Direct Drive turntables

I have had an opportunity to listen to DD, Rim drive and belt drive on essentially the same turntable and there advantages and disadvantages to each.

Belt drive is by far the easiest and least expensive to get good results. The isolation afforded by the belt hides motor flaws and cogging. But it does so at a price.

Direct drive is brutally revealing of even the most subtle problems with the motor. A typical single phase AC motor would be unlistenable on a DD turntable. But a good DD implementation provides some goodness that in my experience you just can't get with belt drive. The goodness includes the much talked about rhythm and pace but also a bunch of other positive attributes like clarity, low level detail, air, etc. It is really difficult and expensive to do DD right. So with a small to modest budget I would choose a belt or rim drive.

I see rim or idler drive as between belt and DD. Rim drive offers some isolation but far less than belt drive. Rim drive offers the same positives of DD but a smaller dose. Rim drive is somewhat forgiving of motor flaws (cogging) but less than with belt drive. Rim drive is a little more complicated than belt and done right will often be more expensive because of the higher demands on motor quality. I think that rim drive is where the best value hits. Rim drive is relatively easy to get right and in my opinion offers significantly better performance.

This is how I believe that the drive topologies stack up. But it actually says little about how turntables using these topologies will compare. With any turntable you are hearing the whole package and the drive topology is just one of many pieces. The well respected SL-1200 gives people a taste of the goodness of DD but at the same time you get the sound of a lightweight resonant base a flimsy platter and an inadequate power supply. Not that I don't like the SL-1200. At it's price point it is very good, but it is not representative of what a really good DD table is capable of.

The term speed accuracy is often misused and misunderstood. It usually is meant as average speed. The speed accuracy numbers quoted are always average speed and are mostly meaningless. Our ears are quite insensitive to average speed and most of us do not have sufficient pitch sensitivity to detect a 1% error and nobody is capable of hearing 0.1% errors.

Speed stability on the other hand is very important and our ears are remarkably sensitive to extraordinarily small deviations. Tiny, short term (less than 1ms) deviations cause a host of problems like smearing, loss of detail and loss of pace. Every turntable has some amount of variation and in my opinion no turntable in existence has been able to reduce speed variations totally below the threshold of audibility.

As Mike mentioned the same principle is in play with digital jitter. It has been well established that jitter in digital audio is clearly audible even though the amount is infinitesimally small (billionths of a second). Great strides have been made to reduce jitter but like analog I doubt that anyone has been able to completely push it below audibility.

I think that there are a number of sound, theoretical reason for the superiority of DD. For example DD in theory is much better able to deal with the effects of stylus drag. This is a controversial subject because it would appear that stylus drag is too small to be audible. That may be the case but there is good evidence that techniques that attempt to reduce stylus drag effect result in better sound.

Mapman, Spinning a platter at an average speed of 33.3333 is very easy. In fact it is fairly easy to get the accuracy to 4 or even 5 decimal points (not that it matters much). The problem is making it constant at a microscopic level, or at least to a level below audibility. This part is very difficult and in some ways at least as difficult than rocket science.

Isolation from the motor is all about constant speed. Motors vibrate primarily because the torque they deliver is not constant. The uneven torque translates into speed instability and speed instability (even tiny amounts) translates into degraded sound.

My experience with turntable drive systems has convinced me that speed stability is one of the most important, and challenging parts of turntable design. I have to admit that this finding was surprising to me so I understand your skepticism.

I think that the topic of record slippage is red herring. I am not aware of any evidence (listening or otherwise) that slippage is happening. However, there is plenty of listening evidence that some phenomenon is at play that fits the concept of stylus drag audibly influencing speed stability. This evidence shows up with or without clamping.

Jj2468, I'll echo what you said about measurements. It is too complex and we don't know what to measure. Both of those statements are often true, that is why you hear them repeated so often. Some things can be measured and some cannot (at least not yet). Some measurements are useful and correlate to good sound and some do not. To the point there is something going on in the world of analog that mimics (or is) stylus drag. It can be heard but nobody to date has been able to measure it, nor have I heard of a credible technique put forth to measure it.

Actually we do have some superb, highly sensitive instruments to measure with, our ears. Too bad many tend to trust them so little.

Dertonarm, An interesting test. I'll have to give it a try. But the test is about record slippage and not stylus drag (at least the way I define stylus drag). I generally prefer clamping. I suspect that this is mainly due to better coupling to the platter but some of the difference may well be due to record slippage. Thanks for the measurement suggestion.

Pryso, It depends on what you are trying to accomplish. If you want to get an accurate understanding of the sonic affect of a particular component then you want everything else to remain constant. In this case it's about gaining understanding and not necessarily optimizing for the best sound. With understanding of the contribution of various components in isolation then you are in a much better position to then optimize.

It is certainly possible to make meaningful judgments without everything else being equal but it is much more difficult and more error prone.

There is usually some synergy between components, but I believe that as the level of quality goes up this becomes less important. I think that a motor implementation that requires a particular tonearm or cartridge to sound optimal should be considered to be flawed. A quality, neutral implementation of any component should work very well with any other quality component. Of course in the real world this all too often is not the case. But it should be a goal of good design.

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.

Raul, I don't think that the measurement vs listening argument is about accuracy vs tastes. Take for an example an early 70's solid state amplifier. Those amps were made in the height of meter watching. They excel at the measurements that you frequently cite, flat response, low distortion, low output impedance. In spite of exemplary measurements they sound terrible and more to the point they sound nothing like the original performance. So good measurements does not necessarily equal accuracy.

You are correct that the target is to reduce colorations. But colorations (or lack of) cannot be defined by a simple set of measurements, in particular the three you just cited. Otherwise those 70s amps would sound both fantastic and accurate.

Measurements are useful but if we were to make the measurements more important than hearing we would all be listening that lovely 70s technology... well actually most of us would have lost interest and found some other hobby.