How to measure tonearm effective mass

Correction to above post:

Where I said "convert the frequency to a period (eg by inversion) and the result is a linear correlation of period shift to equivalnet mass"

I should have said "convert the frequency to a period and square the result. The result is a linear correlation of this quantity and equivalent mass."

My apologies for the error.

Mark Kelly

Lewm,

any arm that has split counter weights can have its effective mass adjusted. Triplanar, Talea, etc.

Mark, My tonearm has the front and rear counterweights riding on separate threaded rods going into the pivot and independent of the arm tube. Thus adjustment of counterweights does not affect the secondary resonance characteristic associated with wand composition or length. Also, its linear design allows separate manipulation of horizontal and vertical mass. Thus far I've found that the vertical mass is the more critical adjustment. Up to two-thirds of the assembly's total moving mass resides in the counterweights-- allowing a wide enough range of adjustment to characterize the tonearm as "universal."

Dan_ed, Indeed a split rear counterweight allows some adjustment. The question is whether the range of adjustment is sufficiently wide for a particular cartridge. In addition, with such an approach it is not possible to separately adjust horizontal and vertical mass; the adjustment of one affects the other.

I would add that with any long pivot arm, the rear counterweight(s) is in the final analysis the slave of the wand/cartridge lever. With a linear wand it is possible to unpack and play with each variable separately.

Dgarretson

Thanks for the clarification. Now that you mention it I saw a phot of your mod to the Terminator (on VA?) and thought it was a very clever solution.

Mark Kelly

Dan_ed, Good point about the tonearms that can accommodate two counter-wts, but as Dave mentioned, the range of adjustment of effective mass afforded by that arrangement would seem to be quite limited.

Mark, Despite the setback you described, do you continue to pursue a Dynavector-like tonearm design where the vertical effective mass could be adjusted? I wonder whether your final solution to that problem would be retro-fittable to my DV505, eventually. Actually, what I do now is just change headshells as and when needed to change EM. That obvious ploy does not allow "on the fly" adjustment to max out the performance of a given cartridge, however.