Modified mechanical anti-skate

Re: Modified mechanical anti-skate

Postby Brf » Fri Mar 22, 2019 12:27 pm

Hi Voltan, you are correct, the VPI anti-skate mechanism does not transfer torque in a linear manner. Conceptually, it made sense to me but when I pulled out the calculator, it proved me wrong.

I used a very simple calculation to illustrate the torque curve. I simply calculated the Force perpendicular to the arm (centre of mass) Fp=Fg x sin(degree)

I will argue that the VPI’s mechanical a/s does "act" in a linear “manner” as illustrated by the below graph but it is in fact not linear by definition.

Sorry for derailing the conversation. Nice work BTW.
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Re: Modified mechanical anti-skate

Postby Votan » Fri Mar 22, 2019 2:25 pm

Brf, please accept my heartfelt thanks.
Honestly, I fully respect and admire your invaluable contribution to this forum, from which I have been taught too much, so the last thing I would like to do is to oppose your statements, knowing that for most members of this forum (including me) this is tantamount to blasphemy. So, your last post greatly relieves me and makes me hope that in the near future we (I mean all the restless minds of the forum) could be able to explore deeper much more relevant issues.
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Re: Modified mechanical anti-skate

Postby Golear » Sun Mar 24, 2019 2:15 pm

I think I wrote in one of my earliest posts that we apply the "cosine rule". It could be a sine or cosine rule depending on which angle we measure (e.g. with respect to vertical axis or horizontal axis). But it's the same curve. So the force varies via a curve rather than a purely linear fashion, as Brf has noted.

My analysis differs from Brf's in a couple of areas: I believe we need minimal mass on the upper arm, and that the thread should be against the hub.

Brf, we need to have an x axis on the graph, that shows how the force varies with the stylus distance from spindle.

It will take hours to read through this thread. Great for entertainment. But there's also an Easy button: just read my earliest posts and try it out for yourself. We need to embrace the spirit of experimentation! We're analog audiophiles, after all!
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Re: Modified mechanical anti-skate

Postby Votan » Sun Mar 24, 2019 3:30 pm

The curve presented in the above post of Brf, is of course for an a/s mechanism with bare poles, with no rubber O-rings or brass washers, etc., so the curve of a/s force tops in the point where lower pole reaches 07:30’ o’clock and then it goes degreasing. But If you start loading the lower pole, then the curve of a/s torque is keeping increasing further before tops. Namely:
As I have explained before, the torque T presented in the tables of my previous posts is the torque created from the a/s mechanism with reference to the pivot of its hub, but this is not the anti-skating torque Ta/s that finally is applied in the tonearm via fishing line.
For my 3DR-10’’ the relevant diagrams of the final Torque Ta/s in the arm for 2 different loadings and for: (a) Not truncated a/s mechanism, and (b) for truncated a/s mechanism are presented below:
A-S Torque in the Arm from Not Truncated a-s mechanism (2).jpg
A-S Torque in the Arm from Not Truncated a-s mechanism (2).jpg (19.99 KiB) Viewed 3029 times

A-S Torque in the Arm from Truncated a-s mechanism.jpg
A-S Torque in the Arm from Truncated a-s mechanism.jpg (19.42 KiB) Viewed 3029 times


