Korf

3 Key Areas of Tonearm Design

In an analog system, a tonearm has a critical role to play. An electrical signal created by the cartridge has to be carefully transported some physical distance over the mechanical system of the turntable to the cabling that transports it to an amplifier. At that point in time, the signal is weak and contains all the vital musical information. During this critical phase, if further vibrational energies (resonance) or electrical interferences are introduced to the signal, the quality of the information is degraded. When setting out to design a tonearm with high-quality sonics, Korf Audio focussed on 3 critical design areas:

Strength
Research shows that high rigidity is the single best prediction of great audio performance in tonearms and headshells.

Korf measured many tonearms and their measurements consistently supported that theory.

For example, here is some measurement data on the Morch DP-6.

Low Resonances
Resonances within a tonearm can reach up to 0.5G!

They are clearly audible. Removing resonances removes distortion, coloration, ringing and sibilance.

Read here how Korf set up measure vibrations.

Read here about vibration analysis of two tonearms with an explanation of how the observed results affect the sound.

Note that Elastic Damping is not desirable. Far from ease resonances, elastic damping stores, transforms and releases the energy back, killing the sonics.

Read here about Constrained Layer Damping.

Quality of Motion
When you measure most tonearms, the bearings ring and ‘chatter’. The bearings stick, release and then slap. This is heard as ‘grain’ and impairs the tracking performance of the tonearm.

Korf designed their tonearms around a revolutionary new bearing design.

Read here about Korf’s new bearing design.

Flexure Pivot Horizontal Bearing

Rather than implement the commonly used rolling bearing, Korf designed a monolithic flexure pivot. It has some significant 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

It sounds smooth! Once you hear a tonearm that is free of bearing chatter, you will recognise the chatter’s sonic signature in most other designs. Flexure Pivot Horizontal Bearings also experience less inner groove distortion and can track warped records much better. ERA built a series of turntables with flexure pivot tonearms back in the 1970s, however they are not consistently used in the 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 Armtube

Korf chose to use steel tube with 0.5mm walls based on an extensive study of armtube materials.

Read here about the results from that study.

• The advantages of using steel tubes are: High bending resistance – 3 times higher than aluminium, 2 times higher than titanium of the same mass
• Low energy storage

Using steelarm tubes brings clarity, tonal correctness, increased dynamics, resolution and bass definition to the sound of an analog system.

Some early SME 3009’s and some Fidelity Research tonearms had steelarm tubes. Today most tonearms are built too light because of poor understanding of low frequency tonearm behaviour. A steel tube cannot be retrofitted to an existing design because its high strength and weight places demands on other parts of the tonearm. For example, if the bearings chatter, they would chatter a lot more with a steel armtube. The tonearm must be purposefully designed around the steel armtube.

Widely Spaced Vertical Hybrid Ceramic Bearings

Korf completed a study of a vertical bearing’s influence on sound and based on the results, chose hybrid ceramic rolling bearings for the task.

Using hybrid ceramic rolling bearings results in better resolution, sharper dynamics, less sibilance and less inner groove distortion.

The advantages are:

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

Clamping Plinth Mounts

Clamping Plinth Mounts give the tightest attachment possible and offer the following advantages:

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

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.

Many tonearm designers use this approach because it’s an effective and sensible design. However, most are uncalibrated or calibrated poorly. Many apply the force that varies a lot between outer and inner grooves.

The Korf magnetic antiskating is calibrated and uniform.

Custom Headshell Connector

Most standard headshell connectors are made out of the wrong materials (usually all aluminium) and don’t sit tightly in the armtube.

So Korf designed their own, which is press-fitted into an armtube, and preserves the mechanical performance of both the headshell and the armtube.

Read here about the development of the Korf Headshell Connector.

Easy to Set Up and Use

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 tigthen. Counterclockwise to release.
Azimuth adjustment	Unscrew to adjust, tighten to set. There’s an azitmuth 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. Korf are keeping their tradition of using the Audio Technica AT7V cartridge for the vibration measurements.

Korf 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. Korf 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, we don’t think it needs any comments.

Korf Audio Website