The importance of VTA, SRA and Azimuth - pics

[J.R. Boisclair]

additional to parallax and optical errors there are also lots of variations mentioned above on measuring angle on a picture taken by a microscope. and still an important problem remains unsolved which is trying to measure a dynamic system with static methods.

on the other hand aligning zenith angle with AM and test record is a dynamic method for a dynamic system. I used AM and test records to setup lots of turntables and cartridges. zenith, SRA and anti-skating adjustments end up at the same alignments using both AM test record and Analogue Productions test record. if you can make record sit perfectly flat on the platter (vacuum hold down) than azimuth tracks give the same alignment too. track locations are totally different on those two records and they're cut at different lathes but software leads you to the same cartridge alignment position in the headshell. which means both records are carefully and rightfully cut and pressed. more importantly it means the alignments you make are correct and also sounds better compared to optical alignment methods.

Hi mtemur,

I am not sure whether you are commenting generally on the challenges of univariate measurements using microscopy or if they are meant for me and my use of them. I imagine you meant your comments generally since we have both functionally eliminated optical distortion error and we also test dynamically those parameters that respond to playback; namely, SRA/VTA and, to a much lesser extent, azimuth. Zenith error is fixed and is not altered by dynamic conditions. However, if your tonearm has excessive (or not enough) horizontal torque force or frictional force any “zenith error” test using a test record will certainly be informed by this condition and the resultant “optimal” setting for zenith error will be influenced by this errant force which will then result in doing the best you can for zenith error with what you don’t know you have at play in your tonearm. Out of control torque force will certainly cause additional issues that must be contended with but unless you measure directly for it you cannot know it exists and to what extent. This is why I tell people that if they get nothing else from WallyTools, get a WallySkater to measure these horizontal and frictional forces to ensure your tonearm doesn’t screw up your cantilever alignment.

Eliminating the variables - that is the value of univariate measurements and why they are the sine qua non of analytical certitude.

so I am not sure what “optical alignment methods” you might be referring to other than those we do regularly - and have been getting progressively better and better at it where as of about 4-5 weeks ago we can safely say our accuracy is down to a small fraction of a degree as per above. In short, I have to contest the assertion that the methodology is problematic or somehow less accurate than multivariate analysis which is fraught with problems starting with the faceting of the cutting styli used to cut the lacquer for the test record and then negatively influenced by a minimum of 14 other variables that we’ve counted to date. But don’t take my word for it: we intend to make it the subject of one of several peer reviewed research papers.

 

[ianm0]

J.R. Thanks for the detailed exposition. I do have a question to raise with regard to Fremer's method of measuring SRA using a CD as the platform. A CD has much finer grooves vs a record. When a stylus rests on either CD or LP, the point(s) of the stylus in contact with the grooves in each is different. Consequently SRA measured will be different. My question is how different? Will it or will it not be significant enough?

I did a quantitive ESTIMATE, as guideline only, on the quantitive relationship between SRA and tonearm height. Please see:

Tonearm height & SRA misconception

It’s been widely accepted that changing a 9” tonearm’s height by 4 mm will approximately alter the SRA by 1 degree. This contradicts valid – to me - claims by audiophiles that they can hear the difference when the aforementioned height changes by a fraction of a mm. Bear in mind the tonearm +...

www.whatsbestforum.com

 

[J.R. Boisclair]

I did a quantitive ESTIMATE, as guideline only, on the quantitive relationship between SRA and tonearm height. Please see:

Tonearm height & SRA misconception

It’s been widely accepted that changing a 9” tonearm’s height by 4 mm will approximately alter the SRA by 1 degree. This contradicts valid – to me - claims by audiophiles that they can hear the difference when the aforementioned height changes by a fraction of a mm. Bear in mind the tonearm +...

www.whatsbestforum.com

Hi ianm0,
Sorry for late reply in giving your point my attention. Just getting my head above water and returning home from being on the road. Will reply in original post when I get time.
J.R.

