LP12 Sub Chassis - The Options

[flatpopely]

Yeah, but that's not really an effective way to balance the load - you'd be better just adding more weight at the other two springs.

I don't understand what you mean by this. How would weighting up the rear and left hand spring counter the effect of a reduced armboard weight over the front spring?

 

[Patrick Dixon]

Umm, who said anything about the front spring? You said

"The rear spring is loaded enough as is without removing the counterbalance of the armboard."

So you are saying that the armboard (as it) is is reducing the load on the rear spring. I don't think that can be correct because it would have to be on the opposite side of the CofG to the rear spring to have that effect. Maybe you mean the rear spring is the one nearest the motor?

 

[pure sound]

Forgive my ignorance. Is the idea with the LP12 that each spring is loaded identically so that it has the same resonant frequency as the other two? ie. w = sqrt (k / m )

Is that achievable with the weight distribution of the suspended part arranged as it is?


EDIT. It looks as though the bearing is at the centre of an equilateral triangle formed by the three springs. However with the mass of the tonearm, the CoG must be somewhere on a line between bearing & arm base closer to the two springs nearest the arm which suggests that they'll be loaded more than the spring to the left & therefore bounce more slowly. Is the LH spring more compliant to compensate?

 

[ChristianThomas]

Why not consider a new top plate and sub chassis combination, that would allow better positioning of the springs, a smaller armboard, and arm weight balancing? You could also put the motor on it's own active subchassis, with sensors on the main subchassis keeping everything in-line.

This would be a new turntable, and much the sort of thing I think I would do. Cover up the exposed surfaces, probably use four springs and have two of them moveable, make a subchassis that is genuinely aligned for strength where it is needed, make sure that its transmission properties don't include any high Q resonances, have the motor on the subchassis but with its own platform (not sure about active?), have the belt give nothing more than a pure couple - meaning it will need another pulley at least, or maybe another motor (??) etc. etc.

And then, if I haven't made a cock-up on the way, it should sound accurate. Whether it will sound good (or to our taste) is harder to know. One has to remember that the LP12 is really a musical instrument that has been tweaked and refined over nearly four decades.

Mind you, having said that, I bet that the first time a stylus dropped onto a record on it, Ivor (or Hamish Robertson) will have gone "Wow, that's good!" (Or, tongue in cheek, "Wow, that's good enough to steal.)

 

[pure sound]

Looking at a pic of the Keel I now see what this issue is with the loading varying from spring to spring. Unless different springs are used in each position or some compensating masses are added (or removed in significant amounts) it's always going to be tricky to get the 3 springs to behave in the same way as each other vertically.

 

[Paul R]

The three springs in the LP12 suspension are interchangeable. The loads on each spring are different. So why does it bounce rather than wriggle?

I think it's partly because of the square root, if the resonant frequency is 4Hz and you double the mass it falls to about 3Hz. So in the real situation, given a normal 'q' the springs are all happy enough to bob at a frequency they all agree on.

Paul

 

[YNWOAN]

EDIT. It looks as though the bearing is at the centre of an equilateral triangle formed by the three springs. However with the mass of the tonearm, the CoG must be somewhere on a line between bearing & arm base closer to the two springs nearest the arm which suggests that they'll be loaded more than the spring to the left

Correct (except I've just seen the pic you have posted and the COG is not quite where you have put it).

 

[pure sound]

I'm sure it isn't. It was only a guess!

I was also disregarding the contribution of the subchassis itself which (in the Keel's case) must be spread about in interesting ways particularly the 'armboard'.




Given the position that the arm mass occupies, the spring location & subchassis layout they inherited from Ariston was some way from optimal imho.

 

[Tony L]

(except I've just seen the pic you have posted and the COG is not quite where you have put it).

It's far higher up, way above the subchassis!

Tony.

 

[YNWOAN]

I wasn't meaning in that respect Tony - though of course you are also correct.

(P.S. Thanks by the way )

 

[per-Sony-fied]

When were talking COG are we talking the balance point of the sub-chassis or the balance point of the sub-chassis with the bearing, platter & arm fitted? It would seem that in those instances it would differ.

 

[sq225917]

COG/pivot point, good spot Tony we all use terms in the same incorrect way to mean the same thing.

PSF you have to really look at the sub-chassis+ arm-board as a combo. You can forget the platter etc as that is already at the point that you really want to be the COG, so it plays little role in reality its mass centered between the springs only helps the situation.

ideally I'd guess you want to reduce the weight out over by the armboard/arm by as much as possible, either using your lightest materials here or machining the armboard away as much as possible. That's why I find an MDF arm-board and composite sub-chassis such an odd combo it seems so unbalanced.

 

[pure sound]

By CoG I mean the point at which you could balance the whole assembly; record, platter, bearing, chassis, armboard, arm & cartridge without it wanting to fall in any particular direction.

The displacement of all the massses in the vertical plane & where (in this plane) they are supported by the springs is a whole other set of things to consider. As Tony points out, on the LP12 its all very high above the point where the springs are themselves supported.

AR, Ariston, Linn & Thorens jumped through alot of hoops when choosing the plinth dimensions they eventually settled on.

 

[Dowser]

Think of it as a boat anchor - it's all that (partially) stops the armboard following the platter!

Tony.

Coming back to this. So are we saying the following is worse than a Linn dressed approach? I guess I should try, but that means finding a more flexible external cable or going back to the stock Linn item.

Thanks, Richard