Analog Corner

If you're going to spend a year-plus in COVID lockdown, it doesn't hurt to have a million dollars' worth of turntables keeping you company, right? That's been my good fortune. Sounds like a roomful, but it's only three: the SAT XD1, the TechDAS Air Force Zero, and the OMA K3 ($360,000, footnote 1).

You'll find this issue's cover girl either strikingly beautiful or homely. Visitor reactions fall strongly into one of those two camps, with nothing in between. I love the looks. Whatever your opinion, the K3's visual distinctiveness cannot be denied. The innards are equally unique.

Looking somewhat like the Guggenheim Museum topped by a heliport and a construction crane, the cosmetically finalized K3 arrived, coincidentally, in the afternoon of a day in which OMA's Jonathan Weiss spent the morning at the actual Guggenheim Museum, installing an array of his OMA Fleetwood Line of loudspeakers in support of "Anthem," artist, filmmaker, and MacArthur Grant recipient Wu Tsang's new commission, conceived in collaboration with singer, composer, and transgender activist Beverly Glenn-Copeland (footnote 2).

The production K3 replaced the cosmetically unfinished but intriguing prototype installed here last fall. The two units measure and sound identical, but Stereophile does not permit prototype reviews; hence the swap—though the word "swap" is inadequate to describe the drama and difficulty involved in lowering the prototype to the ground and hoisting the final curvaceous, slippery, 200-plus pound K3 atop the HRS rack. OMA manufactures a custom stand for the turntable, but space limitations—physical space, not column space—made it impossible to include it in the review.

A backstory worth telling
Much like the gestation of the Continuum Caliburn, the K3's design development and construction resulted from an almost-seven-year international cooperative effort involving industrial-level manufacturing acumen and academic expertise. Auckland, New Zealand–based hydraulics engineer Richard Krebs led a group that also included a Bucknell University team of professors and graduate students from the departments of engineering and physics. Also on the team are tonearm designer Frank Schröder (a familiar name to most Stereophile readers), architect/industrial designer Ana Gugic, and of course Jonathan Weiss, who in 2006 founded Pennsylvania-based OMA, short for Oswalds Mill Audio.

While the company is best known for its compression-horn loudspeakers and tube electronics, OMA has from its inception built custom direct-drive turntables using Technics SP10 variant motors set in a variety of massive, well-damped plinths made mostly of Pennsylvania slate and more recently of hypoeutectic gray cast iron; I enthusiastically reviewed one of those a few years ago.

Krebs is CEO of Hyspecs Hydraulic You want "jaw drop"? Visit the site. The company specializes in hydraulic manifold technology and whaddya know? The K3 sports interior manifolds filled with a special damping fluid. Krebs recently wrote to say he's spent 40 years designing and modifying turntables. Clearly, it's his hobby and passion, guided by engineering expertise, not fanciful ideas.

In fact, Krebs reached out to me a few years ago, asking if I'd spend some time with him and his son at the High End Munich show to discuss turntable design. After perusing his website, I felt wholly inadequate, but in another email, he assured me that my experience compensated for whatever technical accreditation I lacked. We met, and I told him what I knew and believed based on experience and what I'd gleaned from others with mechanical engineering knowledge. He confirmed most if not all of my views based on his technical knowledge and experience. We were on the same page.

Mr. Krebs and Jonathan Weiss gave me separate accounts of the K3's design background, which not surprisingly included the K1 and K2 iterations. The concept derives from vintage vinyl-cutting lathes, which Weiss pointed out were "designed as very expensive tools, built on the level of extreme-precision scientific, laboratory instruments."

Both Weiss and Krebs pointed out that on both the inscription and playback sides, cutting lathes must produce accuracy on the order of 0.005 microns, or, as Weiss put it, "the scale of a large organic molecule" (footnote 3). Only direct drive, the two aver, can track and accurately reproduce a record and not be slowed down, however slightly, by the transients and dynamic contrasts presented in a record's grooves. By the time a belt system's sensors notice slowing and compensate, they claim, the change is well "past due."

