Revelations of Arm/Cartridge resonance features

I recorded the second side of the Hifi News Test LP (HFN 001) on both my Linn Sondek LP12 Valhalla with Ittok XV II arm, and my Mitsubishi LT-3 linear tracking turntable.  Both have Dynavector 17D3 cartridges and used seperate Emotiva XSP-1 Gen 2 preamps.  Both arms have been covered with hockey tape to add grams of equivalent mass to the arms, but I went further with this process on the Linn and added a mountain of hockey tape to the headshell.

Needle Drop on Linn

Needle Drop on Mitsubishi

After the need drop and before the first tracking tone, both turntables show a region of driven LF resonance.

This resonance has an envelope which has a cycle time of 1.8 seconds, which is 60sec / 33.33.

The envelope is significantly greater magnitude on the Mitsubishi, and the resonance(s) within the envelope are higher frequency.  Both undesireable.

All consistent with higher equivalent mass, AND somehow more damping, especially lateral damping, on the Linn.  This results in lower resonance added to playback, which is desirable.  (I realize there many Linn users follow an anti-damping philosophy, but I do not.  I think damping is generally a great way to control undesired resonances.)

My theory has long been the anti-skate mechanism on the Linn adds damping.  I did some tests last year but made no pictures of that.  Otherwise, the Ittok seems to have no damping.  Adding damping through the anti-skate mechanism is a brilliant idea, IMO.

The too-low mass on the Mitsubishi I can fix with more tape, I think.  I had to augment the Mitsubishi counterweight already.  I found an M6 bolt which exactly fit the hole in the back of the arm.  I think I have some more range now than when I quit adding tape, but I could always get a longer bolt.

The damping, well I don't know.  That's at the point I should try to get another turntable working rather than mess too much with the Mitsubishi linear arm.

The cyclic envelope of the resonance on both tables is 90 degrees from it's beginning when the tone burst starts.  That shows it is tied to something on the record.

I wondered for awhile how it could be that the record imposed a cyclic variation, but there was no visible square wave or anything showing where that cycle was being imposed.  Then it occurred to me:

The center hole offset causes the arm to move back and forth from ideal position, synchronized with position on the record.  But the movement imposed by the center hole offset is sinusoidal.  It only gradually reaches the peak movement in either direction.  Nevertheless, there is a distinct point on both sides of the 1.8 sec cycle where the direction reverses.  Even though that position is approached very gradually, the reversal imparts a force driving the next cycle of resonance, and especially on the inside, where it opposes the general direction of arm movement.