Viewing those diagrams we can draw easily the following conclusions during engaging our a/s mechanism, in layman’s terms:
1. A/s force (torque) provided to the arm from the a/s mechanism as it is (not truncated), is not continuously increasing during the movement of the lower pole from 06:00 o’clock up to 09:00; o’clock. I take 06:00 o'clock as a start (though a/s starts increasing earlier as Brf described) because, at list in my setup, a/s mechanism is restricted from the junction box to start earlier (say from 05:00' o'clock).
2. Depending on the loading of not truncated a/s mechanism, (number of rubber O rings and/or brass washers, etc.) a/s force starts increasing from 06:00’ o’clock and tops somewhere between 08:00’ ~08:30’ (depending on the loading, the higher the loading, the higher the top) and then it goes degreasing. This top should be a bit earlier for 9’’ arms and a bit later for 12’’ arms.
3. The above mentioned increase of the a/s force is not linear, but its gradient is decreasing until it tops in the points above mentioned.
4. If you make your higher pole truncated you can move the a/s curve’s toping (depending on the loading) above to 08:30 up to 09:00’ o’clock of the lower pole.
5. If a not truncated a/s mechanism is engaged such a way that its lower pole is in 06:00’ ’o’clock the moment that the arm tracks in the lead in grooves, you can give to the arm a continuously increasing a/s force up to approximately 07:45’ ~ 08:30’ o’clock (of the lower pole) depending on its loading (the more loading, the higher the top). If you do that, then because most LP records have the lead out grooves before lower pole reaches 07:30’ o’clock, you are in the increasing area of a/s all the time.
6. If you make your higher pole truncated, then you must load your lower pole a lot more in order to achieve the approximately similar a/s force to the not truncated one.
7. If in the above toque created from the a/s mechanism, we add the inevitable a/s force from lemo wire, which is continuously increasing, then we can have even better results concerning the toping of the a/s torque.
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Re: Modified mechanical anti-skate

Postby Brf » Mon Mar 25, 2019 9:11 am

Votan, there is nothing that you have wrote above that I disagree with.

For those who are more visual inclined than analytical, there is a very simple way to determine the a/s pole location at which point rotational torque a) starts (a/s engagement) and b) begins to decrease. The a/s path between point “a” and point “b” can defined as the usable/desired range.

1) Add rubber donuts, washers or whatever to the poles in any configuration.
2) Allow the loaded a/s device to freely rotate around its axis. This will align the a/s center of mass directly below the pivot axis.
3) Take note of the poles location, as this will establish point “a” i.e. the a/s engagement location. Point “b” is defined as 90 degrees from this position.
4) When the tonearm is positioned in the lead in groove, the fishing line should start to engage the a/s device at point “a” and when the tonearm reaches the run out groove, the a/s poles should not rotate more than 90 degree from the initial engagement point.

It should be noted that is okay to have the a/s engage after point "a" when the tonearm is in the lead in grove, but the a/s assembly should note rotate more than 90 degrees (from point "a") when it reaches the run out groove.
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Re: Modified mechanical anti-skate

Postby Golear » Mon Mar 25, 2019 12:50 pm

Well said Brf.

Ideally, the range for the anti-skate device should be 90 degrees of rotation. However, because of where the anti-skate thread is attached to the tonearm, the actual range of motion of the anti-skate device, as the tonearm moves across the LP, is about 115 degrees. (This is for the 10 inch arm - there may be a difference with longer or shorter arms.)

If the thread could be attached closer to the pivot, then it may be possible to set the range of motion of the anti-skate device to 90 degrees. This is the key: When the LP moves the arm from outer to inner position, that range of motion needs to correspond to a 90 degree range of motion for the mechanical anti-skate device.

(But the anti-skate thread can't be any closer to the pivot because of the unipivot housing. Perhaps it can be with the gimbal arm. I don't know.)

With the rubber O rings in the default position, the center of mass is quite far past the horizontal plane when the tonearm is in the inner grooves. This results in an appreciable fall in anti-skate force just when you need it most. You might hear it as extra "air" in the high frequencies, but it is mistracking, and this damages the LP. It actually affects all frequencies. You'll hear the difference when you fix it.

Reducing the mass on the upper arm of the anti-skate mechanism will change the location of the center of mass. The center of mass will be closer to the lower arm, instead of midway between the upper and lower arms. So now, when the tonearm is in the inner grooves, the center of mass will closer to the horizontal plane, and this allows the mechanism to apply the correct level of anti-skate. The center of mass will still have gone a little over the horizontal plane, so there will be a small drop off from the max value. But it's ok. I estimate that there's still close to 85% of the max force. (This assumes the thread is also lengthened by a bit, as I have indicated.)
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Re: Modified mechanical anti-skate

Postby Votan » Mon Mar 25, 2019 3:52 pm

Golear wrote:
Ideally, the range for the anti-skate device should be 90 degrees of rotation. However, because of where the anti-skate thread is attached to the tonearm, the actual range of motion of the anti-skate device, as the tonearm moves across the LP, is about 115 degrees. (This is for the 10 inch arm - there may be a difference with longer or shorter arms.)

