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Usually headweights are employed with the tonearm in question when the particular tonearm isn't quite massive enough to fully exercise the suspension of the cartridge that is to be mounted.

There is a difference in terms between effective mass of a tonearm and vertical tracking force.  Vertical tracking force is a static measure of weight (measured in grams) carried by the phonograph stylus.  Effective mass is a calculation that goes like this: "The effective mass of a tonearm is equal to the moment-of-inertia about its pivot divided by the square of its effective length."  

I would further differentiate the effects of effective mass and vertical tracking force by noting that 'effective mass' takes into consideration the forces imposed upon the cantilever and its suspension under the actual conditions of playing a record.  The side forces felt.  The ups and downs of diagonal motion. Tracking over warps, etc.   

What are headweights good for?: In most cases a headweight is mounted between the cartridge and the headshell part of the tonearm in order to add weight directly over the stylus as a record is played.  After vertical tracking force is adjusted the effect of the headweight is to add 'effective mass' to the tonearm so that it may better accommodate certain types of cartridges, usually low output moving coil, that have very stiff suspensions (low compliance).  It is most effective in the case where a medium or low mass tonearm needs to be paired with a cartridge of low compliance. The other, and usually preferred, alternative is to simply use a tonearm that already has enough effective mass to adequately operate the suspension of a low compliance MC cartridge.

Q: are there limits to how much mass may be added?  A: yes.  It is possible to add so much weight over the stylus that the counterweight at the other end doesn't have enough mass to balance or even achieve correct vertical tracking force.  In such cases, it may be possible to find a heavier counterweight.

Typically it is desirable for an arm and cartridge pairing to be compatible in terms of mass/compliance to achieve a static arm/cartridge resonance in the range between 8hz and 12hz.  This can be measured with a test record to verify.  (I use test record HFN 001)

Here is an example of a headweight I have manufactured and used to increase effective mass of various lighter tonearms to the point where they can successfully match-up with a Denon DL-103R low output moving coil cartridge.

Denon DL-103 and -103R low output moving coil.


This headweight is configured into a 'constrained layer' build up. Carbon Fiber / Brass / Carbon Fiber. The dissimilar materials are bonded together and then machined to duplicate the outer dimension of the Denon DL-103 (and R) standard resin body.