Weighted pivot ... Questions

[farflungstar]

I came across a post which mentioned placing a weight on top of a jelco pivot housing. Hugo does this I believe and it has piqued my curiosity. I understand the rationale but wouldn't the extra weight affect the ease of travel of the arm? After all the pivot turns as the arm arcs and so would any extra weight act a little like anti skating...?

[Ammonite Audio]

This tweak is only suitable for arms where there is a resonant structure surrounding the pivot and absolutely not on the pivot itself - as with the Jelco SA-750 series and my FR64S. The weight therefore has absolutely no effect on the arm's movement. Look at this picture of my SA-750E and you can see that the mass damping weight sits on top of the cradle and does not go anywhere near the bearing structure:

https://farm1.staticflickr.com/752/32888243090_7327e9d261_z_d.jpg

Do not under any circumstances think about placing any mass or weight directly on any tonearm's bearing structure or pivot, since this will only result in disaster.

[farflungstar]

You mean like this..

http://i1078.photobucket.com/albums/w483/farflungstar/IMG_20170308_155744_zpsnlefxrvw.jpg

[Ammonite Audio]

This tweak is only suitable for arms where there is a separate structure surrounding the pivot and absolutely not on the pivot itself - as with the Jelco SA-750 series and my FR64S. Look at this picture of my SA-750E and you can see that the mass damping weight sits on top of the cradle and does not go anywhere near the bearing structure (on these Jelcos that cradle does not touch the 'drum' housing the bearings):

https://farm1.staticflickr.com/752/32888243090_7327e9d261_z_d.jpg

Do not under any circumstances think about placing any mass or weight directly on an arm's pivot, since this will only result in disaster (excessive bearing loads, uneven mass distribution, etc). In the case of the FR64S, the flat top of the arm structure does rotate with the arm, but it's only in the horizontal plane, and the horizontal bearings are built like bits of battleship, so adding a mass there does not influence the vertical movement of the arm tube at all.

See http://theartofsound.net/forum/showthread.php?29401-Magna-Audio-News-amp-Product-Updates&p=841721#post841721 for an explanation of why this works, in suitable tonearms.

[farflungstar]

The Siggwan bearing is a little like the FR, it only moves laterally. However that housing must also be sitting on a bearing in order to rotate so maybe best not to - however the same surely applied for the FR and jelco... scratch

[Ammonite Audio]

No, the Jelco SA-750 arms are different because that cradle surrounding the bearing is fixed to the arm's base structure and does not move at all, nor does it make any contact with the bearing housing. I must stress that this tweak only applies to the SA-750 series - the cheaper SA-250 does not have this fixed supporting structure and placing a weight on top would cause damage. In the case of the FR64S, I'm confident that the horizontal bearings are specified like everything else on this arm - eg somewhat stronger than the norm, so I'm happy to take the risk.

If we confine this discussion to Jelco SA-750 arms and their more or less identical derivatives (Audioquest, Sumiko etc), there's no risk in applying a mass damper to the top of the arm. Someone with a suspended deck might have to re-balance the suspension, but that's it.

[paulf-2007]

This " tweak " only benefits the person selling a piece of metal to sit on top the cradle.


This tweak is only suitable for arms where there is a separate structure surrounding the pivot and absolutely not on the pivot itself - as with the Jelco SA-750 series and my FR64S. Look at this picture of my SA-750E and you can see that the mass damping weight sits on top of the cradle and does not go anywhere near the bearing structure (on these Jelcos that cradle does not touch the 'drum' housing the bearings):

https://farm1.staticflickr.com/752/32888243090_7327e9d261_z_d.jpg

Do not under any circumstances think about placing any mass or weight directly on an arm's pivot, since this will only result in disaster (excessive bearing loads, uneven mass distribution, etc). In the case of the FR64S, the flat top of the arm structure does rotate with the arm, but it's only in the horizontal plane, and the horizontal bearings are built like bits of battleship, so adding a mass there does not influence the vertical movement of the arm tube at all.

See http://theartofsound.net/forum/showthread.php?29401-Magna-Audio-News-amp-Product-Updates&p=841721#post841721 for an explanation of why this works, in suitable tonearms.

[Ammonite Audio]

Thank you for that, but who is selling it? Speedy Steve made me one and that is all.

[farflungstar]

Thanks Hugo, I saw your post on speedy Steve's thread and it got me interested. But I'll take your advice and leave well alone.

[Nickfna]

Oh I do love this site

Just tried this little trick on my 750db and it has made a notable difference.:D

Cheers Hugo

Nick.

[walpurgis]

I take it this mod requires resetting of suspension on TT's with floating sub chassis/armboard assemblies.

[Ammonite Audio]

I take it this mod requires resetting of suspension on TT's with floating sub chassis/armboard assemblies.

Correct - see my post #6

[Ammonite Audio]

Oh I do love this site

Just tried this little trick on my 750db and it has made a notable difference.:D

Cheers Hugo

Nick.

Thanks Nick - it's quite good for something that can be tried for nothing!

[karma67]

ive just tried it too,:scratch:

https://s26.postimg.org/g42hwavd5/IMG_1277_1.jpg

[Ammonite Audio]

ive just tried it too,:scratch:

https://s26.postimg.org/g42hwavd5/IMG_1277_1.jpg

A couple of points - this tweak works with the Jelco SA-750 series because the top part of the arm is one-sided and it's therefore free to resonate and a bit of mass controls that resonance very nicely The FR64S is one-sided too, so the same trick works. An arm like yours has a solid and symmetrical cap that will clearly not resonate much, if at all, so I would not expect you to hear any benefit, particularly with a record stabiliser weight that is way too heavy to be sitting on your arm's horizontal bearings. With the Jelco SA-750 there is no additional load on the bearings, so no risk.

[karma67]

yes, it was meant tongue in cheek :)