Hi All,
There is not enough room here for a through discussion of linear tracking. Or for pivoted tracking either. Let's talk about linear tracking only. While this essay discusses the methods from an objective position, I feel obligated to give my opinions where I can. I probably have the most experience with linear tracking on the forum. While you may not agree with me, you at least should hear what I think. There have been four basic systems over the years. Let's look at each one.
* First, and most common, is the electronic servo based control system. In this case, there is a scheme that detects (either optical sensors or mechanical switches) when the arm goes off tangent to the groove, generates a correction signal (the error signal), and causes a motor, which drives the arm carriage, to bring the arm back to tangent with the groove (0 degrees error).This system can be very precise and accurate if it is based on optical sensors (B&O). With mechanical switches (original Rabco), less so. Remember, record anomalies such as off center holes and warps must dealt with by all linear schemes.
This scheme is the one chosen by the Japanese manufacturers. There has been a huge range of quality produced. It's easy to decide which one to get. Buy the expensive ones. Avoid the cheap ones. But this is always true in hi fi. To my knowledge, the European linear servo designs have all been excellent; and expensive.
* Unique is the Rabco/HK method. It is a straight mechanical servo system. I have never owned one of these systems but I have worked on them. I never got the arm to maintain consistent operation. However, I worked on the arm when it first hit the market. From what I have read on this form, others have had more luck. I don't understand the mechanics of this system to knowledgably write about it. I do know I would avoid it like the plague. I know others will disagree.
Like the electronic servo types, the mechanical servo drives the arm across the record with the stylus only providing the error indication.
* There have been non-servo driven mechanical linear arms. The most successful is the Souther design which now made by Clearaudio. In this case the stylus drags the arm across the record. There is no servo help. Marantz, back in 1960's, also marketed a straight mechanical linear tracker. There probably have been others that I am not familiar with.
Obviously, this scheme totally depends on low friction bearings and a very precise/stable set up. The stylus must overcome the inertia of the arm, the carriage, the cartridge, AND overcome the friction of the carriage bearings. It is analogous to the work the stylus must accomplish with a pivoted arm but here there are more bearings. The Marantz arm was a failure. Too much friction. It was an idea ahead of its time. The Souther design has been very problem prone. When set up properly it can work well. But maintaining a proper set up is a challenge. It has one design element that is a big issue. The tone arm is so short that it produces warp wow that is obvious. If used on a vacuum turntable which can flatten most warps, the Souther arm can work but warp wow will still be an issue but less so.
With any tone arm, resonances can determine its audio performance. Ball bearings are very resonant prone. This is why I prefer unipivots in a pivoted arm which has no ball bearings. The Souther carriage runs on ball bearings, lots of them. I can hear those bearing rattle just like I can on most non-unipivot pivoted arms. This is the main reason I don't like the Souther design, clever as it is. It just doesn’t sound very good. And I want a tone arm that does not require constant set up tweaking for optimum performance.
I do think the Souther arm is beautiful. But, the performance........?
I have done a fresh Souther installation and I can report it was a bitch. I learned that I don't want this arm. I came very close to buying one until I discovered air bearings.
* The conceptually simplest implementation of linear tracking is the air bearing arm. Simple from the users point of view and simple conceptually. From an engineering and manufacturing point of view, an air bearing is a very difficult challenge to overcome all the competing requirements for a totally successful tone arm. The devil is in the details. This accounts for the high price of entry. The arms better be good or no rational person would pay the price. But, do remember, the best pivoted arms are also very expensive. Here too, the devil is in the details.
The idea is easy. Mount a tone arm on a carriage and float the carriage on a cushion of air. Friction essentially drops to zero, ball bearing resonances are totally absent and servo complications are not necessary. However, the actual design details are daunting. One must control the air cushion in all directions, the carriage must have a consistent air cushion as the stylus tracks across the record, record anomalies must be dealt with, and user calibration controls must be available, easy to use, and stable. Not an easy set of requirements.
Add to these is the actual design of the air bearing. The necessary mechanical tolerances are very tight. There are trade-offs between bearing stiffness, air pressure, air quantity, bearing size and capillary size. The capillaries are the tiny orifices through which the air escapes the manifold and provides the cushion to float the carriage. Make the capillaries too small and they will clog up. Make the pressure too high and you will hear the air escape. Make the air quantity too great and the compressor or pump will become noisy and expensive. When all these problems are solved, one can have an almost ideal means to play a record.
As with linear mechanical arms, the stylus must provide the force to carry itself across the record. Since there is no friction, the only force the stylus must deal with is the inertia of the carriage (or bearing tube, depending on the design), cartridge, and the arm, (call this total the working mass). These are very small forces because the acceleration of the working mass playing a normal record is very low. It’s just the pitch of the groove. In other schemes, the friction of the system far dominates the force load even with servos. The main negative effect of inertia is the case where the record has an off center hole and warps. In this case, the acceleration operating on the working mass can be quite high. Solution: dump the record. I have done this a few times.
Warps can be tamed using a vacuum turntable such as my Sota Nova.
I don't want to understate the importance of set up. Just like any other first rate arm, set up is critical. The success of the design is largely determined how easily and accurately calibration can be done, and how often the set up is must be done. It is important. The best of the designs do very well with set up.
You might be surprised at the number of air bearing arms that have been marketed over the years-quite a few. Most had problems that defeated the designs. But, a magic few established the standard for analog record playing. There are few air bearing arms on the market now. Analog audio is a small market and air bearing arms are expensive. This makes for a small buying public. I hope some of the new designs prove to be successful. For me, the Eminent Technology ET2.5 is the holy grail of tone arms. I own two.
If you want the best, research and seek out the few used air bearing arms that show up for sale.
Sparky
