SRA, why 92 degrees

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The idea behind checking the SRA with a lateral IMD test tone (4 kHz tone modulated with 60 Hz in this case) is to find where the output is highest. The hypothesis is that mismatch between the stylus line and groove cut would give a slight less output of the 4 kHz signal when looking at the geometries at play. This will separate the SRA from VTA which requires a pure vertical IMD test signal.

In the above case measurements show that the differences are small, very small, but if anything the SRA would be optimal within 0-1 mm tonearm adjustment.
 
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The idea behind checking the SRA with a lateral IMD test tone (4 kHz tone modulated with 60 Hz in this case) is to find where the output is highest. The hypothesis is that mismatch between the stylus line and groove cut would give a slight less output of the 4 kHz signal when looking at the geometries at play. This will separate the SRA from VTA which requires a pure vertical IMD test signal.

In the above case measurements show that the differences are small, very small, but if anything the SRA would be optimal within 0-1 mm tonearm adjustment.
However small the measurements may be, the audible sweet spot was very clearly evident. The difference going from Hi-Fi to actual transparency. Comments were always the same..." sounds like they're there"
 
Changes to rake angle, SRA, have tiny effect on theoretical geometric errors such as IMD which is why ThomasA correctly measures only very small (if any)difference in IMD versus variation in SRA in a lateral test.

However, sometimes that test is known to show differences, otherwise people would not do it. The question to answer is why sometimes, and why when there is no theoretical geometric reason?

IME it can vary from case to case, and this is why people can take vociferous opposing views.


Years ago I found that, for one rig involving a FG stylus, stylus drag coefficient varied significantly with small variation in SRA. My opinion is that any effect follows from this effect and relates to noise floor and mistracing under circumstances of altered stylus-groove friction/drag.


Unfortunately, I assume my posts on this are longer avail, and I can't find them in my archive. But IMO the leading indicator of whether any IMD measurable and audible performance difference is available is whether stylus-groove friction drag varies with SRA.

It's the elephant in the room, and it explains much variance in results and experiences, IMO.


LD
 
Changes to rake angle, SRA, have tiny effect on theoretical geometric errors such as IMD which is why ThomasA correctly measures only very small (if any)difference in IMD versus variation in SRA in a lateral test.

However, sometimes that test is known to show differences, otherwise people would not do it. The question to answer is why sometimes, and why when there is no theoretical geometric reason?

IME it can vary from case to case, and this is why people can take vociferous opposing views.


Years ago I found that, for one rig involving a FG stylus, stylus drag coefficient varied significantly with small variation in SRA. My opinion is that any effect follows from this effect and relates to noise floor and mistracing under circumstances of altered stylus-groove friction/drag.


Unfortunately, I assume my posts on this are longer avail, and I can't find them in my archive. But IMO the leading indicator of whether any IMD measurable and audible performance difference is available is whether stylus-groove friction drag varies with SRA.

It's the elephant in the room, and it explains much variance in results and experiences, IMO.


LD

I don't remember now but did you have those friction data left? Was it the "time-to-stop" method using a free spinning platter to stop?
 
From my archive:

Here's a simple method to measure stylus/groove friction coefficient, which produces results comparable to JVC figures posted on Yosh's site (range c 0.2-0.55 IIRC). Stylus/groove total friction force = this coefficient times VTF. I have tested this method, as have others, and am fairly confident it is correct.

'The stopping method' : A simple way to measure stylus/groove wall friction force using platter rotational stopping time. All that's required is to weigh the platter and measure the stop time a few times, and measure a few distances. Here's the explanation.

First, some definitions:

w(0) = initial angular velocity (rads)
w (stop) = final angular velocity (rads)
a = angular acceleration (rads/s^2)
t = time (secs)
T = Torque (Nm)
I = Moment of inertia kgm^2
F = applied stopping force N
r = radius of applied stopping force F (m)

here's the measurement method

Step 1. Determine T(b) the friction torque in the bearings of the turntable

Weigh the platter and measure the radius. Calculate moment of inertia I = (m*r^2)/2 or however, depending on mass distribution.
e.g m=1.6kg, r = 0.15m I = 0.018 kgm^2

Calculate w(0)
e.g. at 33 1/3 rpm = 2*pi*33.33/60 = 3.5 rads/sec

Put a record on the platter and spin it, but do not play. Suddenly slip the belt off allowing the platter to rotate, and measure time to come to rest t.
e.g = 45 seconds

The frictional torque from bearings T(b) = I*a = I*[-w(0)]/t
e.g T(b) = 0.018* 3.5/45 = 0.0014Nm

Step 2 . Determine T(F) the friction torque due to stylus/groove contact

Play a 33 1/3 rpm record track at a known radius r from the spindle
e.g r = 0.14m

Slip the drive belt off and measure time t for the platter to come to rest
e.g. 23.5 seconds

Calculate T(F) = (I*a)-T(b)
e.g. T(F) = [0.018*3.5/23.5] - [0.0014] = 0.0013Nm

Step 3. Determine F, the frictional force

F = T(F)/r
e.g F = (0.0013/0.14) = 0.009 N

This is all straightforward to do, so I did a quick experiment. OM5E, VTF= 2.0g, I obtained the following measured values for F, based on measured platter weight = 1.6kg, 331/3 rpm, measured stop time = 23.5s (stylus down) measured stop time = 45 seconds (stylus up),

outer track r = 14cm, F = 0.009N
inner track r = 7cm F = 0.009 N

From yosh's recspecs page citing JVC and other figures, the calculated value at VTF = 2.0g should be in the range 0.0044 - 0.011N.

Multiple Friction force in N by 98 to obtain friction force in grams force.

Friction coefficient is simply F/VTF (make sure same units eg gf are used)

On the face of it, this accords with Yosh's JVC etc figures posted on Yosh's site.

LD
 
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