My 2 cents worth.
IMO a perfect TT needs to satisfy three criteria.
1) Perfect DYNAMIC speed stability. No drive system meets this and passing the (in)famous timeline test is zero guarantee of dynamic speed accuracy, only average speed accuracy.
2) Perfect dynamic dimensional stability. Impossible unfortunately. My view is that a skeletal design makes this goal more difficult to achieve since we are introducing the support structure, shelf, platform, into the equation. It effectively becomes the plinth. Related to this is the minimisation of joints and material changes in the platter-arm loop.
3) Perfect stillness. Again impossible, but careful choice of materials and correct implementation of single point mechanical grounding or isolation, would likely be advantageous. This too suggests that a skeletal design is not optimal.
A heavy, inert, one piece plinth is indicated if you agree with goals 2) and 3).
I will not open Pandora's box with any comments about goal 1).
Propagation speeds of the materials chosen also need consideration. |
Parrotbee.
I think that good inspiration can be found if we look at how cutting lathes are constructed or outside of the industry, we can look at how milling machines are put together. These devices are required to hold a fixture precisely in place relative to the work piece, (record blank or machined material), whilst being dynamically loaded. This is exactly what a TT is asked to do.
No issue at all with a suspension if the siting of the TT dictates. My installation is basically fixed to the concrete floor and mother earth and I believe that this approach yields good results. Of course in most situations this technique cannot be used, so a suspension is a good solution.
Interesting re the SME's, ok you can see their entrails but their is a single structure carrying the platter bearing and the arm. In this respect they are pretty conventional.
However the box that encloses it all is missing. This is likely a good thing.
Damping is a real can of worms. It's use can yield fantastic results or it can suck the life out of the sound faster than a death eater in a Harry Potter Movie.
propagation speeds.
Yes in this case transmission velocity of sound thru the material.
for example: acrylic is around 2700m/s
copper is around 4600m/s
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Parrotbee
I look forward to you posting pics of your creation. Good luck with the project. Also can you advise more details on the microscope isolators?
Atmasphere
Dimensional stability.... You said it far better than I
Thanks |
Some 20 years ago when I was building my current SP10 MK3 plinth, I spent a lot of time experimenting with glues.
I was using CLD on the acrylic top plate. In this case a 15mm layer of lead was to be adhered to the acrylic.
I initially thought that a lossy type glue, such as ados, would be optimal but listening to the test pieces thru a stethoscope while tapping it gave a "thunk" type sound. This was not what I expected as I was trying to emulate the ideal water fall type plot as we see published in speaker tests.
That is a sharp rise time with very little tail to the sound.
I then started experimenting with epoxy glues and settled on an industrial araldite epoxy. This lead to protracted experimentation with harder and filler ratios. I found that I could further reduce the tail of the tapped sound by optimising the hardness of the glue.
The final result gave a very short sharp "tic" when the workpiece was struck.
The result was a kind of fusing the acrylic to the lead. In this way they behave as a intimate composite structure.
Aiding this was massive clamping pressure while the glue dried. Over one tonne of cast iron billets were stacked on top of the plinth during the curing time.
Epoxy heats up when mixed, so the glue becomes quite runny. The vast majority of it oozed out between the layers. Unwilling to let this excess set, which would have required expensive remachining, I spent almost all night until just before dawn removing the excess as it flowed out.
Fun. |
Dgarrestson.
I have removed the motor housing on my MK3 from the original square chassis
See the triangular shaped TT on my krebsupgrade.com web site.
The rather flimsy naked motor housing has been laminated ( epoxy glued and bolted) into a machined duralumin housing of 15mm wall thickness. This in turn is adhered to a 15mm thick lead disc. ( the piece that was cut out of the plinth to accommodate the motor)
In my original post on this thread, I talked about minimizing material changes and joins in the platter arm loop along with consideration of propagation speeds, absolute dimensional stability and stillness.
This leads to the conclusion that the chassis has to go.
I also replaced the platter with an acrylic duralumin lead composite.
Regards |
I mentioned the mythical perfect TT earlier on this thread.
item 2) was " Perfect dynamic dimensional stability"
We have seen on another thread here how stylus drag can slow the platter. This even where massive platters and drive systems are employed.
This drag is pulling on the arm and hence its support. In the case of a nude TT, the pod is being exposed to this force. If this heavily modulated force is sufficient to slow a weighty platter of considerable inertia, would it not also be able to "tilt" a free standing pod even, if it is substantial?
Way back in high school we were tasked with this question..
What happens when you throw a snooker ball such that it hits the front of an oncoming train. The answer is that the train slows, and with relative weights and speeds, we were able to calculated how much.
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Timeltel.
Ha.. I marvel at the little things in analog that make big differences. Then we are trying to accurately measure an exceedingly little thing. I also marvel at the existence of intelligible music via a record given the "complications" that exist.
I would say that without stylus drag you have no music. The force it imposes is significant, observable and varying. It is one factor that must be considered when putting together a TT. |
Halcro.
Ok to answer your criticism's of my posts
Most DD TTs use a synchronous motor with some type of feedback or a non synchronous motor also with feedback.
In each case, if all is properly functioning the motor is COMPELLED to rotate at the correct AVERAGE speed. It depends upon the drive design how it reacts at smaller time increments.
I have witnessed this incremental speed change by proxy on a Goldmnud studio by scoping its power supply. There plain as day was a distorted view of the music that was currently being played. Clearly the PS was not stiff enough but that is not the main point here. There can be only one cause of this modulation of the power supply. The platter is momentarily slowing in sync with the music and the motor/ controller assembly is reacting to this by drawing more current to correct the speed drop. The timeline test is showing that your TT is working correctly but do you honestly believe that its servo has some sort of preview of the upcoming modulation and reacts predictively? Of course not, it REACTS to a slow down and corrects. These errors are happening in real time, but it's average speed is correct. The platters inertia alone is insufficient.
