a couple of q's about ladegaard arm

[graeme uk]

Ive been meaning to try out a DIY arm for sometime, ive finally settled on this design to try as an experiment.
I have a couple of questions though that ive not been able to find answers too.
I am going to make it with knife edge bearings.

1. Where should the knife bearing be in relation to the record surface?

2. where should the counterweight be in relation to the knife bearing/record surface?

3. Is there any optimum peramiters for arm lenght? Or is it longer/shorter the better, or just whatever fits your deck best?

These are simple questions i know, which may have universal answers for all arm designs, but i dont know alot about this so thought i'd clear up the grey areas before i start.

Thanx.

P.S. im building this as an alternative to my rega rb250, to use on my modded lenco. I will be using the same ortophon 510mk11 cart, will this cart be suitable for this type of arm?

[hacknet]

cool! i've been experimenting with one on my lenco too! but i havent got any progress! it seems pretty tough unless you have access to a machine shop.

[graeme uk]

thats the beauty of this design though, no machine shop needed.

One more question, where abouts along the arm base should the air enter, im a bit concerned about air pressure varying along the arm base.

[livemusic]

Hi graeme,
Welcome to the Ladegaard's arm builders lonely hearts club!
1. The knife pivot point shall be on the record surface plane, to handle warped records with no error.
2. The counterweight shall be well below the record surface, to minimize VTF changes when the stylus is travelling up and down on imperfect record. The arm shall be balanced in two planes: horizontal and vertical, i.e. in both planes the resulting moment (sum of weights and forces multiplied by distance to pivot) shall be equal to zero.
3. Do not be afraid of long counterweight! By moving counterweight further from pivot, you are killing two birds by one stone: the arm overall weight is getting lower and the moment of inertia (arm effective vertical mass) increases. Both are good things; do not forget that linear arm is shorter than any gimballed arm, so you need sufficient effective mass to get into the natural frequency ballpark (10-12 Hz).
Good luck,
Michael

[graeme uk]

thanx, im going shopping for materials today. hopefully ill have something nocked up in a couple of days and if all goes well ill build a mk2 and make it look good.

I was wondering, the air pump ill be using has 2 outlets. Its suggested these go to the smoothing tank then one outlet to the arm base. Would it be better to run 2 outlets from the smoothing tank to try and even the air pressure along the arm base?

[livemusic]

Would it be better to run 2 outlets from the smoothing tank to try and even the air pressure along the arm base?

I do not believe it does really matter because the air flow is very low and you will naturally have even pressure supplying the air from one end the rail, as I did. The pressure smoothener (surge tank) is essential indeed. I made my from plastic sewing pipe stuffed with polyfill; works perfectly.

[graeme uk]

thats good news as ive already gone too far to switch to two air inlets, although if it shows promise when finished ill probably build another with more attention to detail, im sort of throwing this one together

Could you possibaly explain this a bit more? (the statement below)

'do not forget that linear arm is shorter than any gimballed arm, so you need sufficient effective mass to get into the natural frequency ballpark (10-12 Hz)'.

What sort of amount of mass are we talking, and what do you mean by 'effective'? Is it the overall weight of the whole arm/counterbalance assembaly? If so, what sort of ballpark figure are we talking?

Im working on the arm at the moment, for the arm tube im using a sort of glass fiber composite tube. Not sure how good this will be but its stiff and light and pretty dead as its fiberous. It was however the most suitable thing i could find without resorting to aluminium tube.

[livemusic]

Do search for "arm effective mass" onAudio Asulum:
http://www.audioasylum.com/audio/vinyl/bbs.html
You will find tonns of information. It is essential to get clear idea about this matter for anyone taking burden to build tonearm.
In short, effective tonearm mass reflects the arm inertia, as it is seen by the cart stylus. Effective mass devided by the cart suspension compliance gives you the whole assembly natural frequency. It is important to keep the natural frequency below audible band, but above external ultra-low exitation frequency due to warped/excentric record. The widely accepted natural frequency ballpark is 10-12 Hz. The arm/cart natural frequency is ususlly measured by means of test record.
Hope this help.

[livemusic]

Graeme,
This thread is for you:
http://www.audioasylum.com/audio/vin...es/614350.html

[graeme uk]

thank you, very helpfull. Ill read through it all properly later when ive sorted the carriers air issues which i mentioned in the other thread.