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Adding mass to a tonearm. Best principles?

Just checked and a 1p or 3p coin weigh just over 3grms here. So you can place a coin on top of the headshell and then rebalance the arm. That gives well over 3grms additional effective mass because you've also moved the counterweight. So experiment to see what that does to the HF resonance. Then break the law and cut a bit of the coin, etc. 8-]

A small bit of carpet tile might do better wrt resonances, though. Easier to cut to about 2.5grms if that's what you need - avoids lawbreaking as well. :)
 
jamie123 said:
isnt the mass of a tonearm more to do with its inertia?
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Yes. In this case IIRC the mass of each bit of the arm x its distance from the pivot, then divide the sum total by stylus-pivot distance. A mech eng will be along shortly to point out I've got the maths wrong...
 
jamie123 said:
isnt the mass of a tonearm more to do with its inertia?
Click to expand...

It is EXACTLY that - moment of inertia. For some reason hifi has decided to call it effective mass.
That said, in most tonearms there are two parts that make an overwhelming contribution to the whole - the headshell and the c/b weight.

In essence, correct Jim.

Excellent discussion on Vinyl Engine, if memory is correct. Including worked mathematical examples.
 
Just do the math and work out what mass of the cartridge would need to be to match the tonearm so no mistracking occurs. There are several sites with the formulas on. Then just buy the cartridge with the correct mass...
 
Copied from my 'Audio_Maths' Excel workbook...

=1000/(2 * 3.14 * SQRT(M*C))

Where:
M = sum of tonearm effective mass, cartridge mass, and mounting hardware/additional headshell weights
C = dynamic cartridge compliance @ 10Hz (roughly double the @100Hz value that most Japanese cartridge markers quote, or half of the static compliance if dynamic not quoted)

I've been running tonearm/cartridge numbers since using Lotus 123 for DOS ver 1.0b in the early/mid 1980s.:p
 
Yes it is the 'moment of inertia' that is important.

Moment of inertia is the mass multiplied by the square of the distance between the mass and the pivot point. In a tonearm most of the inertia is in the mass of the cartridge and the headshell, plus the inertia of the counterweight. The downforce on the cartridge does not contribute, but as the cartridge is located much further from the pivot than the counterweight is from the pivot, the cartridge (and headshell) mass is the major component of the inertia. The arm wand usually being relatively light, adds little inertia. Therefore it is more effective to add weight to the cartridge end to increase the effective mass.
 
G T Audio said:
Just do the math and work out what mass of the cartridge would need to be to match the tonearm so no mistracking occurs. There are several sites with the formulas on. Then just buy the cartridge with the correct mass...
Click to expand...
As in the OP then...
I want to mount a Lyra Delos on my SME M2. The maths tells me that that is OK and will have a resonance around 11Hz which is 'ok' but 9Hz is better. So I need to add some mass to the arm to bring it to around 16 or so grms effective mass. It was quite a simple question...I can add mass but where should I add it and how?
Some people above (and it won't do me to mention the parties name) get all sniffy when an Art teacher turns out not to understand (or need to know) the physics involved.
He (they) could of have done something useful like making an actual, practical suggestion but no...
Even if I did read the 'discussion' suggested I'd not understand any of it.


Let's keep it simple.
where do I add the weight.

Now, if we want to erase a certain number of posts above, we can go on from here.
Yeah! :)
 
With my totally unscientific brain I can just about equate inertia with mass/weight. If the cart. end is heavier, inertia will be higher, which is good for many coils like Koetsu, Miyajima (and Denon?) which have quite low compliance. However, this 'adding bits to the headshell' or whatever just seems a bit simplistic; maybe not, though. I have arms (around 14g) which I'd prefer to add mass to (I've a K in my drawer to re-mount) but have always thought that the proper mass arm, so to speak, is the better bet for specific cart's, but is that only for aesthetics?

Interesting thread regardless, though I do struggle with cms/dyne -? and stuff, not to mention the measurement standards of. 100 Hz of Japan and 10Hz here.
 
I see Mike that quite a lot of Japan have caught on and now quote at 10 as well...Dynavector and Lyra both do I think.
 
To the cartridge end as I said above.
You might possibly not want to know the there is another school of thought (Poul Ladegaard of 'Bruel & Kjaer' ) that considers that the ideal resonance should be much higher than the usual 8 - 12 Hz http://www.theanalogdept.com/images/spp6_pics/TT_Design/MechanicalResonances.pdf

I would have thought you should have no trouble at 11Hz (normal band is thought to be 8 - 12) and a couple of my cartridges give that when measured with the HiFi News test record even though the theoretical calculation is different. Usually due the the stated compliance figures not being that accurate.
 
This is the definitive posting on the subject of determining the effective mass of tonearms (assuming the tonearm maker hasn't published a figure, or you'd like to test their accuracy). The only proviso being that it doesn't actually matter if the tonearm is straight, J-shaped, or S-shaped; the maths still work as they are straight line distance vs. mass correlation based.
 
Weren't we just here yesterday, Jacques?

Too low and the possibility of oscillations/mistracking on warped/rippled records; too high and lowest recorded frequencies boosted, perhaps inaudibly, but wasteful of amplifier power, and possibly leading to driver damage at high levels.
 
A practical answer is:

Add a headshell weight like the one in the picture above.
Then add mass to the counterweight (washers or similar) until you get the correct tracking weight with the counterweight in its 'normal position'

So for example say the headshell weight adds 5g to the cartridge end of things. Then with the counterweight in it's 'normal' (non headshell weight) position add enough weight until you get the tracking force you require. You can do this by supergluing on washers (they can be tapped off and the glue removed with acetone if you want to reverse it).

All of this will be easier if you have access to some stylus scales.

With weight added at both ends you will have increased the effective mass.
 
The main cause of the misunderstanding of the term “ effective mass” is that it would be more appropriate to term it “effective inertia”.
To add E mass (inertia) without adding more weight than necessary add it as far away from the pivot as possible ie. the headshell and move the counterweight back to suit. Having the counterweight as close to the pivot as possible is only appropriate if you are trying to minimise the E mass (inertia).

Bill.
 
Jim Audiomisc said:
Just checked and a 1p or 3p coin weigh just over 3grms here. So you can place a coin on top of the headshell and then rebalance the arm. That gives well over 3grms additional effective mass because you've also moved the counterweight. So experiment to see what that does to the HF resonance. Then break the law and cut a bit of the coin, etc. 8-]

A small bit of carpet tile might do better wrt resonances, though. Easier to cut to about 2.5grms if that's what you need - avoids lawbreaking as well. :)
Click to expand...

An eighth of an ounce in fact... :cool::rolleyes:
 
The effective mass will be increased regardless; only the distribution of mass either side of the pivot, and therefore inertia, will have slightly altered
Arkless Electronics said:
Yer just blu tak a penny on yer headshell if yer needles jumping innit:D
Click to expand...
Twist 'n Shout, with nowt jumpin' er skippin'!

tone-arm.jpg
 
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