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The Korf Blog

The inside story: our research,
development and opinions

3 November 2021
The Korf Tonearm in More Detail
The pre-production prototype of the Korf tonearm is ready. I thought it would be good to have one page that has all the data on the new tonearm. Here it is.

The first production batch of the new tonearms is sold out. The preorders for the next series will open in Q2 2022. If you want one, please subscribe to our mailing list — our subscribers are always the first to know, plus they get discounts and offers that are not open to the public.

Specifications
Plinth cutout for Linn/JIS type mount. There's a 28mm diameter hole, and 3 mounting screws around it on a 38 mm circle. Center of the main hole should be exactly 214 mm from the spindle axis.
Plinth cutout for SME type mount allows for a lot more flexibility in placement. Most existing cutouts can be used. If the longer axis of the oval cutout points at the spindle, SME type mount allows for +/-20 mm adjustment in pivot to spindle distance.

What Makes Our Tonearm Different?
From the very beginning, we focused on 3 key things:
Strength

High rigidity is the single best prediction of great audio performance in tonearms and headshells.

Our measurements consistently support that.
Low resonances
(but no elastic damping)
Tonearm's resonances can reach up to 0.5G! They are clearly audible. Removing resonances removes distortion, coloration, ringing and sibilance.

Elastic damping doesn't magically "erase" resonances. It stores, transforms and releases the energy back, killing the sonics.
Quality of Motion
The bearings ring. The bearings chatter. The bearings stick and then slap. This is heard as grain, and impairs the tracking performance.

We designed our tonearm around radically different bearings. How different? A video is surely worth a 1000 words.
This led us to completely rethink tonearm design:
Flexure Pivot Horizontal Bearing

Rather than the more usual rolling bearing, we are using a monolithic flexure pivot. It has some amazing advantages:

  • Zero starting torque/no stiction
  • No bearing chatter
  • Practically zero hysteresis
  • Lasts forever — no servicing, no lubrication
  • Not influenced by temperature/humidity/dust/age
  • Easily replaceable if broken

What does it sound like?

  • Smooth! Once you experience a tonearm that is free of bearing chatter, you will recognize the chatter's sonic signature in most other designs.
  • Less inner groove distortion
  • Great tracking of warps
Why nobody else uses it?

ERA, Etudes et Recherches Acoustiques, built a whole series of turntables with flexure pivot tonearms back in the 1970s. AMG uses two wires essentially in flexure pivot configuration in their tonearms.

Flexure pivots are not mainstream because 1) they are expensive and 2) you can't just replace rolling bearings with a flexure pivot. A whole new tonearm must be designed.


Steel Arm Tube

Based on a large study of arm tube materials, we chose a steel tube with 0.5mm walls.

  • High bending resistance: 3 times higher than aluminium, 2 times higher than titanium of the same mass
  • Low energy storage
What does it bring to the sound?

  • Clarity
  • Tonal correctness
  • Dynamics
  • Resolution
  • Bass definition
Why isn't steel used more often?

Early SME 3009s had steel arm tubes. Some Fidelity Research tonearms had them too. Today, most tonearms are built too light because of poor understanding of low frequency tonearm behaviour.

Steel tube cannot be retrofitted to an existing design. Its high strength places demands on other parts of the tonearm. For example, if the bearings chatter, they would chatter a lot more with the steel armtube. The tonearm must be designed around the steel armtube.

Widely Spaced Vertical Hybrid Ceramic Bearings

We did a study of vertical bearing's influence on sound, and chose hybrid ceramic rolling bearings for the task:

  • Better strength and precision
  • Less chatter compared to closely spaced bearings
  • Low starting torque/stiction
  • Need no servicing
  • Need no lubrication

What do they sound like?

  • Better resolution
  • Sharper dynamics
  • Less sibilance
  • Less inner groove distortion
What does everybody else use?

All-steel bearings are much cheaper. Still, some better manufacturers do use hybrid or fully ceramic bearings.

Placing the bearings far apart can be a bit of an engineering challenge.


Clamping Plinth Mounts

For tightest attachment possible. They give:

  • Better mounting precision
  • Ease of adjustment
  • Better plinth coupling

Why nobody else designs the mounts like this?
Some do (SAT, SME). Generally, clamping mounts are more expensive than traditional ones. And, for reasons I entirely fail to comprehend, many engineers adore set screws.

Magnetic Antiskating

Magnetic antiskating has zero stiction. It would be a real shame to spend so much effort (and money) on those beautiful bearings, and then ruin their performance with a weight on a pulley. Or with a sliding lever.

Does anybody else use it?
Yes, many do. It's a sensible approach. Unfortunately, most are uncalibrated or calibrated poorly. Many apply the force that varies a lot between outer and inner grooves.

Needless to say, our magnetic antiskating is calibrated and uniform.



Custom Headshell Connector

Why not use a standard one? It's made out of wrong materials (usually all aluminium). It doesn't sit tight enough in the armtube.

So we had to design our own. It is press-fitted into an armtube, and preserves the mechanical performance of both the headshell and the armtube.

Why are other manufacturers happy with standard ones?
I don't know. Maybe they value the ability to adjust azimuth at the headshell more than performance?

Easy to Set Up and Use
High performance tonearms are often hard to set up and require tools to do so. We tried to make ours as easy to use as any.
Tonearm lift
  • Industry Standard soft dampened
  • Height adjustable with a set screw
Vertical adjustment
  • Column clamp is tightened and released with 3mm hex key
  • VTA screw for repeatable adjustment (can be removed when not needed)
Antiskating setting wheel
  • With numbers that correspond to downforce setting.
High performance headshell connector
  • Clockwise to tighten
  • Counterclockwise to release



Azimuth adjustment
  • Unscrew to adjust, tighten to set.
  • There's an azimuth marker and scale
Screw-on counterweight
  • Rotate to adjust downforce
  • No tools needed. No set screws, no thumb screws, no tight O-rings.
  • Helpful 0.5 gram (quarter turn) marks.
  • Additional counterweight simply slides in (or out).

Measurements
Here's the vibrometry of V4 (pre-production series prototype) compared to V3 and V2.

The accelerometer is attached to the top front of the headshell, and the vertically modulated 0 to 20 kHz sweep recording is played. The output from the accelerometer is conditioned, amplified and converted to g. We're keeping our tradition of using the Audio Technica AT7V cartridge for the vibration measurements.

We have largely preserved the excellent measured performance of V3.

Here's a comparison with the "standard" removeable headshell tonearm, the Jelco SA-750D/Jelco HS-25 combination. We have switched to a lower noise DAQ a few months ago, and this is why the Korf baseline is so far below the Jelco one. Other than that, I don't think it needs any comments.

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