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Re: Effective mass, moment of inertia and counterweights.

Low effective mass in this context is a good thing. Period. That applies to tonearm and stylus assembly.
That said, when you lower the global effective mass of a tonarm by using an after-market counterweight you don't know what the new effective mass of the arm is going to be. And I'm afraid that the makers of these weights won't tell you.

As far as tonearm resonance is concerned, according to your personal approach the range to aim at is 8-15 Hz or 10-12 Hz. I find 8-15 Hz acceptable in view of the facts as presented here

JAES 1976, p.630 : Happ et al. (Shure) : Record warps and system playback performance

Given the size of the 8-15 Hz window, you can lower the arm's effective considerably before getting into trouble. For my arm/cart combo I can drop the mass from 9,5 (SME 309) to 3 grams (Infinity black widow) and it's still fine.

Another issue may be that when using an after-market counterweight you may be changing damping/resonance behaviour and you may be changing the centre of gravity of the moving part.

I would consider the original design of a tonearm as optimum and would not try and apply "improvements". Unless, of course, there's an obvious problem. If for instance Rega or SME had considered a heavier counterweight closer to the pivot as desirable, I think that they would have used it.


Klaus



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