Tonearm effective mass

Correction. If ALL you change is the counterweight (same cartridge, same mounting screws, etc.):
- a heavier c/w closer to the pivot REDUCES effective mass
- a lighter c/w farther from the pivot INCREASES effective mass

Effective mass is a function of mass X the SQUARE of the distance from the pivot, so changing the distance has a larger effect than a change in mass.

Aside from that, for practical purposes I agree with the rest of Raul's post.

Thanks I thought I had read a heavier counterweight with everthing else the same would lower the effective mass. So it would take a big increase in weight to make very much of a difference? I am trying to get the resonant frequency up to 10 from 8 with a fixed headshell.

Thanks

Doug: Thanks for that information. It is both counterintuitive and very relavant to me at this time as I am in the process of doing a tonearm upgrade and one of the goals is to increase effective mass of the tonearm (cartridge is a wood bodied Denon 103R). I am looking at changing out the stock headshell on the new tonearm (stock headshell weighs about 6-7 grams) with an aftermarket headshell that weighs 16 grams.

I've been assured by someone who owns the same tonearm that the stock counterweight on the arm will balance out the cartridge and heavier headshell. There is also an optional heavier counterweight available for the arm to balance out heavier cartridges. I'm assuming, though, from what you say, that the lighter (stock) counterweight in this situation is going to result in higher effective mass.

Would you concur with that? Thanks in advance.

Dear Doug: The question speaks to add weight to the counterweight, IMHO if you add weight at any position of the counterweight the effective mass goes up against the original one ( obviously with out change in the original counterweight position. ).
A plain answer to a plain question.

Regards and enjoy the music.
Raul.

Raul

Doug is completely correct as long as the dimensions of the counterweight do not change*.

Remember that "effective mass" is actually the moment of inertia of the arm divided by the square of the pivot to stylus distance. Doubling the conterweight mass and moving it half the distance to the pivot halves its contribution to the moment of inertia because 2 x 1/2 x 1/2 = 1/2.

The counterweight is usually responsible for between 20 and 40% of the moment of inertia of an arm, therefore doubling its mass will reduce the moment of inertia by around 10 to 20%.

* If you make the counterweight larger a second effect comes into play: the moment of inertia of the counterweight about its own centre of mass will increase and this will affect the arm as a whole according to the parallel axis theorem. You could double the mass with the same dimensions by making the counterweight from a material with twice the density of the original - changing from brass to a tungsten pseudo alloy would be very close to this.