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For years, I've been trying to discover the effective mass of the Audioquest PT-5 tonearm. No luck. Then I realized I could find it using the calculators at the Cartridge Database. The arm's effective mass was the only variable I didn't know, so I tried to solve for that. (I determined the cartridge resonance frequency at 10 Hz using the old Sure tracking-test record; it's a Grado Gold.) So here's what I plugged into the calculator:
Resonance freq.: 10 Hz
Cart. mass: 5.5g
Fastener mass: .5g (estimated)
Compliance: 20 (listed by Grado as "cu")
Arm effective mass: X
Using those figures, the PT-5 effective mass was calculated at 6.6 grams.
Kinda low, huh? The only info I've been able to find on the effective mass of the PT-5 was from an old "Stereophile" review that listed it as "medium mass." I don't think 6.6g would fit into the medium category.
Is something wrong here? THANKS
-Bob
If that's the static compliance you can bet the dynamic compliance is less. If the dynamic compliance were say 10 um/mN the arm mass would be 19.8 g.
BTW if you have a small speaker and a signal generator you can measure the effective mass of the arm directly.
Mark Kelly
Some makers report the value at 100Hz, whereas the meaningful frequency range is between 5 and 20 Hz.
charlie s.
The Grado specs just list "Compliance," without specifying dynamic or static. I assumed it was dynamic because that seems to be the more common spec.
They also specify it in "cu" (i.e. "20 cu"), rather than the more common "cm/dyne". I don't know if there's a difference or if there is, what the significance is.
-Bob
CU = Compliance Unit. In the old "CGS" system the unit was 10^-6 cm/dyne. In the modern SI system the unit is um/mN. All the units are the same:
1 cm / dyne = 1 g ^-1.s^2 = 10^3 kg^-1.s^2
thus 10^-6 cm / dyne = 10^-3 kg^-1.s^2
1 um = 10^-6m
1 mN= 10^-3 kg.m.s^-2
therefore 1 um / mN = 10^-3 kg^-1.s^2
the two units are thus identical
Mark Kelly
"BTW if you have a small speaker and a signal generator you can measure the effective mass of the arm directly."
this is interesting. can you share how to do so.
Thanks
LT
This was a collaborative effort: I came up with the original idea which was refined by a couple of contributors on DIYaudio.com. Unfortunately I cannot find the original thread so I can't give credit where it is due.
The full description is rather long and involved so I might write it up properly and post it later.
Mark Kelly
Hey, I'd love to see that too!
Thanks Mark
if you can write this things up then it should be benefit for most around here as I found it to be helpful for some tone arm DIYER.
cheers
LT
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