Vintage DD turntables. Are we living dangerously?

07-13-15: Richardkrebs
Fleib.
To add a little more to your question. I took a lot of notes when upgrading a customers LO7D. One surprising observation is that the motor stator is rubber mounted via grommets. Such that it is possible for the stator to twist a little backwards in an anticlockwise direction when applying torque. This would seem to be counterintuitive but it is there in the design.

I don't know about the 3% servo thing, but it has been quoted many times in various posts. If this is how it is built, it would be a reasonable assumption that it is effectively open loop once up to speed. In this way it is relying on the synchronous motor's innate speed accuracy and the platters inertia to maintain the correct RPM.

Richardkrebs,
It is staggering that you would be performing turntable modifications without a thorough understanding how the TT works. It is fanciful to speculate that the Kenwood engineers designed "slippage" when torque is applied by using rubber grommets. It is more likely that they employed the grommets to reduce mechanical and/or electric noise within the suspended board and coils.

To help you understand the L07D servo operation, here is a link to the owners manual - http://www.vinylengine.com/library/kenwood/l-07d.shtml

Page 4 gives you an overview - the L07D uses a dual mode speed control system - if the speed error is below 3% the phase is controlled with a wide lock range and large phase gain. If the speed error exceeds 3% the servo controls speed rather than phase, and applies higher torque. When the servo mode switches from phase to speed control, the coupling changes from DC to AC to minimise influences from the motor drive circuit and motor offset.

Clearly the L07D relies more on platter inertia and phase locking for coping with stylus drag unless the speed error is very high. This is quite a different design to the Technics SP10 solution which relies on a much more aggressive servo speed control system with the DJ market in mind – the ability to start and stop on a dime was more important to the Technics engineers than the Kenwood engineers. I have listened to many L07D’s and SP10mk3’s at length and in my view the difference in servo design and implementation forms a significant difference in presentation, particularly in timing and coherency.

Dover, I've got a fully refurbished L07D sitting right next to a Krebs-modified SP10 Mk3 which I bought NOS (before replacing all electrolytics and having Bill Thalmann perform the Krebs mod). These are easily the two best turntables I have ever heard in my system, yet they sound very subtly different. Before the Krebs mod was performed on the Mk3, I would say the difference between the two was greater than it is now, in favor of the L07D. But of course, there are more differences between the two than those having to do with the drive system: The tonearms, tonearm wire, and phono cartridges are all different as well. They both feed into the same Atma-sphere MP1 phono stage. But it's fair to say that prior to the Krebs mods, I tended to favor the L07D (with an EMI/RFI shield installed between motor and underside of platter). The Krebs mod keeps the virtues and advantages of the Mk3, absolute firm sense of pace, while ameliorating the rather "clinical" nature of its sound, as compared to the L07D, which might in fact err on the side of romantic but intensely "musical". (I distrust that word, too.) These days, I could live with either, happily, but might now give the edge to Mk3. The Krebs mod is transformative.

Richard, I had reason recently to disassemble my L07D motor and then put it back together as well. I do not recall seeing any rubber grommets. What I did see were some nylon grommets and some brass grommets, neither of which afford much elasticity. Is it possible that the motor you looked into had been "messed with" by someone, some time in its past? If you look at the service manual, and if you consult Howard Stearn, the L07D guru who lives in Virginia, you would get confirmation that the grommets should be nylon or brass. Howard talked me through the rebuild process. My memory may be playing tricks on me re the rubber grommets, but I believe this is the case.

Fleib, For all I know, Pierre Lurne' is a genius when it comes to platter design, but I am no fan of any of the turntables with which he was associated. The ones I've heard (Audiomeca, Goldumund Studio, etc) all "suffer" I think both from speed irregularity and an overly spring-y suspension. They are kind of a yin to the yang of direct-drive turntables.

Dover,

"with the DJ market in mind". I trust you were referring to radio station DJs, which were a market for early Technics DD tables, not dance club scratch/mobil DJs. I've read many false assertions the Technics SL-1200s were designed for dance club DJs. I believe the SP-10 and SL-1200 series were designed in the '70s, a time before dance club DJs evolved.

I don't mean to sound picky here but I hate to see the DJ misrepresentation continued for any newbie readers here who may not be familiar with DD design and development.

Lew
The rubber grommets look original.
I will PM you a pic of the stator to compare with yours.
Inserted in the centre of the grommets is a small metal tube stand off to prevent the grommet from being crushed when the bolt is tightened.
There are also a couple of nylon? stand offs to prevent the rotor from contacting the stator windings when the platter isn't in place ( partial mag lev feature)
Anyway, slight stator rotation is permitted by this arrangement and if original it is an interesting design decision.

Lew,
When Lurne was working under contract for Goldmund I imagine there were constraints. The T5 linear arm wasn't held in the highest regard and the DD suspensions were problematic with any arm substitution. Many users including myself defeated the suspension. The Studio came with alternate springs, but most users were clueless about how to substitute.

This is the first I've read of problems with speed stability. These are quartz locked with a JVC motor. Perhaps you're referring to the earlier Pabst motor, but the ones I've heard didn't seem to have speed problems if everything was functional. The Reference was servo controlled belt drive.

Lurne is probably the most copied TT designer in audio. Not sure which Audiomeca you're referring to, but take a look at this design including the suspension:
http://www.tnt-audio.com/sorgenti/belladonna2_e.html

Regards,