general tonearm design question

Nsgarch is correct on all counts (except the mud). Here's an example I often use which seems to help people visualize the stylus azimuth conditions he described.

Sit in a desk chair or rocking chair that can tilt fore and aft. Face directly forward. Extend one leg out in front so your lower leg descends to the floor at an acute angle, with your heel resting on the floor.

Holding your leg in that position, rock the chair back and forth while watching the arc described by your heel. If you're facing directly forward (perpendicular to the chair's axis of rotation) the heel moves straight vertically up into the air and back down.

Your heel is the stylus, your lower leg is the cantilever, the chair is the tonearm. When the tonearm's axis of vertical rotation is perpendicular to the cantilever, vertical arm rotations (as over warps) do not change the azimuth of the stylus.

Now scootch yourself around so you're sitting at a 20-25 degree angle to the chair. Extend your leg again and rock the chair as before. Observe the change in angle between your heel (stylus) and the ground (record). The sides of your "stylus" no longer remain vertical.

When the arm's axis of rotation is not perpendicular to the cantilever, the stylus moves through an arc and its azimuth changes relative to the record. This is the case on all tonearms with offset headshells whose vertical bearings are not offset at the same angle.

When azimuth alters with every change between record surface height and arm height (as over warps or with different VTA settings) the results will include variable crosstalk, muddy imaging and potentially uneven record and stylus wear.

Why build an arm with the vertical bearing axis perpendicular to the armtube rather than the cantilever? Because it's cheaper to design and machine at right angles. The design is inherently flawed, but when building to a price point manufacturers must make compromises. This is one they sometimes choose.

The OL design doesn't make sense to me either. I owned a Silver. It was an excellent arm for the money, really excellent, but it didn't match top level arms for clarity and resolution.

Personally I wouldn't spend $5K (or even $2K) for an arm with this flaw, since the sonic effects are of a type we're sensitive to. It's possible *some* rock listeners wouldn't care, since there are many rock LP's which cannot support too much clarity and resolution. With classical, OTOH, the more resolution the better. I just remounted my UNIverse last night after checking out a friend's cartridge in our system. The tiniest azimuth change I can manage on the TriPlanar's really excellent adjustor is instantly audible. Knowing what's possible, we wouldn't be happy settling for less.

I'm not really sure what happens to unipivot arms when the stylus hits a bump. Anyone?

Most unipivots, it seems to me, rely heavily on stylus/groovewall pressure to help stabilize the arm in the azimuth plane. After all, they're quite "floppy" until the stylus hits the groove.

This has always seemed like a serious design flaw to me. Our poor stylus, cantilever and suspension have plenty to do following groove modulations without having to help stabilize a structure that outmasses them by 100x or more. (Paul and I both think this is a major reason we dislike a certain arm that I won't offend SS by naming. Let's not go there again!)

Someone once tried to convince me that the azimuth rocking of an unstabilized unipivot keeps the stylus in perfect azimuth relative to the changing groove angle on warped LP's. That's nonsense of course. It would be true only if the stylus were capable of rocking the arm to match the groove angle with ZERO time lag and ZERO overshoot, which is patently impossible. The mass of the arm and the compliance of the suspension will always create a delay between the input of uneven groovewall pressures and the readjustment of the arm angle. By the time the arm reacts the warp would have changed the groove angle, you're always behind, and then the arm would overshoot. Adding outrigger weights would not necessarily help, and it could easily make matters worse depending on warp angle, warp frequency, cartridge compliance and the rotational inertia of the arm. That's too many uncontrollable variables. Stability may be a compromise, but it's the best sounding compromise to our ears.

The Basis Vector and Graham Phantom are exceptions among unipivots of course. Each stabilizes itself, in its own way, leaving the stylus/cantilever/suspension to do their primary job. I think this is a major contributor to their increased clarity vs. their unstabilized cousins. The Phantom's non-contact method of stabilization seems especially ingenious, since it preserves the design advantages of a unipivot while eliminating the disadvantages. Kudo's to Mr. Graham on that one.

