DIY tonearm optimal weights?

[mr-mac]

Hi,

I am considering designing and building a DIY tonearm slightly diffrent to some of the ones I have seen on here (I will give more details once I investigate a little....

First off I am looking at armtube materials and wondered before I look at effective mass etc. If there was a good weight just to aim at for the tube on it's own before I even look at counterweight etc.

I currently use a grado red (20cu) but would like to move up to an MC at some point. So I guess a weight for a 9" arm tube that would give me a highish complience arm.

If its a little more awkward than that at least if I could get some advice and direction to help me consider what a suitable material weight for the tube would be.

Thanks Loads for any help

John

[Mark Kelly]

First, catch your goose.

The mass of the headshell is by far the most important determinant of eventual effective mass. Decide how you are going to handle that before you do anything else.

[mr-mac]

Ok so for a highish complience arm what headshell weight should I be looking for?

I may be looking for a way to make the arm tube act as the headshell so not sure where to start with working this out.

So far I am just wanting to assess diffrent materials for the armtube to see if they are a possability or if it's not even worth looking at.

Is there a range of weight that would likely be suitable for the tube? Although I do understand depending on where I go in that range will mean I may need to adjust headshell arrangement to suit.

Thanks

John

[jeff spall]

Hi, "effective mass" is just another way of saying moment of inertia, so it's where the mass is distributed rather than the total that is important.

Working out the static mass is the easiest part of a tonearm layout - it's just mechanics 101: - fulcrum, mass, distance. You can work out your ideal effective mass by calculating back from the range of compliance of the cartridges you are likely to use, starting with a target resonant frequency, maybe 11Hz.
So you just work this backwards:
rf = 159 / sqrt ((eff. mass + cart weight + fastener weight) * (compliance))

There's even a javascript version out there on the web if your maths is as bad as mine:
http://www.mhsoft.nl/MySystem/rf.html

Once you've got that, you can then look at how you distribute the mass in both planes because that's going to affect stability for tracking, warp riding and antiskate control and will also have a major influence on the harmonics of the arm. There are going to be some almost fixed starting points like bearing mass to work from.

Working within the target effective mass, I guess the parameters for the armtube are mechanical stiffness and the harmonic behaviour. These two are often a matter of balancing one off against the other! I think Headshell/tube interface is very important and has a huge effect on the final performance.

Getting all this stuff right is a complicated subject. Mail me at the address on the website below if you need to work out anything specific. I'll help if I can.

[jrevillug]

I think one way of doing it is to bodge together an arm, and see how well it works.

i did this with a Bamboo arm tube, and I haven't changed it yet. It seems to match quite well to my AT-110e, which is a mid-compliance MM cart. For a high-compliance cart, I'd probably be looking at using carbon fibre or similar, and with a low-compliance MC I'd probably stick with the bamboo and make the headshell and counterweight heavier.

A heavier counterweight, mounted closer to the pivot point, will reduce the effective mass - so you can fine tune your choice of arm material using that.

[mr-mac]

Nice one...

Thanks for all the help. I will maybe just get some tubes and give a few things a go.

John

[CG]

Originally posted by Mark Kelly
First, catch your goose.

Sorry to interrupt this thread, but I've been unsuccessfully trying to contact Mr. Kelly through email on various web sites. I may well be caught in his spam filter. Mark, can you please contact me about a classified you have placed? Thanks.