ANGLING FOR 90° - tangential pivot tonearms

[Conrad Hoffman]

Sorry, my dial-up can't deal with so many large images! If you draw a line, or at least a vertical plane, through the stylus/cartridge and it doesn't intersect the arm pivot, you've got forces you don't want. For example, I believe the Zero 100 generates skating force. Most of the arms I saw above don't appear to meet that criteria either. The problems of making decent tiny bearings for a mechanically compensated design are overwhelming (I work with that sort of thing every day), and dragging a big mass around isn't a starter for me, thus my view on driving the pivot support in a track. Yes, it's a bit like the old Rabco, but IMO those designs didn't have a good pivoted arm to begin with. Obviously you want a straight arm and I wouldn't bother with trying to use what's available, as making an excellent straight arm isn't overly difficult. Again, look at what people have built in terms of uni-pivots. If you own a small lathe, any design is within your grasp.

CH

[directdriver]

.

More interesting quasi-tangential pivot arm, this time with strings.

.



.



.

[directdriver]

.

One of the earliest patents that are conscious of the Thales circle in designing a tangential tonearm.

http://www.freepatentsonline.com/2516565.pdf - Patent by Guy



.

[Conrad Hoffman]

I love a neat mechanism and that one's quite ingenious. Still, to beat my dead horse, if the arm pivot is fixed, and if the cartridge tracks tangentially, skating forces will be generated at all but one point. Ain't no escape from that simple geometry problem via mechanical means, at least that I can think of. IMHO, because of dynamic disturbances like record warp and off center spindle holes, any skating force (and its compensation, if used) sets off cantilever and arm motions (or maybe cantilever offsets to the coils) that are more serious detractors of sound than the tracing error we're trying to correct. FWIW, though people find it an interesting curiosity today, I remember when the Zero 100 was in production, knowledgeable people had a very low opinion of its actual performance. Recently, I designed an air bearing arm system and was nearly ready to fabricate it. Though I was able to get the mass quite low, in the final analysis I didn't see any way for it to properly handle a high compliance cartridge and real world (read imperfect) records. Thus my interest in alternate methods.

CH

[directdriver]

if the arm pivot is fixed, and if the cartridge tracks tangentially, skating forces will be generated at all but one point. Ain't no escape from that simple geometry problem via mechanical means

I agree with many things you said. First, on this thread I am trying to make it the most comprehensive gathering of information on this genre and to generate an interest or at least an awareness of the possibility of such device. Personally it's really an intellectual exercise because I already have enough toys to play with in my own audio system, including some servo linear trackers.

I'm glad that you pointed out about the issue when the arm pivot is fixed. Even though the many examples I posted are mostly about pivoting at the headshell, the solution and the idea I have in mind is to move the base, not the headshell. Again, if you look the Thales circle, I want to make the arm to be B to C, so there is no need for offset angle at the headshell at all. I want to separate vertical bearing independent of the horizontal bearing. What I need to do is to come up with a mechanism at the arm base area that allows the arm to swing and shift front and back on a track to create a virtual that can extend and shorten according to the swing motion.

Imagine the Dynavector arm without the offset angle, then you might get an idea of what I am talking about. The Dyna arm has two sections, one arm is purely for horizontal movement and at the end of that is another extended arm purely for vertical movement. Now, just think along the same line in a tangential arm that has a mechanism that allows for guiding. Imagine the effective length is 10" and the actual arm for vertical is about 7" so the track is only 3". After all, a servo motorized arm will require a track anyway but why not take advantage of pivot motion that can inherently allow for guiding the front and back motion. Imagine the Clearaudio Satisfy arm without offset angle at the headshell, and its base is on two rolling ball bearings on a track that pivots and when the arm swings to the spindle the arm base shift forward on the track maintains a Thales triangle inside a semi-circle. I might need to spend some time on some drawings to illustrate my point.

Again, I agree that skating force due to offset is bad for sound. The more I can minimize or eliminate it the better. Thanks for indulging me. I look forward to owning a small lathe one day.

P.S. Sorry to hear that you had to abandon your air-bearing project. Perhaps you might look into the Cantus tonearm in another thread that might inspire you to try a mechanical linear tracker.

[directdriver]

.
Found a rare image of the actual Van Eps tonearm. very cool looking but looks massive.

.



.

[directdriver]

Conrad Hoffman: "keep the standard tonearm, remove the offset and make it a bit longer. Now, just drive the pivot assembly along the back of the turntable in a track. Servo it, probably photoelectrically, but don't try for perfection. Let the tonearm do what it was designed to do. Try for silence and smoothness on the drive. I'm not sure what the best drive method would be, maybe a lead screw and DC motor, maybe a thin thread and drum drive or even a linear motor track. This is basically an electronic forum, so moving the pivot under motor control shouldn't be too difficult."

Looks like someone in 1981 Wireless World was thinking along similar lines. I wish I can find the complete article.

.

[max426]

DD,
This is very interesting to me. Thanks for this thread.
I don't like the idea of an air pump either.

I need to do some more research on these mechanical issues with this type of arm, will post some drawings after some more thinking.

Here is a link to a design not covered in your thread yet.
Equipment for playing gramophone records and method of operation thereof

have you seen this before and what do you think about it?

I am currently working on a diy tt design,even though I don't have a clue about all the technical aspects of such a project. It will be quite some time until anything is realized.

jim

[directdriver]

max426: "Here is a link to a design not covered in your thread yet. www.freepatentsonline.com/4344168 have you seen this before and what do you think about it?"

I never saw that one before. Fascinating! I still can't decipher all the idea from the drawings yet. I need to read further but looks like instead of pivoting the headshell, it's pivoting and moving the armbase. I think it's on the right track similar to what I have in mind. Thanks for alerting me this design.

What I have in mind recently is along similar line. If you look at the Thales semi-circle, the headshell need not to be offset if we can change the effective length simultaneously moving across the arc. In any pivot arm, the swinging is a radius of a circle and we don't want to do that. If we can come up with a mechanism that change the length the cartridge can remain tangent drawing a line to the spindle. Let's say we have an armature that's 10" from the mounting to the spindle to form an 10" effective length arm but the armwand that carries the cartridge is only about 7" so we have about 3" sliding to do and this can be achieve via guiding rails for forward motion and swinging motion in essentially a tricycle platform where the armtube sits. Please look at the crude drawing and I hope it gets the idea across.

It might look complicated by it's actually quite simple. One can implement all the pivot arm concepts we learn in the past into this and the tricky part is the linear motion bearing, the tricycle mechanism. I am constantly thinking ways to reduce parts count and mass. The advantage of this over the Thales tonearm is that there's no pivoting headshell and no bearing near the cartridge, no vibration. Geometrically less ambiguous. Low vertical mass since the arm is only 7" long. Horizontal mass is a touch higher than typical pivot arm and it's similar to the Dynavector. The downside is that we have to rely on linear bearing which can result in high friction. An all pivot approach is doable if we use a parallelogram but that requires many pivot bearings and linkages - I will have to think about that for another project. Since I don't have a small lathe for me to experiment with physical objects, I just have to continue to headbang myself. Sometimes it hurts my brain, but as a masochist it can be quite fun.

Thank you for your interest.

.

[max426]

I think I follow. The pivot at the red arrow is stationary and at the blue area follows the slot or linear bearing as the arm moves across the record?

jim