Is Direct Drive Really Better?

Turntables drive systems is an interesting topic and one that I have been contemplating and experimenting with a great deal lately.

I have concluded that speed stability is one of the most important factors in turntable sound quality. For that matter it is also one of the key performance factors in digital audio. It is well known and accepted that digital jitter significantly degrades sound quality. What is remarkable about digital jitter is that such extraordinarily small timing errors could be audible at all. The message here is that our ears are far more sensitive to timing errors than with amplitude errors. With analog the principle and effects of jitter/timing errors are essentially the same. In both cases waveforms are being reconstructed and timing errors create similar distortions. Distortions that for some reason are much more audible than one might expect.

So when it comes to turntable speed stability it is a much more complex issue than many would think. Technically there is no such thing as “constant speed”. Any drive system will have micro variations in speed. As with digital jitter, both the frequency and amplitude of these variations are important. Wow and flutter measurements only quantify large, low frequency variations and don't seem to correlate well with sound.

The main source of speed variation is the motor. All motors cog, or have variations in torque as they rotate. The correct term is torque ripple. In general it is beneficial to isolate the torque ripple to reduce the effect it has on platter speed. Belts and idler wheels provide some degree of isolation. How much of course depends on how compliant the material is. At the same time it is beneficial to have the motor tightly coupled to the platter and rely on the motors torque to keep the platter speed constant. Tight coupling of the motor is best way to reduce the effects of stylus drag. So we have two opposing objectives, coupling and isolation. It would seem that for any motor, platter combination there would be an ideal compromise between isolation and coupling. For example AC motors have a lot of torque ripple so the best compromise is usually a lot of isolation using a stretchy belt. DC motors have far less torque ripple so they typically sound better when used with more rigid coupling (½ tape). With direct drive there is no isolation so the torque ripple must be very low to get acceptable sound.

Beyond the isolation and coupling issue belt and idler drives are both are susceptible to oscillation. We end up with two rotating masses connected with a compliant medium. The worst case is when both the motor and the platter have the same inertia. At first glance it would seem that the inertias are quite different. However, the motor typically spins much faster than the platter so that the inertia actually ends up being similar despite very different mass. Increasing platter mass and decreasing motor inertia helps reduce oscillation. I believe that this is one of the reasons that heavy platters tend to sound better.

As I said earlier both the frequency and amplitude of speed variations is important. There is considerable evidence that very small, higher frequency variations are particularly audible. Power regenerators for AC motors and batteries for DC motors have consistently provided better sound. My own experimentations has also shown that efforts to reduce high frequency noise results in better sound. This would indicate that higher frequency speed variations are more detrimental to good sound. This kind of error sounds remarkably like digital jitter. It sounds harsh, edgy and smeared. Not an artifact that would typically be attributed to speed stability.

Various techniques may be used to reduce these effects but they are never completely eliminated. A heavy platter will reduce high frequency variations more than a light platter. However, lower frequency problems are less effected. For example the effect of stylus drag is different but not less with a heavy vs. light platter. With a light platter a heavily modulated passage will reduce the speed but because of low inertia it will quickly recover. On the other hand a heavy plater will be slowed less but it will take longer for the speed to recover. So platter mass only changes the frequency of stylus drag effect and does not eliminate it. However, it would seem that lower frequency variations from a heavy platter would generally be more benign.

So after all the ramblings the question is what is the best approach? As many have surmised it mostly boils down to implementation. It also is a matter of compromises. Some will prefer one set of compromises over another based on their tastes. However, I do believe that direct drive has the greatest potential. With careful design and implementation direct drive can result in less compromises. With sufficiently low torque ripple and noise the results are remarkably good. Good enough that a direct drive offering from Teres is in the works.

Dan_ed, We have an early prototype now and hope to have a better version in time for the open house.

Jejune, Not even close to the 1200. The Teres DD table will be flagship offering. It's both difficult and expensive to do DD right.

About stylus drag. I am unaware of anyone who has actually measured it and can certainly agree that it seems far fetched. However, we should not underestimate the audibility of timing errors. Digital jitter on the order of tens of picoseconds has been shown to be audible. A pico second is one trillionth of second! At the 44 Khz sampling rate this is less than one part in a billion. It's easy to imagine that 2 grams of tracking force might have a one part in a billion effect even with a 100 pound platter. Measuring such a small effect would be difficult but not impossible.