Signed,
Anyone...

Doug. Are you saying the azimuth on a uni changes each time you adjust the VTA?

No Pauly, I didn't mean to imply that at all. Sorry if it came through that way.

I was suggesting that IF an unstabilized unipivot relies on stylus/groovewall and cantilever/suspension pressures to help maintain steady azimuth, then:

a) this could make it more difficult for the stylus/cantilever to respond freely to subtle groove modulations and;

b) this would tend to make for constantly changing azimuth on WARPED records. If azimuth depends on the angle of the groove walls, and if that angle changes due to a warp, then... obvious conclusion.

Point (b) is virtually insignificant IMO. An arm with fixed bearings will maintain stable azimuth relative to the platter, but if the record is warped the azimuth relationship of stylus to groove will change because the groove is changing. I only mentioned it to counter the (silly) argument that someone once offered that a unipivot could compensate for warps by changing azimuth in real time.

Of course we should all have SirSpeedy's vacuum platter. Then point (b) would be non-existent!

SirSpeedy,

Glad to hear you're enjoying the EP units. Have you tried the EP-15a without the Ultra-Pure? I'd be curious to know how much additional benefit you hear from the balanced power.

BTW, upgrading the power cord from wall to EP-15 also makes a significant improvement. So does putting it on Rollerblocks. Strongly recommend you try both if you haven't.

Meanwhile, about the "Wobbly" Theorem:

I honestly don't know of "any" unipivot that relies on stylus/groovewall and cantilever/suspension pressure to help maintain steady azimuth,or balancing.That,alone,would be poor design,...

Well, I'm glad you said it first! ;-) But if there wasn't a problem (aka, opportunity for improvement), why did AJ Conti and Bob Graham go to such trouble and expense to give the Vector and Phantom additional stability? You've heard the benefits on the Phantom yourself: clearer, more resolving, less muddy, etc.

...and would surely put a "TON" of pressure on the stylus/suspension

Well, not quite that much. ;-) With a vacuum platter holding the record surface flat, the only impetus for an arm to roll comes from groove transients, which are pretty small events. The pressures being resisted are only significant to a mechanism as tiny and delicate as a phono cartridge. It might not matter to a low resolution cartridge, but to an Orpheus, UNIverse, XV-1S, etc., everything matters.

Who knows?

I admit my little theory is just a hypothesis. But consider the available evidence:
- a unipivot wobbles freely until the stylus touches down
- once the stylus is under full VTF, the arm becomes more stable
- nothing in the tonearm has changed to provide this stability
What other mechanism can we posit for the system's sudden increase in stability than assistance from stylus/groovewall and cantilever/suspension pressures? What else has changed?

Let's do what real scientists do with a hypothesis. Devise a test to prove/disprove it. (I'd do it but I don't have a unipivot lying about, so you're nominated if you're willing.)

1) With the armtube on the cueing support, nudge the finger lift upward with a given amount of force. "Measure" the arm's freedom to wobble by visually noting the amplitude of angular deflection and the length of time/number of cycles it takes to return to stability. Repeat a few times until you're confident you're applying the nudge consistently.

2) Cue the stylus down onto a blank (ungrooved) section of a non-spinning LP. Repeat the nudge test using the same upward force. Note the results.

3) Cue the stylus down into a groove on a non-spinning LP. Repeat as above.

Let us know how the arm's freedom to wobble in response to a given impetus changes under each condition. If my hypothesis has merit, the freedom to wobble will be greatest when the stylus is off the record, less when on the record and least when in a groove.

Doug

Hah! Great idea! Mozart wrote a piece called 'Notturno for 4 orchestras'. I have a London blueback copy in stereo, but I'm sure it would sound more like what he had in mind in quad. Surround sound ain't exactly new! ;-)