I did these tests 20 years ago and they proved to me, back then, that stylus drag exists.
Then their is the subjective test. A frequent comment from my customers who have had MK3's upgraded is that the speed stability is improved yet the MK3 passes the time line test. I do a lot of work on the speed sensing mechanism in the upgrade so this observation does not surprise me.
We should not need remanding that our hobby is subjective by nature.
I do not understand how the time line test proves that the pod is not moving? All it is measuring is the platter's speed.
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Halcro
Humor me.
Explain how passing the timeline test proves that the arm pod is not moving.
Cheers |
Halcro
Yes I get the arm pod high mass and footprint thing.
Again
How does passing the time line test prove that the arm pod is NOT moving.
Cheers |
Halcro
Let's say, just for fun, we replaced one of your very nicely designed arm pods with a block of foam rubber and we mounted the arm on it.
The foam is just stiff enough to carry the weight of the arm.
On your TT...
This set up will play music
It will pass the timeline test.
Are you saying that this pod will not move?😊
Cheers |
Chris.
I re calculated the train thing.
Assuming the following
Make it a 100gm sticky snow ball traveling directly towards the train at 100 KPH, to make things easier.
Make the train, 1000 tonnes, also traveling at 100 KPH
The snow ball hits and sticks to the front of the train.
The train will slow to 99.99998 KPH, more or less. I accept that I may have an extra or too many 9's
Conservation of momentum.
You did ask!
Re the pivoted arm/speed question. It makes sense, since the arm is moving forward relative to the radial line for some of its travel across the record.
"The song, I got U BABE".
My dreams, shall we say, are usually more exciting in content.
Cheers. |
Chris.
We all have songs lurking in our past that in some way stay with us.
Re the number crunching.
The figure given was for a perfectly inelastic collision. If the collision was perfectly elastic the new train speed would be around 99.99996 KPH. 0.00004% speed reduction.
It just goes to show that little things can influence big things, even if they are 10 million times lighter.
cheers. |
While waiting for Halcro to answer Lew's question in his post of 02-13-15, I thought it might to be interesting to look at the pod movement thing from another angle.
To me it is obvious that it will move due to Stylus drag. The question is how much.
In order to calculate this I needed a figure for stylus drag. A search on the web proved inconclusive but then ironically the answer came from the original timeline thread. There, one TT is mentioned where specific data is given on the amount of laser pointer movement per revolution and its distance from the centre spindle.
This TT is a beautifully engineered machine with, from memory a 22 kg platter driven by a fractional horse power motor via a thread. Hereafter I will call this TT. "TD"
The specifics were 2 mm movement on a distance of 400 mm per revolution.
With this information it is possible to calculate the retardation torque and hence the drag. From this it is possible to calculate how much the pod moves.
So assumptions......
A pod identical to Halcro's is used on TD
Platter 22 kg of uniform section
Pod plus tone arm 11.5 kg
Height to record surface above mounting surface 150 mm
Pod feet 100 mm spacing in a equilateral triangle
Pod/ arm CofG, 75 mm above mounting surface
Pod CofG Central inside the mounting feet
Pod feet are not adhered to the mounting surface. ( no penetration of the cones into the shelf )
The same arm and cartridge used on TD is used on the pod.
TDs motor only provides enough torque to maintain original speed before stylus is lowered, after it is lowered.
Stylus is lowered at a radius of 140mm
Platter has a diameter of 320mm
The first answer is the force applied to the platter to cause this retardation torque. This works out at 0.0031 newtons. Actually a large number under the circumstances but it is slowing a 22 kg platter!
Using this force and applying it at a height of 150 mm to the pod we get a tilting of 8.2 microns towards the platter.
Observations.
With the stylus at a radius of 140 mm, the two front feet of the pod do not appear to be a right angles to the arm when viewed from above. This will reduce the tilt a little. It is unknown what happens to the magnitude of the stylus drag as the arm tracks towards the centre, so it is possible that the pod will tilt as calculated once the arm is at right angles to the feet assuming that this happens before the end of the inner grooves.
As an aside the tilting at a radius of 140 mm produces a yawing effect on the pod such that the arm rotates approximately along its axis. This effect is caused by the configuration of the feet. It is tiny and likely insignificant.
The calculations assume that the platter motor assembly cannot move. It can and will, but much less than the pod becaue of its wide footprint.
The calculations assume that TDs motor does not sense the slow down and produce restorative torque. Since it is a synchronous motor it will act to try and maintain speed. This will put more energy into the system and increase the tilt.
The pods appear to be slightly crescent shaped. If this is the case the CofG will be biased towards the two feet closest to the platter. This will increase the tilt.
If TDs arm and cartridge was fitted to a pod and used on Halcro's TT, things would be different again. This because the TT-101 does NOT slow down. It is putting more energy into the system, so the tilt would be larger.
The calculations assume that TDs platter has a uniform section. If its radius of gyration is larger or smaller than this suggests, the tilt will similarly be larger or smaller.
Actual dimensions and weight of the pod will materially change these numbers.
I do not know if the amount of movement is of any significance but for sure it moves. This compromises one of the three ideals I mentioned featured in the mythical perfect TT. This was my starting point in these discussions. |
Parrotbee.
Really look forward to seeing pics of your TT creation.
There are so many ways to build a TT, all with virtues and compromises. The art, I guess, is maximising the virtues and minimising the compromises. |
