Does any Audiogon member have a Holbo air-bearing turntable?

@lewm 

The problem you are writing about does not apply to the Holbo air bearing LT tonearm. The Holbo tonearm is an air bearing that floats on the air, has no friction and the needle is always exactly in the middle of the groove.

@lewm you stated: for some LTs the stylus travels in tiny arcs across the LP, in which case there would be a tiny skate force constantly changing in direction (toward the spindle then toward the outer grooves, back and forth).

Curious which LT arms do the above? 

@frazeur1 

getting the foundation right with the table upfront is best anyway

I asked Bostjan what his turntable is sittng on.  He told me he has a big block of Medium Density Fibreboard weighing 20-kgs which sits on four Sorbothane 50-mm hemispheres.

I got a chance to weigh one of my sandstone slabs, and my estimate of 20-kgs was wrong - try 27.5-kgs.  Two of these plus the turntable will exceed the rating of 60-kg per shelf for my SolidSteel S3-3 rack by about 7-kgs.

It was an interesting exercise with Sorbothane working out what size, shape and "durometer" to choose.  Their Engineer in Ohio suggested four 1" disks, each 1/2" thick with Durometer of 70, giving a resonance around 12-Hz.  This would be roughly where I would expect arm resonance to occur.

The local distributor had stocks of 50mm hemispheres in Durometer 50, so I have ordered eight - another A$100 down the gurgler!  My guess is the natural resonance will drop to around 3-Hz if I use them all.

My partner has had a cancer scare with one of her hemispheres, durometer unknown, which has delayed me going home and setting up the Holbo and hence delayed my report back to you.  I can say that the ordering and delivery process was seamless.  The box just arrived, no customs interaction, much quicker than I expected and exactly as Bostjan had promised.

On another thread about cartridge tracking weights, there was a reference to an Audio Technica webpage with a link to a cartridge review.  I was thrilled when I learned that it was reviewed in a Holbo Mk2

To respond to a few questions: I am aware that the Holbo air bearing arm is likely not traveling in tiny arcs across the LP, depending upon the stiffness of the air bearing to prevent that, and I assume it is stiff enough.  LT tonearms that are much more prone to do that jig across the LP surface are the servo controlled ones, like the original Rabco SL8 and copies thereof, including the Goldmund T3F and I think the Revox LT tonearm on the Revox TT.  There may be others. These rely upon a motor to pull the pivot, which is riding on a solid rail, across the LP. To activate the motor, a tiny switch is situated so as to sense when the stylus tip gets minutely ahead of the pivot. The switch makes contact and then activates the motor which pulls the pivot a minute bit ahead of the stylus and then switches off, etc. The Clearaudio LT which once had another name that escapes me, and which has virtually no arm wand, as I recall relies upon gravity to help the cartridge traverse the LP.  The platter is dished toward the center.

@lewm 

I am aware that the Holbo air bearing arm is likely not traveling in tiny arcs across the LP, depending upon the stiffness of the air bearing to prevent that, and I assume it is stiff enough

Hi Lew

I can confirm that the Holbo tone arm feels incredibly stiff.  The bearing tube, which is bolted at right angles to the tone arm tube, is quite long.  I have gently held the head-shell and the bearing tube and tried to twist and rotate the arm to no avail - could feel no movement.  All it will let me do is raise and lower the head-shell vertically and slide the whole arm assembly sideways - something that should only be done with the air on!

Mind you, I can feel no platter movement at all when I switch the air supply on and the platter lifts 10 microns.  If the drive belt is fitted, there is no perceptible delay between switch on and the platter starting to rotate.  No air, she no rotate.  That is how it is shipped!

I can also confirm that I cannot hear any sound at all from the combined electrical and air supply unit, unless I make bone contact with my skull, in a pretty quiet environment.

Any servo-controlled tangential tracking arm will move in small arcs as you describe, because there has to be an error signal to get the servo motor to kick in, and another to switch it off.  I think B & O made one.

From my understanding, at least some Clearaudio tangential arms are carried on gantries above the record, which have to swivel out of the way.  The arm is mechanically supported on a glass tube. 

The Holbo tone arm is long enough to completely clear the record when in its home position and there is no mechanical connection once it is lowered except a tiny, soft air tube, less that 2-mm diameter, and four silver Litz wires.