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RE: Arm mass categories?

Posted by John Elison (A) on June 6, 2017 at 15:10:54

In Reply to: Arm mass categories? posted by M3 lover on June 6, 2017 at 14:05:36:

My source is basically common sense. The lowest mass arm that I know of is the Infinity Black Widow with effective mass of 3-grams. Another low-mass tonearm that everyone accepts to be low-mass is the SME III with effective mass of 5-grams.

The SME V, the Technics tonearm and Rega tonearms are all accepted to be medium mass tonearms. These have effective mass in the range of 10 to 12-grams. Therefore, the division between low-mass and medium-mass is somewhere in-between 5-grams and 10-grams.

I think most people would consider a tonearm with effective mass of 20-grams to be a high-mass tonearm. Therefore, the division between medium mass and high mass must be somewhere in-between 12-grams and 20-grams. I chose 15-grams.

However, regardless of how you choose the divisions, a tonearm like the FR 54 with effective mass of 16-grams would never be considered a light arm or low-mass tonearm when 12-grams is clearly defined as medium-mass.

> Common headshells can weigh at least 9-10 grams which must have an affect.

I'm not sure what you're getting at. The headshell is always included in the effective mass of the tonearm. For example, the weight of the Technics headshell is 7.0-grams without cartridge wires and between 7.5 and 8.0-grams with cartridge wires, depending on the specific cartridge wires being used. I just weighed a Technics headshell and several different types of cartridge wires.

The effective mass of a headshell is actually slightly less than its total mass because effective mass is the headshell's moment-of-inertia about the tonearm pivot divided by the square of the tonearm's effective length. Therefore, a 7.5-gram headshell might have effective mass of only 5.75-grams, depending on its mass distribution. Consequently, a Technics tonearm might have effective mass without its headshell of about 6.25-grams.

Do you know the weight of the FR 54 headshell? Whatever it weighs, the tonearm's effective mass of 16-grams includes the headshell.

Best regards,
John Elison

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