In my 10” tonearm, the actual range of motion of the anti-skate mechanism, as the tonearm moves across the LP from the lead in groove up to the LP’s label, is (for the lower pole) from 06:00’ up to the 08:00’ o’clock, namely only 60 degrees, as you can see in the attached photos.

Outer Groove.jpg
Outer Groove.jpg (66.84 KiB) Viewed 2969 times

Inner Groove.jpg
Inner Groove.jpg (73.52 KiB) Viewed 2969 times

Btw, I have totally replaced the factory fishing line (which was too short for the above exact engagement) with a new one longer, exactly the same cross section (0,230 mm).
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Re: Modified mechanical anti-skate

Postby Brf » Mon Mar 25, 2019 4:56 pm

Votan wrote:In my 10” tonearm, the actual range of motion of the anti-skate mechanism, as the tonearm moves across the LP from the lead in groove up to the LP’s label, is (for the lower pole) from 06:00’ up to the 08:00’ o’clock, namely only 60 degrees, as you can see in the attached photos.


My 12-3DR Uni and my 12 Fatboy Gimbal both have the same engagementpoint and 60 degree usable range as noted by Votan.
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Re: Modified mechanical anti-skate

Postby Golear » Mon Mar 25, 2019 6:05 pm

Golear wrote:Well said Brf.

Ideally, the range for the anti-skate device should be 90 degrees of rotation. However, because of where the anti-skate thread is attached to the tonearm, the actual range of motion of the anti-skate device, as the tonearm moves across the LP, is about 115 degrees. (This is for the 10 inch arm - there may be a difference with longer or shorter arms.)

If the thread could be attached closer to the pivot, then it may be possible to set the range of motion of the anti-skate device to 90 degrees. This is the key: When the LP moves the arm from outer to inner position, that range of motion needs to correspond to a 90 degree range of motion for the mechanical anti-skate device.

(But the anti-skate thread can't be any closer to the pivot because of the unipivot housing. Perhaps it can be with the gimbal arm. I don't know.)

With the rubber O rings in the default position, the center of mass is quite far past the horizontal plane when the tonearm is in the inner grooves. This results in an appreciable fall in anti-skate force just when you need it most. You might hear it as extra "air" in the high frequencies, but it is mistracking, and this damages the LP. It actually affects all frequencies. You'll hear the difference when you fix it.

Reducing the mass on the upper arm of the anti-skate mechanism will change the location of the center of mass. The center of mass will be closer to the lower arm, instead of midway between the upper and lower arms. So now, when the tonearm is in the inner grooves, the center of mass will closer to the horizontal plane, and this allows the mechanism to apply the correct level of anti-skate. The center of mass will still have gone a little over the horizontal plane, so there will be a small drop off from the max value. But it's ok. I estimate that there's still close to 85% of the max force. (This assumes the thread is also lengthened by a bit, as I have indicated.)


I stand corrected on the above. My turntable is packed away right now, so I was going by memory. I then looked at some photos on my phone, and can see that my arm also has the 60-odd degree range of motion. So I stand corrected on that.

What I can see though is that there's something different in my set up. It might be that I could lengthen my thread quite a bit more. This would make the lower arm point down to 6 o'clock at the start of the LP, where it now starts at about 7 o'clock. It's also possible I have an older arm base, and the junction box is in a slightly different spot.
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Re: Modified mechanical anti-skate

Postby Brf » Tue Mar 26, 2019 8:50 am

Golear wrote:I stand corrected on the above.


You're allowed, and you have company ;)

I also took a mulligan on one of my statements later last week.
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