Servo Controlled Arm

Dan_ed...If groove spacing changes abruptly there will be a momentary change in tracking angle WITHIN THE SPEC RANGE OF 0.5 DEGREES. (Remember, the arm pivots). A properly designed servo does not overshoot or hunt. Digital servos make it particularly easy to implement the necessary algorithms, which may be difficult or impossible to do with analog components.

Again, why do it at all? I would file this under "gimmick". That's all I'm saying.

IMHO, of course. :-)

Why do it at all? Because a conventional 8.5" or 9" tonearm can have a tracking error up to 3 degrees compared to the linear arm error of less than 0.5 degree. So the "gimmick" would be to lower harmonic distortion. As for the comment on servo damping, dan_ed makes it sound more complicated than it is. The tonearm, cart/stylus, LP groove form a sytem which will resonate at one particular frequency which is variable. Detection of resonance is easy and almost instantaneous due to a very large increase in amplitude. With the servo detecting and changiing some variable, it keeps the sytem away from the resonance point. Theoretically, it should produce cleaner sound.

Rotarius...I think you are talking about the vertical "Biotracing" servo, which is quite separate from the linear tracking arm. Sony made pivoting Biotracer arms also.

By the way, I made a typo in my last posting. The spec tracking angle error is 0.05 degrees, not 0.5.

The Biotracer arm tracks very warped LPs with not the slightest problem. (That's why). If you lift the arm manually is has a strange "limp noodle" feel to it. The downforce remains constant dispite movement of the arm up and down as with a warped record. The downforce, VTF, is applied electronicly, so it can be tweeked while a record is playing. The arm itself is mass balanced (no downforce) and this minimizes sensitivity to vibration.