Measurements on LP12 with Ittok arm and additional headshell mass

Additional Headshell Mass*

Upon getting my Linn LP12 fixed, and finding that I liked it slightly better with my 6.1g additional headshell mass, which is a squished roll of hockey tape,  I decided it was time to do some measurements with and without the extra mass, and with different levels of antiskate.

The results seem to indicate that the mass DOES improve the low frequency tonearm/cartridge resonance by lowering it considerably in frequency and slightly in level (except it increases the amplitude of the resonance in the right channel).  But it seems to have no measurable impact on anything else, including the 20-20kHz response using the 20 second sweep on Hifi News test record (HFNTR).  So how does reducing the resonance from 13Hz to 9Hz make the bass above 20Hz sound better?  Apparently not by a straightforward effect on the frequency response as such, but by reducing resonances that are intermodulating with the rest of the sound.  Note that a 20 second 20-20kHz sweep will also tend to downplay low frequency resonances which take a while to develop, a pink noise test (not yet done) may be more revealing.

Meanwhile, cranking the antiskate compensation up to 3.1 g (the maximum) seems to have no effect on the vertical resonance.  

The 3.1g antiskate setting also the cleanest performance on the maximum trackability test, which sounds slightly distorted at the 2.2g setting which would nominally match my 2.2g tracking force.  I have been using the 3.1g setting since discovering that it worked best two years ago.  The Ittok antiskate mechanism seems to add desireable horizontal damping as well as antiskating force, and both are useful to me up to the 3.1g setting.

I was motivated to test vertical resonance vs antiskate compensation specifically because of a strange channel imbalance in the vertical resonance.  There is much more vertical resonance in the right channel than the left channel.  This could mean the stylus is climbing the outermost side of the groove, which is what excess antiskate force might tend to do.  But no antiskate setting made any measured difference on the vertical resonance.

Here is a picture showing first the horizontal and then the vertical resonance test in the No Added Mass condition (on top) and the 6.1g Added Mass condition (on bottom).  Both resonances are clearly lower in frequency (desireable) with the added mass.  However, strangely, the vertical resonance in the right channel gets lower and perhaps slightly larger.  The marker is at 9Hz in the horizontal resonance test.

Horizontal and Vertical Resonances vs Added Mass (bottom)

You can see why I was motivated by this result to see if excess antiskating force was causing the increased right channel (bottom in Audacity) resonance amplitude with added mass.

But antiskate does not appear to be causing this issue.  The image below shows the result of testing vertical resonance vs antiskate.  There appears to be no change in the vertical resonance while varying antiskate compensation from 3.1g to 0g.  And it's always worse in the right channel anyway.

Vertical Resonance for 3.1g, 2.2g, 0g, and 1g antiskating compensation

Meanwhile, the Frequency Response sweeps show no bass difference from added mass (on bottom).  In the highs the added mass slightly changes the high frequency measurement (I thought it sounded better WITH the mass, but the graph does not make that look obviously better):

20-20kHz sweep without and with (bottom) added mass

Also the frequency response sweeps show no low frequency dependency on antiskating force level.  Once again I thought the highs sounded better in the 3.1g antiskating condition, but this is not obvious in the measurements.

Frequency Response vs Antiskating level (2.2, 3.1, and 0g)

These results suggest to me that the added mass should be somewhat smaller than 6.1g, perhaps 4.5g or so.  With 6.1g, the horizontal resonant peak is 9 Hz which is fine but has legs reaching down to 7 Hz, which is undesirable.  Also a lower mass might reduce the 9Hz or so increase in amplitude of the vertical resonance.  But without the mass, there is resonant garbage as high up as 16 Hz, which is clearly undesirable.

(*The photo is to show what the 6.1g mass looks like.  It is not fully authentic, because by the time I remembered to take a photo I had already removed the mass again and had started replacing it with a stack of hockey tape, which should work better.  I didn't remove the beginnings of that stack to take the photo,  I simply stuck the mass back onto the headshell again, so it shows quite a bit more hockey tape underneath the roll than there was before.  Prior to this week, I had never even measured the weight of the hockey tape roll either.   When I came up with the idea of rolling up a bunch of hockey tape and squishing it on top of the headshell 18 months ago, I had conceived it as only a temporary measure.  It would enable me to quickly figure out exactly how massive the mass had to be.  Or so I thought.  Somehow all the tests I did back then seemed inconclusive.  Especially just listening to the "warbling" as the test record instructs you to do is quite ambiguous, and then to compare one such ambiguous listening session with a much earlier one is just confounding.  I knew the mass was an improvement but it would be better larger or smaller?   I couldn't tell, and I didn't dare to make it much larger.  I had never gotten around to the much more precise visual analysis in this post, until now.  And so I like to think that in some ways I'm getting better.  And of course I rebalanced the arm whenever I either added or removed the mass.  The mass is to increase the tonearm+cartridge Effective Mass, not the tracking force.  The Linn Ittok is a low to medium mass tonearm.  For low compliance cartridges such as the Dynavector 17D3 moving coil, a high mass (most importantly, high lateral mass) tonearm like the Dynavector DV-505 or Fidelity Research is called for.  One can fudge this a bit with tonearms that have adjustable damping, like my medium mass Jelco 750L, which I haven't tested yet.)