Direct drive vs belt vs rim vs idler arm

Spirit, Have you tested the Salvation TT speed stability with Timeline?

I agree with SYSKOS, that Timeline results of members' various TTs would be a very useful grade scale (pass or fail).

For a Timeline test to be truly useful, some standard should be applied. For example, I saw a DD table tested with the Timeline. The owner held a piece of paper behind the turntable and demonstrated that the red dot did not drift during the 20 second test. I asked him what would happen if he let the dot fall on the wall about 4-5 feet away and waited five minutes. He admitted that a further distance would show a drifting over time. A video with a set distance of the laser dot to a wall, say 5 feet away during a period of ten minutes, or with the stylus at the beginning, middle and end of an LP would be good, but I don't want to have to watch a 20 minute video showing the entire length of the LP. Perhaps a fast forward feature with a time stamp and mark on the wall for some kind of consistency would work. Also the test should start with the stylus not contacting the LP and then during the test, it should be lowered onto the LP.

I agree that the results would be interesting, though I really wonder if the speed variation of a successful KAB strobe result versus an unsuccessful Timeline result would be audible. My table shows a steady result with the KAB and I have not had the opportunity to borrow a Timeline.

I would think that any manufacturer who has a design which passes a Timeline test would want to show such a video for marketing purposes. This issue came up in another thread about speed accuracy and someone asked about the accuracy of the Timeline device itself. If the strobe is slightly off, but the table is accurate, the red dot would also drift slightly. Measurements are only as accurate as the measuring devices themselves.

The timeline has a quartz clock. It is accurate, according to Ron Sutherland, of two parts per million. That's very good, and I have no reason to doubt him.

I don't know the limits for other drive types, but it is theoretically possible for an idler to be accurate to around one part per million, so my controller was designed for that. You could go further with a GPS referenced OCXO clock in the controller, but could anyone hear it, and could the device keep up with the clock?

At some point we have to question the merit of it all because there are mechanical limits. And, just because a turntable has a great clock doesn't mean the turntable spins accurately, and just because it spins perfectly in the lab doesn't mean it will do it under every condition, in every environment. Still, I believe it is the responsibility of a manufacturer to get as close to perfect as he can given the price point of the machine he is selling. If you pay small money, you aren't going to get super accuracy, but you may get a very musical turntable that is consistent. To me, that's worth a great deal, although I personally try to push the limit to its threshold. Essentially, you should get what you paid for.

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I made an error in my last post. The Timeline is accurate to one part in two million.

Gentlepeople
I have not used the timeline but understand that if a TT was to say AVERAGE 33 1/3 rpm over a LP side, it would pass the timeline test. Am I correct in my assumption?
If so we can easily discount any TTs that are not averaging the correct speed. But this test does not take into account dynamic speed changes.
With most DD TT's using a DC motor, a controller produces a rotating field at the desired speed. The magnetic poles are pulled around at this speed. Contrary to some opinions expressed in these forums, the servo does not intervene unless an external load causes a speed change. ( this assumes that the TT has been put together as the designer intended). If the load does change, the magnetic poles will move back in phase slightly until output torque matches the new higher load. The magnetic poles will still be following the rotating field so it will be again rotating at the correct speed. But while this is happening the motor tends to self correct as its torque increases with the phase shift between the rotating field and the magnetic poles. AC motors exhibit the same self correcting characteristic. For a DD TT to fail this average test it would have to be incorrectly designed.
A good string drive with zero slip and idler drive would I imagine also pass this average speed test.The only way for these to fail would be for the motor to be under such high load that it jumps a pole. No normal retardation torque would be sufficient to cause this.
But here is the tough one, the motor does slip back in phase due to stylus drag without jumping a pole.Even at treble frequencies with one DD TT I have tested. This can be seen by scoping its power supply. And watching the current draw. It was actually possible to recognize the music being played at the time on the scope. The speed sensor was measuring these tiny errors and correcting. This could be observed by watching the motor draw current in sync with the music. An amazing observation.
The conclusion is that some form of speed correction is required if we are to seek good dynamic speed stability. High moment of inertia platters will of course help but only for momentary load changes. Sustained increases in load will cause the same phase shift. If no speed correction is employed, we are relying on the motor self correction characteristic. This may be enough but note the motor needs to move back in phase, I.e momentarily slow down before the automatic increase in torque output comes into play. As with all things engineered, compromises are a sad fact. We each need to decide what compromise best meets our biases.
Some of you may have been asked the thought experiment at college where a tennis ball was thrown directly at the front of an oncoming train.
The train in theory slows down.