SA-1.2 BCS



Produsent: ABIS

Pris: 25800


Beskrivelse

Abis SA1-2BCS (Black, Convertible, Silver Wiring)

The Abis SA1.2BCS is a unique alternative to the inflexible arms that make up the usual analog suspects.

 The SA1.2 already allows users to experiment with effective mass by changing the relative positions of the main counterweight, sliding VTF weight and by changing headshells.  

The SA1.2BCS offers the additional option of changing the center of gravity, by changing counterweight stubs.  Also, the arm wand is wired (internal wiring) with high quality Japanese produced ultra-soft annealed silver wire, and the supplied headshell leads are silver.

“Positive” versus “Negative” Balance

It’s difficult to come up with the right word for it, but the Abis SA1.2BCS can be configured for “negative” balance.  Another way of saying “negative” is “unstable”.  Imagine a pyramid, like The Great Pyramid of Khêops.  In its current configuration, it is absolutely positively in a positive state of balance.  It is unconditionally stable.  It won’t turn over!  Now, take the pyramid and flip it over.  It becomes unstable, or negatively balanced.  If you could keep it balanced, a stiff wind or small earthquake would make it fall over, eventually.  

One thing to remember is that all tonearms are intentionally out-of-balance, to apply tracking force.  Once VTF is added, this causes one end of the arm to fall, weather or not the arm is positively, negatively, or neutrally balanced.  The 1~3 grams of VTF at the end of a tonearm dominates the static characteristics of the tonearm at rest.  The reason to explore a negatively balanced arm is how negative balance affects the dynamic tracking characteristics of the arm, and especially how the cartridge will track warps.

The vast majority of tonearms are positively balanced.  Why?  It is because it has always been done that way (and some tonearm designs would be impossible with negative balance)! A neutrally balanced tonearm is possible, but it requires a specific cartridge in a specific offset to maintain the proper distribution of mass; change to a taller or shorter or more massive or less massive cartridge, and it will depart from neutral balance.  With the SA1.2BCS, you can convert from positive to negative balance by changing counterweight stubs.  See below:  The stub in the foreground provides for positive balance.  The stub in the background provides for negative balance.  Note:  The stubs in the picture are upside down from their regular orientation.

sa1.2bcs convertible stubs

 

Almost everyone will ask:  “Why would I want a negatively balanced arm?”  The simple answer is “warps”.  If all records were perfectly flat, it would not matter.  Because most records are slightly warped (or worse, the platter or mat isn’t flat), it becomes an issue.  With a negatively balanced arm, as the tonearm rises above the record surface, as the cartridge tracks a warp, the stylus pressure decreases.  That decrease is the opposite behavior of most arms.  Conversely, as the arm drops, stylus force increases.  Therefore, a negatively balanced tonearm actually compensates for record warps.  Instead of mistracking when the cartridge dips, the negatively balanced arm will supply slightly more tracking force.

With a positively balanced configuration, as most tonearms are, you lose tracking force as the tonearm drops, and gain tracking force as the tonearm rises.  This isn’t a great thing.  With some arms that have very low slung counterweights, the difference in tracking force can be quite dramatic, causing the stylus to lose contact with the vinyl as the tonearm drops (mistracking), and to dig into the vinyl as the tonearm rises (hard on records and cartridge suspensions).  

Someone might ask:  “If negatively balanced tonearms offer better performance, why don’t more tonearms come with negative balance, and why aren’t all Abis tonearms negatively balanced?”  The best answer is tradition.  Most designers and buyers will consider that the traditional method is best, because it’s always been done that way.  It’s the same reason American car buyers put up with oil leaks and constant maintenance until people noticed that Japanese cars ran longer between servicing, and Japanese car dealerships didn’t need pieces of cardboard to protect the showroom floor.  Oil leaks were a tradition.  

In the case of the SA1.2 tonearm, converting to negative balance will make the practice of “zero balancing” the arm a little more random, and potentially confusing for some.  Therefore, the SA1.2 and SA1.2B are shipped with the positive balance configuration.  Because it is like an upside down pyramid, it will not return to the same place when trying to set zero VTF.  It could tip forward or backward, though the effect is actually very subtle.  And since the designer of the SA1.2 provided a sliding VTF weight to make setting VTF easier, it is Abis’ policy to provide the positively balanced models for our consumers (so they can use this feature with more confidence).  Positive balance makes it easier for the user to set the zero balance, then set the VTF with the sliding weight.  It isn’t difficult to find nearly zero VTF when the arm is negatively balanced, but it will never be certain.  Regardless, whether positive or negative, you need to use a scale to fine tune the VTF.  When you change cartridges and headshells, the relative distribution of mass changes, and this affects the calibration of the sliding VTF weight.  

