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How do I calculate the effective mass of a tonearm

djnagle

djnagle

AK Subscriber
Subscriber
Hi All, my little project is turning into a big project. I am thinking of lengthening my toneare from 9" to 12" so how do I calculate the effective mass so I can match the right cart, and, how do I figure out the new geomotry for proper placement in the new plinth? Thank in advance.
 
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Hey Dennis! what kind of mad scientist TT project are you working on now? I must have missed it.

Answer to your question: I have no flipp'in idea :D

RC
 
It might be easier to build it and figure out the effective mass by using a cartridge with a known compliance and a test record. As for placement geometry, I would copy the offset angle and effective length from a well known 12 incher (like an SME 3012) and then use the placement recommendation for that arm. There's a boatload of manuals and mounting info for the 3012 over on vinyl engine.
 
For arm mass you need a digital pocket scale--cheap on ebay or Harbor Freight. Use a cartridge without the stylus and weigh it. Mount the cartridge on the arm. Remove the counterweight. Let the front of the cartridge land on the scale. Subtract the weight of the cartridge from that reading and that is a quick and dirty estimate of the arm mass--close enough to make cartridge/stylus matches.

For geometry, you need to decide what null points you want to use (e.g. Baerwald, Stevenson, etc) then you need to mount the cartridge you'll most likely be using, and position it in the middle of the slots--just estimate the angle or keep it parallel to the slots. Then measure the straight line distance from the stylus tip to the middle of the pivot point. That gives you the effective arm length. I have a database, and if you give me that info I can give you a mounting distance. Or if you prefer, I'll email you the database and you can mess with it yourself. You can PM me if you like.

EDIT: The only problem with scuzzer's recommendation is that the arms are called 12", but the slots aren't in the same positions so the effective lengths vary. You could end up with no room in the slots. Unless, however, you are using one of the arms for which there is a manual--in that case, your info is all there.
 
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I was stuck on this myself and have looked through quite a few confusing posts from other sites. No answers over there. With what I know already, Hackaplan is dead-on of course. Thanks for the formula. I can now fit my old TT beater Toshiba SR-230. Wish me luck on that one.
 
hakaplan said:
EDIT: The only problem with scuzzer's recommendation is that the arms are called 12", but the slots aren't in the same positions so the effective lengths vary. You could end up with no room in the slots. Unless, however, you are using one of the arms for which there is a manual--in that case, your info is all there.
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He said he was lengthening the arm, if that's the case he can make it as long as it needs to be for the midpoint of the slots to be equal to the effective length of the arm he's matching.

Edit: How do you use arm weight to figure out effective mass?
 
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I have found it is better and more precice to ignore effective mass and compliance alltogether!!!

Not that I consider the issue redundant, but the Ortofon test record with a frequency sweep from 5 to 16Hz or so seems more ... effective as it gives me, visually, the system's resonance frequency and oscilation width.
 
Thanks everyone. This is all great information and the more I read the more I understand. Even thought I am making a plinth out of marble, I will have the old box plinth to use for the mounting geomotry before I start cutting marble.
 
doucanoe said:
Hey Dennis! what kind of mad scientist TT project are you working on now? I must have missed it.

Answer to your question: I have no flipp'in idea :D

RC
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Hey Dou, I got this old Rek-O-Kut TT and I am going to shine it up real pretty then make a swoopy plinth out of mable for it. I also got a broken Infinity Black Widow and since I have to replace the wand I will make it a 12" arm vs the 9" it came with stock.

I found a company that sells the right size woven Carbon Fibor shinny tube and will use that.

My intent is to polish the platter up to chrome spec then have the pulley and belt cover black anodized then have Rek=O=Kut K33H incraved on top where the lettering is now.

The only thing I am now sure about right now is the type of feet I can use.

NewImage.jpg
 
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Scuzzer said:
He said he was lengthening the arm, if that's the case he can make it as long as it needs to be for the midpoint of the slots to be equal to the effective length of the arm he's matching.

Edit: How do you use arm weight to figure out effective mass?
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Well, an arm of mass 10g weighs 10g. I tested arms of known mass and I came within .5 to 1g using this method. And that's close enough for arm/cart matching.
 
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SaSi said:
I have found it is better and more precice to ignore effective mass and compliance alltogether!!!

Not that I consider the issue redundant, but the Ortofon test record with a frequency sweep from 5 to 16Hz or so seems more ... effective as it gives me, visually, the system's resonance frequency and oscilation width.
Click to expand...
Well that's nice after you have the cartridge, but not much help in deciding what to cartridge to buy. What you can do, if you have the Ortofon record and an arm of known mass, is calculate the actual compliance of the stylus. Then you can use that stylus on other turntables with the record to calculate the arm mass. Problem is that the Ortofon test record is extremely rare, and when you do find it it is extremely expensive. The tones on the HiFi News record are not accurate, and the Shure record only does tones on the even frequency (6, 8, 10) so not much accuracy there.
 
The hifi news one is not off by much in the area of interest 8-11 hz. There's a chart around on the net somewhere that details the actual vs. reported frequencies.

Found it:

gi.mpl


I can't vouch for the accuracy of the above numbers since I've never measured them myself. It's from John Elison over at Audio Asylum.
 
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With the advantage of having my AudioQuest PT-5 tonearm and Grado Gold cartridge in place, I calculated the arm's mass in reverse using the effective mass calculator at:

http://www.cartridgedb.com/tools.asp

I determined my Grado Gold's resonance frequency at 10 Hz using the old Sure Era IV tracking-test record, and 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 to be 6.6 grams. I'm still not too sure I did everything right, because that seems pretty low. The only info on the tonearm I could find is that it's a "medium mass" arm, but 6.6 grams seems lower than "medium." Maybe the Shure test record is off??
-Bob
 
hesson11 said:
With the advantage of having my AudioQuest PT-5 tonearm and Grado Gold cartridge in place, I calculated the arm's mass in reverse using the effective mass calculator at:

http://www.cartridgedb.com/tools.asp

I determined my Grado Gold's resonance frequency at 10 Hz using the old Sure Era IV tracking-test record, and 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 to be 6.6 grams. I'm still not too sure I did everything right, because that seems pretty low. The only info on the tonearm I could find is that it's a "medium mass" arm, but 6.6 grams seems lower than "medium." Maybe the Shure test record is off??
-Bob
Click to expand...

Odd you came up with 6.6grams EM. The arm is listed as being 10gr EM which should put the 20CU right about 10hz.
 
tnsilver said:
Any comment are welcome.

Cheers

Tom
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I think that your post is worthy of it's own thread. You have gone to a lot of work and trouble to document some excellent information.

To attaching it to a four year old post does not do you or it justice. Maybe Moderator Mr. Lin can do that for you ...if you want. Preston
 
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