 

[ianm0]

Hi JR
With your sophisticated equipment, I hope you can shed some light on:
When a particular tonearm height is changed by say, 2 mm and other values, how much is the height of the rear end of the cantilever has changed for a particular cartridge? That will be an interesting academic exercise. Without doubt that depends of effective mass of tonearm and compliance of the cartridge. In my original post, I did not take into account the tonearm's rotation about the pivot. That certainly could thrown some doubt on my assertion. Clarification will be most welcome. After all, all theories are subject to experimental verifications.
Hope you have time for it.

 

[Johan K]

Hi ianm0,
Sorry for late reply in giving your point my attention. Just getting my head above water and returning home from being on the road. Will reply in original post when I get time.
J.R.

Hi J.R.

Nice to see you on the WBF… I just join the forum 1-2 weeks ago, so I’m pretty new here.. but I like it.. Hope you’re good my friend .

All the best

Johan in Sweden

 

[J.R. Boisclair]

Hi J.R.

Nice to see you on the WBF… I just join the forum 1-2 weeks ago, so I’m pretty new here.. but I like it.. Hope you’re good my friend .

All the best

Johan in Sweden
View attachment 102505

It's always good to hear from you and see your FUN listening room, Johan!!

 

[scericson]

What software are you using with your digital camera? I've seen photos taken with angles superimposed but I have not found the software that does that.

 

[J.R. Boisclair]

What software are you using with your digital camera? I've seen photos taken with angles superimposed but I have not found the software that does that.

The software comes with the WallyScope camera. It is made by the same company that makes the microscope camera. If you want a simple app to lay down angular measurements in photos, just type "angle measurement app" and see what comes up.

 

[Calle_jr]

.
After quite extensive testing of VTA/SRA changes, I would conclude that:
If you manage to install your tonearm parallel to the record within +/-5mm (which is very easy), and if the cartridge is not poorly designed, poorly manufactured or broken, VTA and SRA is not critical. At least with a line contact stylus.
Actually, it is more important to make the plinth and platter in level and to adjust the VTF after the VTA/SRA is set.

Again in my opinion, below is summary of proof that this is so:
- A vertically cut 1kHz sine to check differences in waveform and harmonics due to changed VTA.
- A laterally cut 60Hz + 4kHz sine ratio 4:1 to check differences in ratio and side lobes due to changed SRA.


1kHz sine vertical, UATLP track B4. Measured with 0 VTA, -4.8mm VTA and +4.8mm VTA

FFT:




Closeup of FFT at fundamental and 2nd harmonic:




Waveform:






60Hz + 4kHz sine lateral ratio 4:1, UATLP track A9. Measured with 0 VTA, -4.8mm VTA and +4.8mm VTA

Waveform:




Closeup of waveform:




FFT:




Closeup of FFT at 60Hz and 4kHz:





If anyone has something different to show I would much appreciate it and I will be more than happy to change my opinion.
I really tried to find a change.
There's tons to read about VTA/SRA, but it is very difficult to find concrete results really showing the phenomenon of changed VTA/SRA.


Notes:
- "0 VTA" represent visually set tonearm parallel to 180g record.
- UATLP = Analogue Productions Ultimate Analogue Test LP AAPT 1
- Tonearm: Kuzma 4Point (one turn = 0.8mm)
- Cartridges: Audio-technica AT44GPT/II (VTF=2.02g) and Lyra Atlas Lambda (VTF=1.72g)
- VTF was set for "0 VTA" and not changed for each change of VTA.
- Recorded with a chain of Pass XP25 (160ohm/100pF) + 19dB OP-amp gain + HP-filter (-3dB @60Hz) + Tascam DR-40X pcm 24/96.
- Zenith error was <15µs for a 10kHz lateral sine and azimut was adjusted to give less than 30dB crosstalk @1kHz for the cartridges.

 

[J.R. Boisclair]

.
After quite extensive testing of VTA/SRA changes, I would conclude that:
If you manage to install your tonearm parallel to the record within +/-5mm (which is very easy), and if the cartridge is not poorly designed, poorly manufactured or broken, VTA and SRA is not critical. At least with a line contact stylus.
Actually, it is more important to make the plinth and platter in level and to adjust the VTF after the VTA/SRA is set.

