Servo Controlled Arm

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.

Actually, not a problem at all . . . there are countless electronic products around you every day that use electro-mechanical servo mechanisms that respond several orders of magnitude faster than the speed of sound. Think about the focus servo on your CD-ROM drive, happily maintaining a lock on a target the size of a few smoke particles, with the disc careening around at 48x speed.

A servo-damping system is basically an electronic implementation of the typical gooey-fluid tonearm damping trough - and likewise can only affect a very limited set of tonearm performance parameters. And similarly, the shortcomings of these servo-dampened tonearms isn't necessarily the servo . . . it's the tonearm.

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.