4yanx, I have never thought of myself or my customers as being supporters of belt drive. It's what we have used and it has and continues to be a very good drive system. I suspect that our customers are not married to a particular methodology and will accept whatever delivers the best sound. But I hasten to add that Teres Audio is NOT abandoning belt drive. The new Teres DD setup will be considerably more expensive to produce and required a much bigger development investment. And while it does deliver considerably better performance it does so at a price. I think it unlikely that we will ever be able to come up with DD system that will compete, at the same price point, with our current belt drive motors. Of course we will try, but don't hold your breath.

So is DD superior? I think that the answer can only be yes and no. When it comes to the best money can buy, then yes I believe that DD has the potential for performance that cannot be matched with either belt or idler drive. But be prepared to spend a lot to get there. When it comes to more reasonably priced turntables then it comes down to the quality of implementation and also personal tastes. As a generality I think that belt drive tends to offer better sound for the money at all but the highest price points.

Those coming to Denver will get to hear the exact same table, arm and cart with belt vs. direct drive. It's a simple matter with the new prototype to switch between belt and direct drive. But I hasten to add that this still will not mean much about the debate at hand. The motor and controller topology is completely different. So people will be hearing motor differences along with the belt vs direct differences. I suspect that in this case the motor/controller differences will have a larger impact than the drive method.

There seems to be a lot of religion about drive methods, but I see it as just a small part of the equation. The motor/controller design and characteristics are at least equally important. Rather than taking our direct drive development as an endorsement of the drive method it would be more accurate to say that we have developed a promising new motor and we are implementing it with direct drive. A suble but important difference.

Zaikesman, I have to admit that the direct drive choice was based almost entirely on theoretical reasoning. The initial quest was to produce a better quality motor using some new techniques that I had been musing about for some time. When examining drive methods it looked like DD had the most potential (ie lack of compromises). But at the same time it also looked like the most difficult and risky approach. A little isolation can cover a lot of motor sins and with DD you get none. I also looked closely at idler drive. From a theoretical perspective it seems that idler drive is somewhere between BD and DD. Some isolation but less than with BD. I happen to think that idler drive has a lot of potential and suspect that at some point I will experiment with it.

In the end I settled on DD because I believed that this new motor would have low enough torque ripple that DD could be used without compromising the smoothness that is characteristic of a good BD table. Our first DD incarnation confirmed my theories. It had great pitch stability, drive and rhythm but sadly lacked smoothness and refinement. But with some considerable effort the smoothness and refinement has now surpassed our best BD motor. So is the success due to the motor or the drive method? The answer must simply be yes.

Fortunately, or unfortunately, marketing had little to do with the decision. It's going to be difficult to recoup the development costs for the DD motor. Sadly the motor is expensive to produce so I doubt it will ever be sold in volume. But I can say that it has been one of the most personally rewarding ventures I have embarked on.

+++ do you think that in this moment your non-metal DD design is a better one? or do you make it in that way because looks better than an all metal one? +++

Good question Raul. The short answer is that we use hardwoods primarily because of the sound, looks are secondary.

The first turntable to come out of the Teres project was my design that used a well damped all aluminum base (http://www.teresaudio.com/fame/1.html). The very first base that Teres offered was 3" thick acrylic. It looked good and was both inexpensive and simple. The sound was OK but was not nearly as good as aluminum version. On a hunch we decided to try making a base out of rosewood loaded with lead shot. To our great surprise the sonic result not only exceeded the acrylic base but also far exceeded my all aluminum base.

I have experimented with many materials and have yet to find anything that delivers subtle midrange detail and texture like dense hardwoods. Carbon fiber comes close but lacks some of the warmth. I find that aluminum is detailed and pristine sounding. However, compared to hardwoods it sounds whitewashed in the midrange and there is a lack of "life". I think that there are a couple reasons for this. The first is rigidity. Most people are surprised to learn that hardwoods like cocobolo are much more rigid than aluminum. A 1/8 inch thick ½ inch wide strip of cocobolo cannot be bent with bare hands. However strip of aluminum of the same size is easily folded in half. Also hardwoods have a chaotic grain structure that varies greatly in density. This varied density is good at both dissipating and breaking up resonance.

I have also experimented with other metals and prefer both mild steel and stainless steel to aluminum. The steels offer a more clarity and detail but also sound a bit too harsh and analytical. Of the metals I find that brass is far superior to steel or aluminum. It is detailed and rich without ever sounding harsh like steel or smeared like aluminum. We found that replacing all of the aluminum parts on the Teres 340 with brass was a major upgrade. Brass is very expensive so it is not often found in turntables. However, I believe it's properties easily justify the cost. The new direct drive Teres will continue to use hardwoods but more brass will be used.