Again in my opinion, below is summary of proof that this is so:
- A vertically cut 1kHz sine to check differences in waveform and harmonics due to changed VTA.
- A laterally cut 60Hz + 4kHz sine ratio 4:1 to check differences in ratio and side lobes due to changed SRA.


1kHz sine vertical, UATLP track B4. Measured with 0 VTA, -4.8mm VTA and +4.8mm VTA

FFT:

View attachment 134041


Closeup of FFT at fundamental and 2nd harmonic:

View attachment 134046


Waveform:

View attachment 134047




60Hz + 4kHz sine lateral ratio 4:1, UATLP track A9. Measured with 0 VTA, -4.8mm VTA and +4.8mm VTA

Waveform:

View attachment 134044


Closeup of waveform:

View attachment 134045


FFT:

View attachment 134042


Closeup of FFT at 60Hz and 4kHz:

View attachment 134043



If anyone has something different to show I would much appreciate it and I will be more than happy to change my opinion.
I really tried to find a change.
There's tons to read about VTA/SRA, but it is very difficult to find concrete results really showing the phenomenon of changed VTA/SRA.


Notes:
- "0 VTA" represent visually set tonearm parallel to 180g record.
- UATLP = Analogue Productions Ultimate Analogue Test LP AAPT 1
- Tonearm: Kuzma 4Point (one turn = 0.8mm)
- Cartridges: Audio-technica AT44GPT/II (VTF=2.02g) and Lyra Atlas Lambda (VTF=1.72g)
- VTF was set for "0 VTA" and not changed for each change of VTA.
- Recorded with a chain of Pass XP25 (160ohm/100pF) + 19dB OP-amp gain + HP-filter (-3dB @60Hz) + Tascam DR-40X pcm 24/96.
- Zenith error was <15µs for a 10kHz lateral sine and azimut was adjusted to give less than 30dB crosstalk @1kHz for the cartridges

.
After quite extensive testing of VTA/SRA changes, I would conclude that:
If you manage to install your tonearm parallel to the record within +/-5mm (which is very easy), and if the cartridge is not poorly designed, poorly manufactured or broken, VTA and SRA is not critical. At least with a line contact stylus.
Actually, it is more important to make the plinth and platter in level and to adjust the VTF after the VTA/SRA is set.

Again in my opinion, below is summary of proof that this is so:
- A vertically cut 1kHz sine to check differences in waveform and harmonics due to changed VTA.
- A laterally cut 60Hz + 4kHz sine ratio 4:1 to check differences in ratio and side lobes due to changed SRA.


1kHz sine vertical, UATLP track B4. Measured with 0 VTA, -4.8mm VTA and +4.8mm VTA

FFT:

View attachment 134041


Closeup of FFT at fundamental and 2nd harmonic:

View attachment 134046


Waveform:

View attachment 134047




60Hz + 4kHz sine lateral ratio 4:1, UATLP track A9. Measured with 0 VTA, -4.8mm VTA and +4.8mm VTA

Waveform:

View attachment 134044


Closeup of waveform:

View attachment 134045


FFT:

View attachment 134042


Closeup of FFT at 60Hz and 4kHz:

View attachment 134043



If anyone has something different to show I would much appreciate it and I will be more than happy to change my opinion.
I really tried to find a change.
There's tons to read about VTA/SRA, but it is very difficult to find concrete results really showing the phenomenon of changed VTA/SRA.


Notes:
- "0 VTA" represent visually set tonearm parallel to 180g record.
- UATLP = Analogue Productions Ultimate Analogue Test LP AAPT 1
- Tonearm: Kuzma 4Point (one turn = 0.8mm)
- Cartridges: Audio-technica AT44GPT/II (VTF=2.02g) and Lyra Atlas Lambda (VTF=1.72g)
- VTF was set for "0 VTA" and not changed for each change of VTA.
- Recorded with a chain of Pass XP25 (160ohm/100pF) + 19dB OP-amp gain + HP-filter (-3dB @60Hz) + Tascam DR-40X pcm 24/96.
- Zenith error was <15µs for a 10kHz lateral sine and azimut was adjusted to give less than 30dB crosstalk @1kHz for the cartridges.

