Direct drive vs belt vs rim vs idler arm

I was wondering if someone with two tonearms can do a simple test with the Feikert Android/iphone app. Play the test tone with the first tonearm and begin recording. After some data has been collected, drop the second stylus onto the record and see what happens to the tone. If the app is sensitive enough, this might show how much the platter slows due to the additional stylus pressure, how quickly it recovers, and how much it overshoots. It would be really interesting to do the exact same test on a number of different TTs.

Dover. I tried to get out of this thread by offering an olive branch since I sense that we are going nowhere.... oh well.
The argument that servos cause overshoot followed by a period of slowing and then repeat does not hold up to analysis. This pitch was likely put out into the market by BD manufacturers and it has taken root in the collective thoughts of the audio community.
We use servo control here almost daily on small and large machines. If they behaved as you describe the machine performance would be totally unacceptable in some cases destructive and dangerous. Servos are not fully on, fully off devices. They have response curves, gain, ramp rising and falling, dead band and frequency responce adjustments. These parameters are talored to the task. We tune then for this. Properly implemented they do not overshoot and as we apply then here they achieve a staggering level of accuracy. The same applies to DD and ironically this is proven by the scope tests I did on the Goldmund. You will recall I could see the music being played at the time on the scope, even treble information. If the servo was correcting, over shooting, correcting undershooting..etc, what I would have seen would be a series of square wave like pulses with little relation to the music as the platter acceleration/ decelleration time constant would smother the individual current draw/music waveform. (I have said before. A correct match of motor capability, platter inertia and controller) Say what you will about the Goldmund, but one feature it has is a very mechanically stable speed measuring system. This along with the servo iteslf was doing its job correctly. ( lets keep physical motor cogging out of this)

I hesitate to use the "spurious" word again, but while loop rigity, energy dissipation etc are topics dear to my heart, as you well know, they are spurious to the discussion on platter speed stability.

Wow and Flutter, stylus drag speed change. Yes I knew the moment I pressed submitt that this would draw a response. I will concede that they are both troublesome. That said as per my earlier post, stylus drag induced speed changes are below the threshold of measurement with the SP10 MK3 and most likely many other DD tt's. On the other hand the time line is ineffective in measuring your type B stylus drag because, by its very nature, its effect is transient and the time line is measuring an average.

Agree, quality power supplies are critical to the correct operation of....just about everything in our hobby.

Ketchup. I like your idea of using two tomearms. I think that the test disc is 10 inch diameter, havent seen one. So could it be placed on top of a LP? Use one arm to measure frequency and use the second arm to play music on the outer track of the LP. That would be intersting.

Richardkrebs
I seemed to have missed the olive branch. Was it before or after "end of story"and "for the last time".
I have continued the discussion as you have misconstrued some of my comments and not fully addressed some of my queries.
I cannot understand how the servo/speed correction system knows whether you are going to play Mahlers 2nd Symphony or a Beethoven Sonata.
You seem to be certain that there is no measurable stylus drag on the DD by measuring the power supply. This simply doesn't quantify the the stylus lag in real time that is occurring at the stylus tip.
The comments on loop rigidity and energy dissipation were put forward in the context of maintaining the attack, intensity and decay of each note. You have overlooked the fact that even if you had perfect speed stability, the attack, intensity and decay of each note can be distorted by an inadequate plinth that is not rigid and doesn't deal with the energy reflected into the platter.
It is of no consequence to me really, but you assumed I measured the variation in stylus lag using the Timeline. That assumption is not correct.
Finally, I am trying to understand how Direct Drives address these issues, but you have offered no explanation for the differences I heard and described between the Goldmund and the Kenwood L07D, particularly in speed, timing and coherency.

Dear Halcro, what an excellent job you did testing your turntables. Your dedication to the hobby is impressive. It looks like the DD table actually increased speed just a bit with the 2nd tonearm while the belt drive dropped by about the same amount. The magnitude of change being just at or under 0.1%. If I were to guess, it looks like you dropped the 2nd tonearm at around the 10 second mark? And on the Raven at about the 11 second mark? Notice that the speed on the DD recovers after 2 rotations, but the belt drive speed remains about 0.1% lower. As for being able to hear that differnce in terms of pace/rhythm, I don't know. Keep in mind too that this is an extreme test beyond any music- dropping a 2nd tonearm onto a platter.