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The Korf Blog
The inside story: our research,
development and opinions
development and opinions
14 April 2017
Armtubes. Have I Missed Anything?
We are continuing our big tonearm experiment, and today I will talk about armtubes.
Theoretically, the job of armtube is an easy one. Together with a headshell, it has to firmly hold the pickup assembly in place, and deal with whatever mechanical energy the pickup cartridge transmits through its mount. This suggests that the best armtube is a shortest armtube made of the stiffest material with a highest damping capacity η (ability to dissipate energy during vibration). By the way, here is an excellent article listing η for a variety of materials.
As magnesium alloys are quite strong and have the damping capacity at least an order of magnitude higher than other comparable materials, a tonearm with a short magnesium armtube should be by far the best. Unfortunately, it isn't quite so simple.
Experience tells us that there is no direct linear relation between stiffness or damping capacity one one side and the quality of the tonearm on the other. Such theoretically unsuitable materials as steel, titanium, ceramics and wood were often used with great effect.
Theoretically, the job of armtube is an easy one. Together with a headshell, it has to firmly hold the pickup assembly in place, and deal with whatever mechanical energy the pickup cartridge transmits through its mount. This suggests that the best armtube is a shortest armtube made of the stiffest material with a highest damping capacity η (ability to dissipate energy during vibration). By the way, here is an excellent article listing η for a variety of materials.
As magnesium alloys are quite strong and have the damping capacity at least an order of magnitude higher than other comparable materials, a tonearm with a short magnesium armtube should be by far the best. Unfortunately, it isn't quite so simple.
Experience tells us that there is no direct linear relation between stiffness or damping capacity one one side and the quality of the tonearm on the other. Such theoretically unsuitable materials as steel, titanium, ceramics and wood were often used with great effect.
Why is it so? Is it because damping capacity η doesn't adequately describe the damping properties of an armtube in a typical tonearm configuration? Or does the tonearm construction influence the sound more than the choice of materials? To find out, I am going to test the following arm tubes:
Some of what I am going to test
- The default choice found in what seems like 80% of tonearms. Anodized aluminium, 10mm outside diameter. 1mm wall thickness
- Thick-wall aluminium, 10mm diameter, 2mm wall. We'll get the raw tubes and test how they sound. If time permits, we'll anodize a few and compare.
- Thin-wall steel, 10mm outside diameter. Deployed to great success in early SME 3009. Is it still the better option?
- If weight permits, a slightly thicker-walled steel tube.
- Thin-wall aluminium, undampened. Ok, I know it is probably not going to sound acceptable, but it might be a good baseline for comparison.
- Thin-wall aluminium, dampened with bonded foam. Should add a lot of rigidity too.
- Thick-wall carbon fiber, outside diameter 10mm, wall thickness 2mm.
- If time permits, thin-wall carbon fiber, as is and dampened with bonded foam.
And last but not least, wood. I've tried it before, and here is the picture of one such attempt. It was interesting and educational. All types of wood I had on hand sounded unacceptable without surface treatment. But even the thinnest layer of lacquer transformed the sound. Shellac was the best. Will wood be competitive with our metal and carbon tubes, I wonder?
Am I missing anything? Oh yes I am. There's a secret sauce I am going to try: constrained layer damping. But this warrants a whole post of its own. See you next week!
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