I suggest you review the many studies that were published in the 1960s through 1980s on the subject. You will see that they conclude differently but none of them, as far as I remember, used tonearm height as a means with which to impact VTA. It’s just the wrong thing to do as you are introducing too many changes in multiple vector forces which will certainly impact the data due to its multivariate nature (when changing tonearm height).

Further, the distortion characteristics for VTA increase at the inner area of the record, so you might try that out if you haven’t already. Use only vertically modulated track, of course. Horizontal modulation won’t show you a thing about VTA impact.

I can certainly tell you that, say, 3° of angular change at the head shell is very clearly audible even to an untrained listener. Of course, we controlled for the addition of the material in this testing by first listening to the same shim material at 0° VTA correction.

By the way, the sound of changing VTA/SRA in isolation doesn’t create any tonal changes easily noticeable. It’s just an improvement in clarity, image, separation, soundstage size, bass solidity and transient attack. Chances are that your cartridge needs plenty of downward adjustment. I own an atlas myself and have analyzed well over 50 of them. In most cases I can’t get them down as far as I would like them before the body starts getting too close to the record I regularly bring them within a half a millimeter from the record surface under stylus drag. Just shim it and listen for yourself. Keep that tonearm level.

By the way, the comment above isn’t a slam Lyra. They are absolutely fantastic, but they have the same problem almost every other cartridge manufacturer does: VTA is too high.

 

[ianm0]

.
After quite extensive testing of VTA/SRA changes, I would conclude that:
If you manage to install your tonearm parallel to the record within +/-5mm (which is very easy), and if the cartridge is not poorly designed, poorly manufactured or broken, VTA and SRA is not critical. At least with a line contact stylus.
Actually, it is more important to make the plinth and platter in level and to adjust the VTF after the VTA/SRA is set.

Again in my opinion, below is summary of proof that this is so:
- A vertically cut 1kHz sine to check differences in waveform and harmonics due to changed VTA.
- A laterally cut 60Hz + 4kHz sine ratio 4:1 to check differences in ratio and side lobes due to changed SRA.


1kHz sine vertical, UATLP track B4. Measured with 0 VTA, -4.8mm VTA and +4.8mm VTA

FFT:

View attachment 134041


Closeup of FFT at fundamental and 2nd harmonic:

View attachment 134046


Waveform:

View attachment 134047




60Hz + 4kHz sine lateral ratio 4:1, UATLP track A9. Measured with 0 VTA, -4.8mm VTA and +4.8mm VTA

Waveform:

View attachment 134044


Closeup of waveform:

View attachment 134045


FFT:

View attachment 134042


Closeup of FFT at 60Hz and 4kHz:

View attachment 134043



If anyone has something different to show I would much appreciate it and I will be more than happy to change my opinion.
I really tried to find a change.
There's tons to read about VTA/SRA, but it is very difficult to find concrete results really showing the phenomenon of changed VTA/SRA.


Notes:
- "0 VTA" represent visually set tonearm parallel to 180g record.
- UATLP = Analogue Productions Ultimate Analogue Test LP AAPT 1
- Tonearm: Kuzma 4Point (one turn = 0.8mm)
- Cartridges: Audio-technica AT44GPT/II (VTF=2.02g) and Lyra Atlas Lambda (VTF=1.72g)
- VTF was set for "0 VTA" and not changed for each change of VTA.
- Recorded with a chain of Pass XP25 (160ohm/100pF) + 19dB OP-amp gain + HP-filter (-3dB @60Hz) + Tascam DR-40X pcm 24/96.
- Zenith error was <15µs for a 10kHz lateral sine and azimut was adjusted to give less than 30dB crosstalk @1kHz for the cartridges.

