Video of Technics SL1200 - looks to be running too fast. The "33" keeps lurching forward and then slowing, repeatedly. It is non linear, like it's hunting or slewing around. Watch closely.
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Gentlepeople. Isn't this an interesting thread. Ct 0517, that was a great video. Thank you for sharing it with us. Good to see what a well engineered closed loop speed control can achieve. It was also interesting how fast the high inertia platter slowed in the translator clip. Tonywinsc has put very clearly the concept of inertia. In several of my posts I used the terms "radius of gyration" and "moment of inertia". I apologyse for not making clear what I meant by this. Radius of gyration is the distance where the mass of a rotating body appears to be concentrated out from axis of rotation. In the. 2001 space example this is almost out at the circumference on the station. Most DD TTs engineer the radius of gyration to be at some mid point out from the axis. In Dover's Final TT and most other belt drives or their derivatives, it is engineered to be further out. By calculating the radius of gyration and them knowing the weight of the platter we can calculate its moment of inertia which Tonywinsc clearly describes. I have the moment of inertia figure for the SP10 MK3. This is 1,100 kg/ cm. In other words in a rotational sense, the platter behaves as if it weighed 1,100kg with all of this weight at a radius of 1 cm. this is quite high for a DD. Good BD TT's as implied above will have higher moments of inertia than this. This is I think what Dover meant when he referred to "effective mass" . The key here is that for a closed loop drive, the motor torque capability, servo response and platter moment of inertia all need to be in harmony. The video the Ct0517 posted clearly shows the positive effect of this. Dev, you have asked for a list of TT's that approach the 3 rules. We could all build such a list. It is quite simple if you examine the TT under scrutiny. Remember this is a list of rules that would apply to TT that is impossible to build. Rule 2) is a catch all as it requires absolute dimensional stability between platter and arm board. It actually covers almost all of the requirements of a plinth design. This eliminates all TT,s that have, say, soft thrust pads in their bearings ( a trick employed to improve rumble figures )...allow any flex or bending of the plinth, have a soft Matt, excessive bearing clearance etc. The examples of breaking rule. 2 are many. Rule 1) has been covered in this thread quite well Rule 3) opens another can of worms. To suspend or ground. A tricky one as it depends upon where the TT is sighted. We also need to look outside out hobby field for examples of engineering where similar design criteria are required. High powered microscopes. Large telescopes. Aeroplane propeller balancing tables, are all areas where rules 2 an 3 come into play. My fear is that today, basic engineering is being ingored in favor of fashion. If we look back at the.. 70s and 80s we see fantastic examples from companies . Micro Seiki, Final, Technics, Sony, Luxman, Onkyo,Victor, et el, producing flagship models that tried to address at least some of these requirements. It is mouth watering to think of what the engineers involved in these designs could come up with today if they were given the R&D dollars. We live in ( naive) hope. |
Richardkrebs Can you please review the video again. From 0-24 seconds you can read the 33 quite clearly, but the 33 is moving forward and back, it is not stable. From about 57 seconds on - he now has a constant load on the platter. 33 is unreadable, but it looks to me as though the surging has ceased. The conclusions I see from the video are : Unloaded, it is sort of speed stable but "hunting". Can you not see this ? Loaded - appears to me to suggest this deck is not accurate with a load ( blurred 33 ) but ironically the long term speed variation seems to have improved with the finger load on it ( less drift ). To be fair to the deck, the load on it probably takes the deck outside of its error correction operating parameters. |
OK just for fun here's something to try on a Linn/Oracle/Clearaudio...take your pick http://www.youtube.com/watch?v=lwugFlbCOww I've recorded the KAB speed strobe on my Final Audio Parthenon VTT1 thread drive TT while I bash the record with my knuckle. Anyone else want to have a go - I used a Carol Kidd record.... from Linn. |
I agree with Dover. I watched Ct057's video, blew it up to full screen size, place the point of my cursor on the "33" and it is clearly lurching forward. However, I notice some lurching on my KAB disk which is a result of the disk not being perfectly flat on the LP surface. The warp, if you will, effects the movement of the "33" precisely the same once per revolution. So some of this observed lurching is from the imperfection of the KAB disk itself. But the lurching that is in sync with the finger tapping in the video seems to be the result of the platter motion and motor correction. What turntable is in the video? |
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