Burne Jones vintage tangential tonearm

[troglodyto]

Hy!

I'm currently looking to rebuilt & instal a vintage Burne Jones tangential tonearm.
Here are some pictures : http://www.diyaudio.com/forums/analogue-source/165878-angling-90-tangential-pivot-tonearms-34.html

Problem: I don't have any template, and I've no idea about the proper way to calculate the mounting distance...

Any idea?

Thank's!

[duficity]

typically, the stylus overhangs the center spindle by about 15mm. So, measure your stylus to pivot distance, subtract 15mm and you will have your spindle to pivot distance.

[dtut]

Very interesting arm and since it's a tangential arm, if you mount it so the stylus is tangent to the first groove and perpendicular to a line drawn from that point to the spindle, the stylus should be tangential to the groove and perpendicular to the radius at any other point across the record.

Doug Tuthill

[ripblade]

Very interesting arm and since it's a tangential arm, if you mount it so the stylus is tangent to the first groove and perpendicular to a line drawn from that point to the spindle, the stylus should be tangential to the groove and perpendicular to the radius at any other point across the record.

I would say this is very close to the correct positioning, but since the arm still pivots in an arc and has apparently little or no offset, the arm likely uses underhang similar to the RS Labs unipivot, and some DJ arms used for scratching. If this is the case, try an underhang of 19mm (or add 19mm to the stylus to pivot distance) to see if this works.

The stylus should align with a theoretical null somewhere in the middle of the playing area, with the shell appearing straight inline with the wand(s).

[vinylrayk]

I would say this is very close to the correct positioning, but since the arm still pivots in an arc and has apparently little or no offset, the arm likely uses underhang similar to the RS Labs unipivot, and some DJ arms used for scratching. If this is the case, try an underhang of 19mm (or add 19mm to the stylus to pivot distance) to see if this works.

The stylus should align with a theoretical null somewhere in the middle of the playing area, with the shell appearing straight inline with the wand(s).

I think there are several things going on here at once. The arm actually does have an offset angle, it just varies depending on where it is on the record. I would expect the offset angle to be zero somewhere near the midpoint, increasing positive offset angle towards the outer groove, and increasing negative offset angle towards the runout groove. This would also make for some interesting skating effects: inward at the outer groove, zero near the center, and outward at the inner groove.

In principle, the articulation is supposed to give us an error null across the entire surface of the record, not just at one point. That's the whole objective of the articulation anyway, isn't it? It does seem that this arm, when optimized, would end up with an underhang at the spindle instead of an overhang.

[nat]

I don't think error angle would affect skating. I think it's simply a matter of the difference between the tangential force of the movement of the record, and the less than tangential angle of a line from arm pivot to stylus tip compared to the line from center of record to stylus tip. Linear arms don't need antiskating because the (theoretical) angle of the arm is always tangetial, but all pivoted arms produce skating, regardless of whether antiskating is a good thing or not in terms of sound.
I think with the B&J that friction, mass, and resonances are more likely to be an issue than skating is.

[vinylrayk]

I don't think error angle would affect skating. I think it's simply a matter of the difference between the tangential force of the movement of the record, and the less than tangential angle of a line from arm pivot to stylus tip compared to the line from center of record to stylus tip. Linear arms don't need antiskating because the (theoretical) angle of the arm is always tangetial, but all pivoted arms produce skating, regardless of whether antiskating is a good thing or not in terms of sound.
I think with the B&J that friction, mass, and resonances are more likely to be an issue than skating is.

I didn't say that the error angle affects the skating, I said that the B-J has a variable offset angle, and it is an offset angle that causes skating, as it does in pivoted arms. Otherwise I agree with your description of the forces not lining up with the pivot, and the comments on the friction and mass issues. I am a big linear arm fan.

[ripblade]

Linear arms don't need antiskating because the (theoretical) angle of the arm is always tangetial, but all pivoted arms produce skating, regardless of whether antiskating is a good thing or not in terms of sound.

Yes, but underhung arms w/o offset offer a degree of skating reduced to a point it can be disregarded....it will even behave as a true tangential at one point in the playing area.

The concept behind the RS Labs unipivot considers the large variability of the dynamic skating force coupled with the futility of precisely nulling it using a constant side bias produces a large enough compromise in tracking ability that a slightly wider range of distortion is relatively benign in comparison.

[vinylrayk]

Yes, but underhung arms w/o offset offer a degree of skating reduced to a point it can be disregarded....it will even behave as a true tangential at one point in the playing area.

The concept behind the RS Labs unipivot considers the large variability of the dynamic skating force coupled with the futility of precisely nulling it using a constant side bias produces a large enough compromise in tracking ability that a slightly wider range of distortion is relatively benign in comparison.

What I think I'm hearing you say is that, the tracking angle error in a straight arm without offset produces less distortion than the residual side-thrust left over from sub-optimal anti-skating bias. The concept also implies that the distortion from an elliptical or microline stylus being misaligned at a significant tracking angle error is an acceptable compromise.

I don't think the general VE jury will give you a favorable verdict on that one.

[ripblade]

Perhaps. It may be the swivelling headshell that is a unique feature of the arm is intended to alleviate the worst aspects of misalignment.