I tried to use the same technique to set up VTA, as per Analog Productions's Set up disc Side 2/Track 9. I can confirm that the harmonics' amplitudes hardly change with your range of arm heights - as you showed in the plots. The question arises: is that a valid way to set up VTA? JR raised the same concern. I can hear the effects of arm height difference +-0.5mm. The tone of vocals is a good indicator of the change.

 

[Calle_jr]

Thank you for pushing me to the relevant issues, @J.R. Boisclair .
I really mean that.

I suggest you review the many studies that were published in the 1960s through 1980s on the subject.

I did.

It’s just the wrong thing to do as you are introducing too many changes in multiple vector forces which will certainly impact the data due to its multivariate nature

I totally see your point, but it is not too difficult to bear those in mind I think.
I uploaded a sketch just to show the impact of VTF here a few years ago (copy below).





If VTA/SRA is so sensitive, it really should show in at least some of the tests. How unlikely is it to have parameters sensitive to everything but not showing anything?
I know that VTA is an angle defined at the cartridge, but 1° at the cartridge is the same as 1° at the tonearm pivot. The difference is that the forces change. That's why I wrote to adjust VTF after VTA/SRA adjustment.

Still, those are the means I have to change VTA/SRA if I want to keep especially zenith and do measurements in a controlled way.
Your point is valid, but nevertheless, a VTA/SRA change by +/-5mm at tonearm pivot doesn’t impact waveform or FFT, at least not for a line contact stylus.

Sorry if I seem stupid stubborn, but if I’m wrong or correct in the statement above, A LOT of discussions in this topic would be clarified.

I tried to use the same technique to set up VTA, as per Analog Productions's Set up disc Side 2/Track 9. I can confirm that the harmonics' amplitudes hardly change with your range of arm heights - as you showed in the plots.

Thanks.

The question arises: is that a valid way to set up VTA?

No.
My intention was that UATLP track B4 is for VTA adjustment and track A9 is for SRA.
If VTA is not sensitive for the current cartridge, SRA can dictate the adjustment and vice versa.

It’s just an improvement in clarity, image, separation, soundstage size, bass solidity and transient attack. Chances are that your cartridge needs plenty of downward adjustment. I own an atlas myself and have analyzed well over 50 of them. In most cases I can’t get them down as far as I would like them before the body starts getting too close to the record I regularly bring them within a half a millimeter from the record surface under stylus drag. Just shim it and listen for yourself. Keep that tonearm level.

I have the same experience, especially with Lyra which I am very familiar with. By ear I always ended up with a smaller VTA than a level tonearm gives.
But if I can’t see it on the waveform or FFT, how can I see it?



Again, thanks for your replies.

 

[sbo6]

For all interested - I have been astounded by the improvement of properly setting SRA around the optimal 92 degrees, and perfect azimuth as shown below... The geometry of the arm-cartridge is anything but appealing now, but the results are well worth it.

1) Proper SRA has dramatically reduced intermodulation distortion, which can be easily verified with large scale choral music - grainy choruses now sound extremely smooth:

View attachment 223

If you were to look at the stylus with a 10x loupe, you'd see the end result, which unfortunately I cannot photograph.

2) Perfect Azimuth has expanded the soundstage and improved channel separation:

View attachment 224

After setting azimuth correctly, the results can be verified by playing a 1kHz mono tone:

View attachment 225

After these adjustments, I went ahead and realigned the cartridge with my Mint LP tractor again. The overall improvement is phenomenal. One of the more important decisions I made last year when I got back into analog was to get an arm that supports all these adjustments. I would now not be able to go back to any ordinary arm!

I hope you calibrated those meters beforehand. ;-)

 

[J.R. Boisclair]

Thank you for pushing me to the relevant issues, @J.R. Boisclair .
I really mean that.



I did.


I totally see your point, but it is not too difficult to bear those in mind I think.
I uploaded a sketch just to show the impact of VTF here a few years ago (copy below).


