The Korf Blog

The inside story: our research, development and opinions

21 March 2018
A Tale of Three Counterweights
As you might remember from our tonearm wand experiment, the counterweight design we chose wasn't really good. I basically recreated Ortofon AS-212's decoupled stub without any decoupling, and it resonated a lot. I suspected this was one of the main reasons the performance of our arms wasn't as good as it might have been.
A counterweight from a 1970s Telefunken tonearm
I've taken Prototype 5 (medium thick-walled steel tube), as it is the one I find more pleasant to listen to compared to others.

First, I've decided to try 2 different counterweights. A lighter one from an AS-212 (95 grams), and a heavier one from some Telefunken all-in-one music center (115 grams). I've attached the accelerometer to the end of the stub with a heavy one, and to the top of the lighter counterweight.
A counterweight from a Lenco L50 on a cut-off steel bolt stub
Then I cut off the sad aluminium stub, drilled a hole where it was and cut an M8 thread in the hole. This allowed me to screw in a mild steel Ø8mm rod. I've used a 125 gram counterweight from a 1960s Lenco L50 tonearm. The accelerometer went on the end of the rod.

Below are the vibrometry results of all 3 counterweight configurations.
Please note that the vertical scale is different to other charts. As the counterweight stub is shorter than the main armtube, the displacement produced by vibration is smaller.

First thing I see is that headshell vibrations don't enter the counterweight. When measuring the Ortofon counterweight, I attached the accelerometer to the weight itself rather than the stub. You can see that result is basically the same as with heavier Telefunken, except for some residual ringing from the headshell.

And, of course, the steel stub makes a world of difference. The main counterweight stub mode is all but gone. But will it reflect in the normal measurement taken at headshell? I have revisited our old Prototype 5 measurements and compared them to the same armtube measured in our chopped-off counterweight arm.
There are some easily explained changes here, and some more challenging ones. The main counterweight stub mode at 800 Hz is clearly gone, and its 1600 and 2400 Hz 2nd and 3rd harmonics are much lower.

The bearings resonances are lower as this particular prototype uses a better tighter bearing. It's amazing what difference better tolerance balls make.

This leaves us with 2 unexplained artifacts. First, the much higher headshell resonance. I've used a different Picomin 22 this time, and it might be that its behaviour close to its own 11 kHz resonance is different. But I suspect it's not the only reason.

And second is the main armtube mode. It's a bit more pronounced, and this change can't be explained away by different calibration or measurement error. It looks like increasing the rigidity of whole tonearm has a side effect of making main armtube resonance more pronounced.

Today's measurements gave us some answers and 2 good questions. I'll try to formulate an experiment to understand why the main resonant mode becomes stronger with stiffer tonearm construction. As for the headshell, I'm afraid the resonance frequencies of the sensors that I have do not allow for thorough investigation.

One option is getting access to a Doppler laser vibrometer that can measure vibration into the ultrasonic range. Unfortunately, it's prohibitively expensive for the kind of business we are in. Second option might be in using an ultrasonic microphone or tensometer. Watch this space!
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