By all accounts it's a very good sounding arm. I have one but haven't set it up yet as it's precarious nature scares the hell out of me

[troglodyto]

Thank's for all these informations!
However, as I'm only a beginner, I'm a little bit lost by differing answers as these ones:

typically, the stylus overhangs the center spindle by about 15mm. So, measure your stylus to pivot distance, subtract 15mm and you will have your spindle to pivot distance.

and

the arm likely uses underhang similar to the RS Labs unipivot, and some DJ arms used for scratching. If this is the case, try an underhang of 19mm (or add 19mm to the stylus to pivot distance) to see if this works.

--> stylus to pivot distance -15mm OR +19mm???
What to chose?

[ripblade]

Depends on the effective length of the arm, but assuming zero offset it will be underhang. Take a look at the following drawing:

zero offset underhang.jpg (195.79 KiB) Viewed 1328 times

Using Armtrack.xls to balance the tracking error across the playing surface I determined the null to be at approximately 93 mm. Drawing the arc trace of a 229 mm arm from this point the underhang becomes 18.16 mm.

This is only an exercise if the BJ arm does in fact have an offset at the shell, and is other than 229 mm effective length, but it clearly shows the relationship between zero offset and overhang.

Can you confirm an offset angle (if any) and the length of the arm?

edit: Balancing peak distortion instead of peak error, the null moves to 75mm and the underhang becomes 12 mm. These numbers are probably the safer bet.

[dtut]

Look at how the arm is built. Look at the geometry. There are four pivots, two unequal length arms, and a "head shell" that is part of the mechanics creating the geometry. If the stylus transcribes an arc across the record, it is very shallow or complex, or both and the "pivot point" will be a geometric product and well behind the support - in other words, a very long effective length, but if this is in fact a tangent arm, concepts like "under hang, over hang, offset," and even, to some extent, "pivot point" are moot.

The OP can establish this for himself by cutting a strip from an old calendar page so that there is a long line down the center of the strip with perpendicular lines at even spacings and putting a hole at one end centered on the long line which will fit over the spindle. Then he can put the stylus down on the long line at one of the cross lines at any point on the record and temporarily place the support so that the stylus/cart is tangent and perpendicular. Once that's done, he can move the stylus/cart to any other point - here's the tricky part - the strip may have be rotated to meet the stylus/cart, but the stylus/cart should still be tangent and perpendicular. I think I would start with the arms as parallel as possible, the cart carrier directly in front of the support, and the stylus in the middle of the record. Misalignment can be dealt with by careful repositioning or twisting of the support.

This is actually pretty easy and very understandable with pictures, but I've never figured out how to post pictures on VE.

I also think it might be worthwhile for the OP to ask his question on the "Angling for 90 degrees" thread on DIYaudio Analogue where the original pictures came from.

Doug Tuthill

[vinylrayk]

Look at how the arm is built. Look at the geometry. There are four pivots, two unequal length arms, and a "head shell" that is part of the mechanics creating the geometry. If the stylus transcribes an arc across the record, it is very shallow or complex, or both and the "pivot point" will be a geometric product and well behind the support - in other words, a very long effective length, but if this is in fact a tangent arm, concepts like "under hang, over hang, offset," and even, to some extent, "pivot point" are moot.

The OP can establish this for himself by cutting a strip from an old calendar page so that there is a long line down the center of the strip with perpendicular lines at even spacings and putting a hole at one end centered on the long line which will fit over the spindle. Then he can put the stylus down on the long line at one of the cross lines at any point on the record and temporarily place the support so that the stylus/cart is tangent and perpendicular. Once that's done, he can move the stylus/cart to any other point - here's the tricky part - the strip may have be rotated to meet the stylus/cart, but the stylus/cart should still be tangent and perpendicular. I think I would start with the arms as parallel as possible, the cart carrier directly in front of the support, and the stylus in the middle of the record. Misalignment can be dealt with by careful repositioning or twisting of the support.

This is actually pretty easy and very understandable with pictures, but I've never figured out how to post pictures on VE.

I also think it might be worthwhile for the OP to ask his question on the "Angling for 90 degrees" thread on DIYaudio Analogue where the original pictures came from.

Doug Tuthill

Well said! I, too, think an empirical approach is the most straightforward. I was able to get a copy of the original B-J patent and the patent drawing:



I then traced the B-J profile and scaled it best I could as a cad drawing on my computer. Having done that, I then manipulated it into 3 positions on the record surface:



The base of the overall arm does not rotate, but only pivots up and down. It's the 2 arms that pivot horizontally. The concepts of overhang and underhang really don't apply to an articulated arm like this. It DOES sort of have an offset angle, but it changes as it moves across the record so as to maintain tangency. It would be very interesting to see how accurately it can do that. The sight lines are perpendicular to the headshell at my best guess as to where the stylus would be in the headshell. The lines all pass pretty close through the spindle center, which is an indication of tangency. The patent defines 6 mathematical equations to describe the geometry of the arm movement, but I haven't tackled those yet.

Troglodyto: If you could precisely measure the distances between the 4 pivot points on the arm and the distance of the stylus from the front pivot points behind the headshell, I may be able to make a model accurate enough to plot on paper as a full size template.