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dario54 4th August 2008 07:47 AM

Effective mass??
 
I'm building my own tone arm and found all considerations about the so called EFFECTIVE MASS of the tone arm related to the interfacing of a specific cartridge.

Can someone explain in detail how to get this value?

I can ONLY get the WEIGHT of the arm, section by section, in grams.

Any help will be apreciated, thanks.

Dario54

sreten 4th August 2008 11:27 AM

Hi,

Starting with the headshell =100% multiply each section by its
% distance away from the pivot points at the base of the arm.
This will give you very near the total effective mass.

:)/sreten.

h_a 5th August 2008 06:43 AM

Effective mass is just the inertia of the arm. You could get this value with respect to the tonearm axis by applying a specific torque and measuring the resulting oscillation frequency.

But I guess that's maybe beyond you want to do.

As a simple estimate, the weight of the headshell will largely dictate the effective mass, since its distance to the axis is largest. However this precedes that the weight of the arm (w/o headshell) is not significantly large.

Have fun, Hannes

sreten 5th August 2008 11:34 AM

Hi,


My post has a mistake, the method works out how to balance the arm.
The % distance needs to be squared for each section for effective mass.

:)/sreten.

AMV8 5th August 2008 04:25 PM

Hi Dario

Good to hear that you are building an arm.

Are you thinking of including any damping of the arm at the pivot point or next to the cartridge? Silicon fluid is easy to buy as a damping fluid.

Some people, myself included, think that damping the arm is the easiest way to avoid movement of the arm. The arm movement that normally affects the sound most is the movement of the arm along its axis. A pivot point is not very good at resisting longitudional or lateral movement unless damped at the pivot point. Many manufacturers of expensive arms use a very tight tolerance bearing to resist arm movement. Unless you can find such a bearing ( it will be expensive ) then I think that damping is the best way to go.

Don

Steerpike 5th August 2008 09:55 PM

Somewhere on the WWW I read a technique to measue the Eff Mass very acurately. I cannot now re-find it, but in summary it is:

Get a small, speaker - say 2-inch, a cheap fullrange, as used in budget headphones.

*Carefully* stick a knob of soft stationery putty (BluTak in the UK, Prestik locally, or Pritt GlueMagnet) onto the cone.

Connect to an audio oscillator and millivolt meter, and measure the resonant frequency.

Now place the speaker cone upward on the turntable and stick the cartridge - MINUS THE STYLUS, for safety) into the putty, so the arm & cone move as one.

Re-measure the resonant frequency, and do the necesary calculation to find the increased mass. This will be the effective mass, and not the 'gravitational mass' of the arm.

dario54 6th August 2008 07:40 PM

Thanx all fr the suggestions...

My problem is simple:

I got a used DIY unipivot and I'm trying to somehow update it fr the use with my Denon 103.

Actually it is quite short, maybe 8.5 inch, while I intend to have it much longer, maybe 11 or 12 inch.

The older arm (fm the cup holding the pivot) + shell (very minimalist) weight around 12 grams, the new one without shell, cable is 22 grams.

When thinking about the shell, I'm in doubt what solution prefer, considering the EFFECTIVE MASS of the whole arm.

Only thing that relievesme is the fact that 103 prefer heavy mass arms...

Will try to post some pictures to make myself more clear.

Dario54


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