DIY Tonearm questions:

[pixpop]

I'm gradually working on a DIY TT and Tonearm. Currently I'm thinking about tracking error and how I will handle it. I have a few ideas, but wanted to know if there are any standards concerning the mechanical design of cartridges.

I'm thinking of the 1/2 inch mounting hole design. Are there any standards or conventions regarding where the stylus is relative to the two mounting holes? How far below? How far in front, etc.?

For cartridges with a flat top surface, is that surface intended to be parallel to the record surface?

Also, I've seen references to some standard regarding where the recorded area lies on an LP record. Something like 66mm to 148mm radius. Anyone know where I can find this specification?

Thanks,

Neil

 

[jeff spall]

Hi, try this:

http://www.william-reed.net/audio/tonearms.zip
or
http://www.william-reed.net/audio/tonearms.sit

one of the best references to geometry and setup I've come across.

The distance of stylus to mounting holes doesn't seem to be standardised, but you should set up the correct overhang for the arm's effective length. I think that the conventional slot adjustment is a good practice as it allows you to compensate for minor inaccuracies in the arm mounting point and also for the fact that very often the sytlus isn't perfectly aligned along the axis of the cartridge.

It's convention that when the cartridge mounying surface is parallel to the record and the tracking weight is at the published value the VTA/SRA is correct so that's a good starting point for VTA. As there's a big variation in the height of cartridges and the thickness of records, VTA adjustment is a must.

regards, jeff

 

[pixpop]

Jeff, thanks so much for the pointer. That's a really great article, very clearly explained.

Neil

 

[jeff spall]

Hi, yes it's got all the essential info to get the geometry right.

One thing I've found to be very good if you are building/modifying an arm is to make use of the front edge of the headshell. Make it exactly the arm effective length and pointing to the spindle axis. You can use this as a datum to set the stylus up exactly aligned with the edge with the cartridge square to the front edge. You can do this before mounting the arm and get the setup almost spot on.
regerds. jeff

 

[pixpop]

Jeff,

do you mean that I would make it so that the contact point is directly below the center of the front edge of the headshell? That's a great idea. And make that edge point to the spindle. I like it!

 

[jeff spall]

Hi, yes that's right. The distance from your pivot point to the end of the headshell = effective length. Make the headshell end parallel to the line from the stylus to the spindle.
Built-in setup guage.
I came across this because the Rega arm is very nearly right as it comes.

Here's my home-mod arm stuff:
http://www.william-reed.net/audio/

regards, jeff

 

[EC8010]

Another thing that's worth considering if you're making your own arm is to position the slots in the headshell so that when the cartridge is correctly aligned the stylus tip is aligned with the central axis of the arm tube. Doing this with a unipivot means that warps cause the arm to be lifted up and down rather tending to twist.

 

[pixpop]

Would you assume that the stylus is centered between the mounting holes?

Otherwise, it would mean making the slots match a particular cartridge, right?

 

[EC8010]

Yes, the stylus is centred between the mounting holes, but when you angle the cartridge, the stylus tip swings away from the axis, so you need to move the slots to one side. In general, the distance from stylus tip to mounting hole axis tends to be about 0.35", so if you put that tip on the axis of the tube then mark out the mounting holes you find that the headshell tends to be asymmetric. I posted a thread about my arm somewhere on this forum, but I can't quite remember where. It shows how the headshell should be done (in my opinion, of course).

 

[pixpop]

In general, the distance from stylus tip to mounting hole axis tends to be about 0.35"

But aren't the mounting holes 0.5" apart? So if the stylus was centered between them, the distance would be exactly 0.25" ?

Obviously, I'm confused about something

Oh wait... you're talking about the axis that runs through both mounting holes, across the cartridge. So this 0.35" is a good value to use then. I should make my slots so that the center of the slot will be 0.35" back from the stylus position.

Actually, now that I think about it, my current TT is set up the way you describe. The tonearm is straight, and the mounting slots are angled, and the contact point could be made to lie on the arm axis. When I get home, I'll have to check whether it actually is.

 

[EC8010]

Yup - you've got it.

 

[jeff spall]

Hi,
would I be right to think that the stylus-on-centreline would only apply to a unipivot arm? It seems very good advice for a unipivot. I guess the theoretically *perfect* point might be slightly different because there's already torsion forces on the arm caused by the lateral forces on the stylus (tracking, bias etc)

On a pivoted arm, the vertical force acting on the arm is offset from the right angle centreline of the bearing axis by the same angle as the cartridge offset angle. Doesn't matter to the bearings as they still "see" the load exactly the same and the the torsion forces on the arm are negligible compared to its stiffness.

The more I look into this stuff, the more my head hurts.

regards, jeff

 

[phn]

Currently I'm thinking about tracking error and how I will handle it. I have a few ideas, but wanted to know if there are any standards concerning the mechanical design of cartridges.

47 Laboratory handles it best. It simply ignores the tracking and anti-skating mechanism and focuses on the things that matter -- attack, imaging and overall musicality. Is said to blow the several times more expensive SME V clear out of the tub.