Antiskating and Frequency Modulation Distortion of Different Tonearm Geometric
Hi Guys (gals?)
There has been a few different threads on the Gon recently about turntables maintaining their speed - Speed Stability.
The effect of stylus drag with turntable speed is always mentioned and acknowledged.
I continue to go back to Bruce Thigpens ET2 tonearm manual, and find information that I had either overlooked or forgotten about. IMO - for anyone in this hobby this manual contains important data. Here is BT's information on this subject, the forces involved and some real measurements.
The Et2 manual on page 51 says that an LP record contains approximately 40% vertical energy and 60% lateral energy.
These are significant and different forces at play.
BT did testing of these forces as they applied to different tonearms. The manual section that summarizes these findings follows.
Taken from “the Eminent Technology ET2 Tonearm Manual - Pages 46 and 47”.
Disclaimer - This is public information and is available online at the Eminent Technology website under support/manuals/ET2 owners manual.
Antiskating and Frequency Modulation Distortion of Different Tonearm Geometric
Pivoted tonearms are designed so that the head shell holds the cartridge at an “offset angle” with respect to an imaginary line drawn through the tonearm pivot to the stylus tip. The arc traced by the stylus tip extends past the record center and is defined as “overhang”.
This design approach minimizes tracking error. There have been many articles written about the geometry of this design approach. Pivoted arms create several side effects which reduce phono cartridge performance. The first is a skating force which results from two different parts of pivoted arm design.
There is a force component (vector) that is directed toward the center of the record. It results from the stylus drag force vector not falling in line with the pivot point of the arm. This force pulls the tonearm inward and the stylus can be observed as bending outward. This force and the resulting bending can be demonstrated by connecting a rubber band to a pivoted arm around the cartridge body and pulling it straight (away) from the tonearm. Note: the motion of the tonearm is inward and results in bending of the rubber band (cantilever).
If you corrected these forces with an anti-skating mechanism such that the stylus did not bend (you can not really do this because the frictional force and resultant bending varies with groove modulation, stylus shape, tracking force, etc...) there still exists another component of skating.
This second skating force results because of overhang. There are frictional force vectors that result which are not directly ahead of the stylus. The surface of the record is not really moving straight ahead with respect to the stylus tip. As a result, there are force components directed ahead and toward the center of the record. The magnitude of the inward force depends upon the degree of overhang.
This means that any given cartridge works against lower horizontal forces in the Model Two tonearm (.1 gram or less) compared to a conventional arm (.2 grams/gram vtf). These figures apply if you use records that are not severely out of round. If you like to play severely eccentric records, ones with runout of greater than 1/8”, then we suggest you use a low mass pivoted arm.
For vertical forces while tracking warps, the cartridge suspension system must work against the tonearms moment of inertia about its vertical axis of rotation. For the Model Two, these forces will be similar to those of a conventional tonearm which has low to medium effective mass (10 grams).
Fwiw - As an owner of both pivot and straight line tonearms I am aware that stylus drag differences differ with each. I never did take any measurements. It is now possible with software, a test disc and your Android or Iphone to measure things like turntable speeds and tonearm resonances, as discussed in this thread. See the post from
Halcro