With the negative balance counterweight stem in place, the arm is fairly close to neutral balance when a heavy cartridge is used.  So, the center of gravity is only slightly above the axis of the vertical bearing.  Another note: this is not something a unipivot can do easily.  Unipivots require a low center of gravity to increase stability. Otherwise, a negatively balanced unipivot would simply roll over on its side, like a ship taking on water. Because gimbal bearing arms are held stable by the bearings, it allows gimbal bearing arms to exploit the goodness of being negative balance.

*I first heard it described as “negative versus positive balance” by Michael Fremer, when he was commenting about another company’s tonearm.  There are more accurate terms, but I think the description is easily understood by audiophiles.  I am sure a mechanical engineer could come up with correct terminology.  I am not a mechanical engineer, and I assume neither is Mr Fremer.

Silver

The SA1.2BCS comes with silver internal wiring, and the headshell leads are silver.  The conductivity of silver is approximately 5.4% greater than copper.  This will improve signal to noise ratio, and increase the current transfer from the cartridge to the phono stage.  Subjectively, and sometimes objectively, silver wiring will equate to higher cartridge output, or more gain.  Also, silver withstands work-hardening better than copper, which means that a silver wired tonearm will last longer, maintaining its performance.

 

Bearings and Arm Structure

Key to the performance of the SA1.2 and SA1.2B is the use of very high grade ball bearings. Bearings used for horizontal movement are axial-loaded, angular-contact, thrust-bearings.

These thrust bearings have zero play and are held in contact by gravity.  You can think of them as acting like a unipivot, but with more than one contact point.  Because they have two tapered seats, they will self-align in the presence of a load (gravity), have exceedingly low moving friction and are especially suited to angular and axial loads.  

The angular error needed to misalign these special bearings, commonly used in high precision machinery, are so severe that they fall well outside the operating conditions for a turntable (if you need an arm for playing in zero gravity, or turned at 90° to earth’s gravity, you will be forced to use a different style of tonearm).  Traditional deep-grooved ball-bearings are not specifically designed to take an axial load, being better suited for radial loads like a wheel bearing, and they suffer from higher rolling resistance.  While high quality bearing materials are a key to success, the proper choice and loading of bearings is just as important.

angular contact thrust bearing

                           Angular Contact Thrust Bearing

Also key to the performance of all tonearms, and especially the SA1.2 and SA1.2B, is the resonance characteristics of the arm “tube”.  

The SA1 design is built from four pieces of billet aluminum, milled to tight tolerances, finished, fitted by hand, and torqued to settings determined by experimental observations.  The torque values chosen introduces preload on the fasteners, and produces hysteresis stresses (basically, inner friction) in the crystalline structure of the metal.  These stresses damp vibration by forcing the crystalline structure to be more tightly bound, than when in a free state.  T

he elastic and anelastic properties of all four pieces are slightly different, preventing dominant acoustical nodes from developing. 

When properly machined, fitted and torqued, a unit composed of several milled pieces will have lower maximum resonance, with several smaller resonance frequencies, compared to an armtube composed of one piece.  These statements are backed up by scientific study into weapons systems, rockets, engines, structural elements, etc..  Resonance is a key source of failure, and the research proves the point:  a single piece of billet material can have 1~3 dominant resonance frequencies, with one resonance frequency dominating the performance of the structural member. If you think about the bars that form the notes on a xylophone, you will have a practical understanding of the limitations of “one-piece” arm tubes.  

In practice, a combination of surface finish, damping materials, shape, density, alloy, crystalline structure, fastening technique, and fastener preload, among others, will determine the relative distribution of acoustic energy in a tonearm.  

Since extremely dense (heavy) materials, like depleted Uranium, don’t easily lend themselves for use in tonearms, a holistic approach is necessary.

The rectangular shape, the tight assembly by hand, and the nature of billet aluminum creates a low-resonance design that is essentially quiet.  

The preference for 12″ arms has more to do with compliance match, than the theoretical advantages of a 12″ arm (which are nullified by the use of a spherical tip).  The SA1.2 and SA1.2B maximize the possible geometry at 9.4″, while excellent damping and high mass give it the tonal balance of 12″ arms.  

With the SA1.2 and SA1.2B, the tracking error is very low, while the superior sonic signature of the billet aluminum arm parts, and the mass equivalent to most 12″ arms, gives 12″ arm sound in a compact form.  It is ideally suited for low to medium compliance cartridges, especially those that are particularly problematic when it comes to mistracking.



Spesifikasjoner

Technical Data
Total Length: 310mm
Spindle center to arm pivot distance : 223mm
Stylus to pivot distance: 239mm
Overhang: 16mm
Offset Angle: 22.25°
Cartridge weight: 1545
grams, including
headshell
Antiskating
Device 03
grams
Net weight 750g (with headshell: 767g)
Custom made in Japan for Sibatech, Tokyo, Japan. www.sibatech.co.jp