How to change effective mass of a SME V.

Holly mo!
Nick this sound very involved indeed.
I think a reasonable and quick approximation is to take off the counterweight and cart, then place the arm on a micro-scale. I did this with a 9c (Pro-Ject) arm and is was the exact weight (mass) as stated it should have been, 11g.

Go figure,

It's actually very easy.

You know the effective mass of the SME V.

You know its effective length. Call this Xel.

You know the mass of the extra headhsell weight.

You can take the counterweight off and weigh it (in grams). Call this M.

You can measure the counterweight's distance from the pivot before the additional weight and after it. Call these Xb and Xa.

The counterweight's additional contribution is then given by

M . (Xa^2 - Xb^2 / Xel^2)

The new effective mass of the arm is the sum of this number, the original effective mass and the additional headshell weight.

Mark Kelly

BTW for a first approximation you can simply assume that the counterweight's contribution will be approximately 10% of the added headshell weight's.

If you wish to add say 5 g overall, you would add a headshell weight of about 4.5 g and expect the counterweight movement to get you (most of) the rest.

Mark Kelly

God I hate that I can't edit

The bracket is in the wrong place, the equation is

M . (Xa^2 - Xb^2) / Xel^2

Mark Kelly

I don't want to beat a dead horse, but for good measure. After some careful consideration the calculation I proposed above is wrong.

It appears as though it could be okay in practice, at least with my technics tone-arm. That is when one pull the balance weight back by .5mm the tone arm move into a new inclined equilibrium.

In theory, this should not occur unless the pivot point is displaced to the new center of mass. I assume that the fact that in practice a new equilibrium point is found is due to friction in the pivot point and that the change of the center of mass is minimal.