understand the aversion to measurements. If you don't like measurements, you don't have to do them or pay attention to them.

If measurements don't correlate with perceived improvements in sonics, then the measuring tool needs improvement. The poor correlation of THD and perceived sonics is a good example.

Audiophiles have differing tastes. The assumption that measurements cannot accomodate differing tastes is probably untrue. In fact, if an audiophile was able to correlate measurements with his or her sonic preferences, it would likely improve the his or her ability to predict whether a particular purchase will be satisfying. Just a hypothesis.

Regarding belt drive slippage. Let's assume for the moment that a) the stylus places meaningful drag on the platter, and b) such drag varies in intensity with the shape of the grooves of a record, and c) the variation in intensity affects rotational speed to a degree that it creates an objectional variation in pitch. It would seem, to me, that compensating for the problem by "belt slippage" is akin to a servo motor that always reacts after the fact. So, to compensate for the inaccuracy of the servo motor (belt slippage) you use a very heavy platter so that the affect of the stylus drag upon the rotational speed of the platter drops below an objectionable degree of variation in pitch. I understand the hypothesis that the mass of the platter minimizes pitch variation.

I don't understand the hypothesis that belt slippage does something to minimize pitch variation. a) Why? b) How would you control belt slippage so that it acts predictably and with repeatable results?

In regards to testing drive systems, here are a couple of ideas. Test 1. Place very tiny hash marks next to a groove of a record. Play it and film it with a high speed camera or strobe (using a macro lense or microscope). Measure the speed of the stylus against the hash marks. Switch turntables and repeat. Compare the speed of the stylus over the same section of grooves. Test 2. Take a teres turntable. Play a short, dynamic, section of a record. Record the output from the loudspeakers using a microphone and input it into a computer. Do it for the belt drive version and the rim drive and use the same record grooves for each. Load the sampling into a computer software program that charts frequency and transients over time. Compare the durations and frequencies of the samplings by graphing one sample over the other where the x axis is time and the y axis is frequency.

Jeff

Lew, you offer a great example of comparison and I should have remembered it. A friend of mine owned a VPI Scout and became curious about the current rim drives. Although it cost about the same as his table and arm, he ordered the VERUS rim drive/controller system. There has been full agreement by everyone who has heard it that this rim drive plays more realistic and satisfying music. Interestingly he took this system to his VPI dealer where they compared the original belt drive to the VPI rim drive to the VERUS. Each of the three produced a different sound.

So I guess there are limited opportunities to compare different drive systems. But I still feel in most cases one could not make a fair comparison as to which system is "best" or "most musically satisfying" when the set up for each system is not optimized for the best results with each individual drive system.

J2468, No aversion to measurements here. It's just that I tend to put more trust in our ears.

I find the propensity to associate stylus drag with slippage curious. Although there will always be some amount of slippage with any friction drive (Mark Kelly calls it scrubbing) I do not think that it has much if anything to do with stylus drag.

I believe that an important issue in drive design is delay of torque delivery from the motor. This is akin to what you posted about servo reacting after the fact. A motor connected to a platter via a compliant belt is unable to apply torque to correct short duration speed fluctuations. If the platter decelerates slightly the motor applies more torque to compensate. But the belt simply stretches a little more. The energy ends up being stored in the belt causing a delay before it affects the platter speed. So the correction ends up arriving at the wrong time often making matters worse.

A heavy platter changes but does not solve the problem.
A massive platter will reduce the magnitude of a short term variation but extends it over a longer period of time. A light platter will conversely allow a larger speed variation but it recovers more rapidly. Heavy and light platters sound different but neither solve the problem.

I like your idea of using high speed photography to measure speed variations from stylus drag. But perhaps there is a better way. Rather than use a camera a reference track with a precise, constant tone could be used. This would require two tonearms and a test record with a steady tone and a track with variable modulation. The two tonearm part is easy but I am not sure if a suitable test record could be found.

It is expected that the magnitude of speed variation from stylus drag would be extremely small. To detect and measure the variation would require high precision. It may well be that audible speed variations would be too small to detect with a setup that is not prohibitively expensive. But thanks for the idea. I am interested in pursuing it.

Teres, thank you for a very articulate explanation.

I agree that our ears and our perception are more important than a number on paper. I do believe that with study, we will learn to improve our correlation between measurements and sonic satisfaction. If a measurement does not correlate with perceived sonic satisfaction, then the measuring system needs improvement and such theory does not bar individualism.

In other words, if a measurement not help then throw it out.