Servo Controlled Arm

Heh! If avoiding skipping were the only criterion for tonearm performance, virtually every tonearm would be a contender. ;-)

The theoretical problem with servo control of a tangential tracking arm is that, by definition, correction doesn't kick in until the arm is OUT of tangency. Also, depending on the design, the drive may then scoot the arm right past the tangency point until it's off in the opposite direction.

How far out of tangency any particular servo-driven arm gets before correcting, and how far it overshoots, depends on design and build quality. But any servo controlled arm must always be hunting for (and may rarely find) the true tangency point.

Throw in variable groove spacing and off-center LPs that can send the tangency point rapidly forward or even backwards, and true tangency may be an infrequent event at best. A good pivoting arm may actually cope with these challenges better.

The inherent impossibility of maintaining tangency theoretically limits servo designs to less than high end performance levels. Whether any non-tangencies will actually be audible depends on the resolution of the system and the ears of the listener, so YMMV - as usual.

This was one of the stupidest ideas of all time, winner of the uncoveted GOLDEN TURKEY award from HIFI NEWS. Instead of letting the grove pull the arm across the record it drove the cartridge across it. If it happened not to be in alignment presto, a new groove. As HFN put it, a solution for which no problem exists. Bear in mind that these were , at best, mid priced tables and were not , even then, using the last word in technology. They sounded , well, to be kind, I won't say. Ranks right up with the Yamaha "EAR SPEAKER" and other marketing derived ideas.

Guys, you are barking at the wrong tree here. The JVC QL-Y3F turntable is NOT a tangential linear tracking arm. It is a pivot arm with what is called an ED(electro-dynamic)Servo arm to control damping and behavior of arm movement in using electronic means.

vinylengine.com/library/jvc/ql-y3f(DOT)shtml

Here's a blurb from The Vintage Knob site: thevintageknob.org/VICTOR/QLY7/QLY7(DOT)html

"The goal was to eliminate as much as possible the ED Servo (Electro-Dynamic) tonearm's own resonances and stray vibrations : there is no bearing (per se) but two motors which drive the arm's movements vertically and horizontally."

"Tracking force, anti-skating and Q Damping can therefore be operated from elsewhere than the tonearm's base : the minutious mechnical controls included in the tonearm of the QL-F6 were scrapped for "micro-computer" control - a very cool word in 1980!"

"VTA is adjustable with a lock on the non-electronic side of the tonearm."

JVC was an innovative company and their turntables are excellent quality. Their more conventional tonearms that came with their QL series turntables are true sleepers.

By the way, I am not a fan of linear tracking servo arm either but they still have merits even if they can't keep tangency through out but the elimination of anti-skating force and it has its sonic merit and the evenness and consistency of sound through out the entire record side is not to be sneered at. Of course you can argue it still exhibits miniscule force. Playing a record in the real world situation very often negates absolute tangency so any idea or innovation to help the needle navigates this dynamic journey is welcome.

But back to Drewmbl's question as to why don't we see more servo arms in the way JVC implement, I don't have an answer. But I suspect cost and complication is the reason. JVC was a big corporation that could afford to spend millions on R&D on such technology but certainly not for the now niche market of tonearm, which is a cottage industry now. A lot of Japanese turntables got criticized because because they are direct drive and many features automatic mechanism which to the audiophile community that's losing "street cred" right there. Many never even bother to listen to some of these gems.

One thing I don't understand is that why can't more people come up with a passive tangent arm, not the servo kind, without using the damn air-bearing. I know Clearaudio is the lone manufacturer here but surely there's gotta be another way of dragging the needle across the record without using a pivot arm. I saw an experimental turntable once that uses water to flow the arm as bearing. Hmm....

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.