The human ear registers these speed anomalies as "softness, smearing," or as an impediment to "natural, lifelike musical flow." Neither platter weight nor bearing implementation can compensate, they say. So, rather than concentrate on average speed accuracy, the goal was to maximize "moment to moment" accuracy.

In addition to rotational accuracy, Weiss added, "any movement of the tonearm components relative to the platter" will result in apparent platter speed change—something SAT's Marc Gomez pointed out to me as an issue with the Continuum Caliburn's "floating armboard," though he acknowledges the tradeoff with the floating system's unrivaled isolation. All of these concepts, and others, are typically discussed by designers of ambitious turntables. Implementation is key, as is weighing the various tradeoffs incorporated into every sound engineering plan (as opposed to harebrained scheme).

A massive, cast-then-machined plinth
Cutting lathes were almost always made of cast iron; in the case of Neumann lathes, even the 66lb platters were cast iron. All were direct drive. To learn more about cutting lathe construction, refer to pspatialaudio.com/lathes.htm.

The K3's massive cast plinth structure, the design of which was aided by finite-element analysis (FEA) performed at Bucknell, is said to be critically damped, which means no oscillation and the fastest possible approach to equilibrium.

The casting is of the aforementioned high-graphite-content hypoeutectic gray iron. The plinth, the platter, and the circular armboard all feature internal chambers (the plugs of some of which are visible around the platter and armboard periphery) filled with a mixture of a special oil and particulate matter designed to deaden vibrations and kill resonances. The platter hides a matrix of vertically oriented chambers filled with this oil-and-particulate mixture, all located based on the FEA analysis.

Vibrational energy entering the system encounters the pools of oil and a sandlike material and dissipates as heat. The plinth features few joints and material dissimilarities so as to better evacuate noise-producing energy instead of reflecting it. Three feet, one terminated in a minimal-contact metal ball, act as efficient drains for vibrational energy.

Casting these parts to form the K3 chassis, Weiss told me, required "ultrasophisticated 3D-printed sand molds" (footnote 4) involving numerous iterations and final 5-axis CNC machining at aerospace mil spec levels. Weiss says the machining and metal casting could not have been accomplished with the technology of even 10 or 15 years ago. Any machinist looking at this complex, obliquely shaped casting would immediately understand the difficulties involved in machining the post-cast chassis. Bucknell University's Small Business Development Center awarded the K3 its 2019 Product Innovation Award.

K3's ultrahigh-torque motor
The direct-drive motor is a "bespoke" 18-pole, coreless-slotless, zero-cogging design based upon rotors and stators purchased as parts (after a six-month search for the right ones) and assembled with large neodymium magnets to produce what OMA says is "the most powerful motor ever used on any turntable." OMA says it's more powerful than the Lyrec motor used on Neumann lathes. OMA also says it has developed superior motors to drive lathes, but that's a story for another column.

In this ingenious design, the stator clamps securely to the massive chassis bottom, which then becomes the motor housing. The rotor attaches to the bottom of the bearing sleeve. An unusually large-diameter (25mm, almost 1") hardened stainless steel alloy shaft bolts directly to the chassis, helping to maintain a high level of dimensional accuracy between the rotor and stator and between the platter and the armboard.

The unusually long bearing puts the powerful motor magnetics, which are located at the bottom of the platter/ bearing assembly, farther away from the cartridge than is typical on direct-drive designs.

OMA was after exceptional dynamic speed stability; the goal was a system that would perform optimally while playing a record. Krebs said that, with finesse, higher motor torque makes it easier to precisely manage speed fluctuations under dynamic conditions.

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Footnote 1: OMA/Oswalds Mill Audio, Fleetwood, PA, with a showroom in Dumbo, Brooklyn, NY. Tel: (917) 743-3789. Web: oswaldsmillaudio.com.

Footnote 2: See guggenheim.org/wu-tsang-anthem.

Footnote 3: Unless someone has made a bigger one in the last few years, the largest known molecule is PG5, which has a diameter of 10nm, or 0.01 microns.—Jim Austin

Footnote 4: Sounds weird I know, but it's an actual thing: 3D-printed sand molds.