Servo Controlled Arm

The JVC turntable's tonearm of which you speak is pivoted, and probably more accurately described as "servo-dampened" . . . it's a completely different concept from a servo-controlled linear-tracking turntable to which Dougdeacon refers. The Sony Biotracer is probably the most famous of these machines, but IIRC Denon, JVC, and others made variants as well.

The whole idea is that by using some form of motion sensor and actuator on the vertical and/or horizontal tonearm pivots, the resonance and damping characteristics of the tonearm can be altered by changing the electrical response of a feedback loop between said sensor and actuator. The main shortcoming these systems is that the resonance mechanism of the cartridge mass and the tonearm/headshell/bearing flexibility is completely out of the feedback loop - so even if the servo system is perfect, it cannot compensate for this main resonance mechanism.

I have set up and measured a handful of these types of turntables . . . and when adjusting the electronic damping system, it's easy to make a radical change in the way the tonearm feels when hand-cueing. But I've never seen it have much if any effect on the measured resonance of the cartridge/tonearm combination, in either the peak amplitude or the frequency.

Now with regards to a linear-tracking system that uses a pivoting tonearm on a servo-controlled "sled" . . . this is a system that can indeed work brilliantly or poorly, depending on its design and execution. Dougdeacon correctly points out the usual audiophile objection - that "true tangency" isn't maintained at all times. But the actual possible tracking error of a system is easy to measure -- it's simply the relationship between the groove runout in the LP, the servo gain and sledge speed, and some basic trigonometry.

If you perform these measurements on a well designed servo-sled system (I suggest Beogram 4000, 8000, et. al), and compare the results to the tracking error of the Baerwald/Loefgren geometries . . . you get a different picture. And it does a great job avoiding the difference in horizontal-vs-vertical resonance envelopes inherent in most air-bearing linear tonearm designs.

Kirkus, thank you for your insightful comment. Very informative on the shortcomings of these servo arms regarding the cartridge mass interacting with the arm mechanism.

You are also correct on the linear tracking servo arms like the Rabco and Goldmund. I used to object to such design for the same reasons the above members mentioned but I have since revised my thoughts on them. Doing away with anti-skating and tracking consistency across the entire side of a record and less stress and wear on the cartridge cantilever are all positive features to me. After all Goldmund is planning to release their $30,000 Reference 2 table with a linear tracking servo arm, T-8, a T-3 arm on steroid.

Sorry to the OP for my generic response, not related to his arm. I read "servo controlled" and assumed we were discussing a linear tracker. "Servo dampened" would have clued me in to googling first. Thanks to Hiho and Kirkus for addressing the real question.

I also wonder how an active servo mechanism could respond effectively to the varying resonance damping characteristics of different cartridge suspensions. Every adjustment it would make would be after the event.

A servo could be adjusted to control horizontal and lateral resonance frequencies of the whole arm, but that doesn't address the real challenge in cartridge/tonearm interaction - controlling internal resonances that feed back into the cartridge and alter the signal it generates.

The internal energies in a cartridge/tonearm occur at all sonic frequencies and with constantly varying amplitudes, all at the speed of sound. Quite a challenge for an active system to deal with.

I have my suspicions about this approach. The easiest way to "break" this concept is to put on an LP that is not perfectly centered. Each time the eccentricity comes around the stylus gets slammed from one side to the other because the servo cannot react properly. Now it may very well play perfectly pressed LPs beyond amazement, but the design does seem to limit what LPs could be enjoyed.

To all who have responded,

Thank you for the time and education. So much to learn....

Sincerely,