The ‘improvements’ being observed with ‘truly straight underhung arms’ in this thread and elsewhere are related to the geometric effects of eliminating the offset angle. Conventional pivoted arms include an offset angle to minimize tracking error and "overhang" the spindle on their inward swing. There is a not-so-obvious and undesirable consequence of the offset angle,
besides skating. When the arm and cartridge ‘bob’ sideways, such as can occur during jarring bumps or footfalls, the left and right sideways deflection of the stylus that occurs from horizontal motion of the cartridge is
dynamically decreasing and increasing the effective length of the arm. This translates into a fore and aft motion of the stylus tip, or ‘scrubbing’, along the groove. The amount of horizontal stylus displacement that gets translated into ‘scrubbing’ is mainly proportional to the Sine of the offset angle (Sine of typical 24 degrees = 0.4), and is a constant ratio. There is also
some dynamic length altering or scrubbing effect from the horizontal stylus displacement about the
cantilever pivot, but this is small in comparison to that from the offset angle, and this contribution approaches zero for very small amounts of cantilever deflection (Sine of 0 degrees = 0). The lion’s share is from the offset angle.
Look at this sketch from Shure of an offset pivoted arm:
http://shure.custhelp.com/app/answers/d ... b0ZiWWs%3D
This scrubbing motion of the stylus tip along the groove causes frequency modulation (FM) of the signal in the groove. This form of FM distortion has the same audible effect as flutter in a tape recorder or a movie film projector. The warble-like sound of flutter in a malfunctioning tape deck or old projector is a familiar and easily recognizable audio effect. Unlike tape decks or projectors where the effect is somewhat constant due to problems in the mechanism, the flutter effect with FM distortion in disc playback is typically
transitory. The effect can be demonstrated with a sustained source of stimulation at the mass/compliance resonance frequency, such as low frequency resonance sweep signals found on certain test records. Jarring and footfalls aren’t the only dynamic sources of stylus scrubbing. Dynamic variations in stylus friction drag force caused by rapidly changing groove modulation levels can also cause a dynamic variation in skating forces. This dynamic change in stylus friction drag can translate into dynamic horizontal vibration of the arm which, in turn, can cause stylus scrubbing and sometimes audible flutter effects. All things being equal (which is rare), longer arms with less offset angle have an advantage.
In the case of a straight arm with no offset angle, the friction drag force vector on the stylus is in line with the pivot. As such, dynamic variations of friction drag force on the stylus don’t translate into dynamic horizontal/sideways vibration. No sideways movement, hence, no induced FM or flutter on a crescendo. I think this is the operative factor being observed in reviews of the RS-Labs and other ‘straight’ arms. As was the case for offset arms, straight arms too will see
some scrubbing from the horizontal stylus displacement about the
cantilever pivot, but this is small in comparison to that caused by an offset angle, and this contribution approaches zero for very small amounts of cantilever deflection (Sine of 0 degrees = 0).
Here is my own sketch of what happens with a ‘truly straight’ arm:
For the same ‘lateral vibration’ or horizontal deflection of the stylus, the only scrubbing that occurs with the straight arm is that from the horizontal stylus displacement about the
cantilever pivot. The absence of an offset angle
greatly reduces the total magnitude of fore and aft scrubbing along the groove. I think the positive comments from reviewers of RS Labs and similar straight arms stem from this reduction of FM distortion caused by stylus scrubbing. The tradeoff with ‘truly straight underhung’ arms is high tracking error distortion. A straight underhung arm
will also exhibit skating effects dependent on the amount of tracking error at a given point. Interestingly though, while the friction drag force on the stylus will cause skating everywhere except at the null point, the resultant stylus friction drag force vector on the cantilever is
in line with the cantilever and the arm pivot, so stylus friction drag doesn’t cause any side-force vector on the cantilever or sideways deflection of the stylus cantilever and, hence, no scrubbing. Oddly enough, whatever skating forces do exist on a straight arm will show up as increased stylus contact pressure on one side of the groove or other, depending on the null point, but
will not show up as a side-force on the cantilever!
And, what about vertical scrubbing effects? The RS-A1 arm design does something similar in the vertical direction that ‘truly straight’ arms do in the horizontal direction:
Here is another sketch:
The RS-A1 arm pivot is very high above the record surface. As the arm pivot gets raised higher, it decreases the angle ‘beta’ in the figure, which has an effect in the vertical direction that is similar to decreasing the offset angle in the horizontal direction. This would decrease stylus tip scrubbing effects caused by dynamic deflections of the stylus in the vertical direction. The relative effect of the high pivot would depend on the geometry fixed by the arm pivot height and the cartridge VTA. The results would vary greatly from user to user, depending on individual arm setup and actual cartridge VTA. The high pivot may help reduce stylus tip scrubbing from vertical vibration but the tradeoff is that it makes warp wow effects worse. More ways to pick your poison…
So, a given point on a disc might sound better/worse with a conventional pivoted arm having lower tracking error distortion, or a straight arm having lower scrubbing/FM distortion. It depends on the program material, null points chosen, and perhaps other factors. Of course, the tracking error issues of ‘truly straight’ arms
could be addressed by moving the arm pivot with a servo to maintain tangency, aka make it into a linear tracker, but that would be getting off topic and risk starting a food fight
.
Disclosure statement: I don’t own, use, or endorse RS Labs or DJ straight arms.
Ray K