View attachment 134061


If VTA/SRA is so sensitive, it really should show in at least some of the tests. How unlikely is it to have parameters sensitive to everything but not showing anything?

I claim that VTA and SRA are NOT as sensitive on a degree for degree basis as zenith error correction and azimuth. I see no reason why it cannot be measured. It certainly has been in published studies.

I know that VTA is an angle defined at the cartridge, but 1° at the cartridge is the same as 1° at the tonearm pivot. The difference is that the forces change. That's why I wrote to adjust VTF after VTA/SRA adjustment.

Still, those are the means I have to change VTA/SRA if I want to keep especially zenith and do measurements in a controlled way.
Your point is valid, but nevertheless, a VTA/SRA change by +/-5mm at tonearm pivot doesn’t impact waveform or FFT, at least not for a line contact stylus.

Sorry if I seem stupid stubborn, but if I’m wrong or correct in the statement above, A LOT of discussions in this topic would be clarified.



View attachment 134067
Thanks.


No.
My intention was that UATLP track B4 is for VTA adjustment and track A9 is for SRA.
If VTA is not sensitive for the current cartridge, SRA can dictate the adjustment and vice versa.




View attachment 134067
I have the same experience, especially with Lyra which I am very familiar with. By ear I always ended up with a smaller VTA than a level tonearm gives.
But if I can’t see it on the waveform or FFT, how can I see it?

This is where I lean on the WAM Engineering engineers, particularly our EE PhD who spent his career in Fourier Transform and optics. I can say that with our zenith error studies he is managing the analysis at the smallest discretization possible. When I record a sample for him to analyze we spend a great deal of time making sure we are maximizing bit usage, highest sampling rate, etc., etc.

I'm really sorry, but I'm not your resource on this as I am not an FFT expert by any stretch of the imagination. There are likely to be some hints for you when we release our paper on zenith error. It will most certainly be written with such a fine degree of detail such that is likely to melt most brains - including mine!

Again, thanks for your replies.

 

[Calle_jr]

I claim that VTA and SRA are NOT as sensitive on a degree for degree basis as zenith error correction and azimuth. I see no reason why it cannot be measured. It certainly has been in published studies.

Most of what I found deal with VMA and the problem of deviations in manufacturing, or it involves only one measurement not a variation of VTA/SRA. Also, it was not line contact styli.

James White and Arthur Gust did tests on different carts and varied VTA.
I don't have those papers anymore, but they varied VTA by 3° and showed changes in the range of 1dB for a vertical cut 1kHz sine. We can't use an adjustment method based on such insestitive results.

However, they got a 5dB change with a vertically cut 1kHz square wave by changing the VTA by 3°.

The audio-technica AT33PTG/II has a VTA=23° (mean value stated by a-t).
The vertical cut square wave I have (CBS STR-112) is cut with 15°, so I should get the best result by lowering VTA by 8°. I don't think that will work.

But I found some hope after a nights sleep (it's morning now in Europe). If I zoom into the FFT 2nd harmonic for track B4, there actually is a 1.6dB difference:



If I use the current "6 turns CW" (the green curve) as "0 VTA" and lower the whole tonearm by some 10mm, it may show correlation. I.e. it may be that I was too far from "correct VTA/SRA" in the above measurements and that the distorsion level differences sort of flattens out. Maybe.


Does anyone know the VMA on UATLP?

I'm really sorry, but I'm not your resource on this as I am not an FFT expert by any stretch of the imagination. There are likely to be some hints for you when we release our paper on zenith error.

There are definitively pitfalls in FFT, e.g processing gain, size, windowing, energy distribution etc. But I'm quite familiar with it and in the tests above it is relative measurements, all made in the same way.
I really look forward to the paper!
.

 

[mtemur]

Check this one @Calle_jr
It's a very informative article about VTA/SRA.

I agree with you @J.R. Boisclair on VTA and SRA not being as sensitive on a degree for degree basis, at least not as sensitive as azimuth and zenith error. This is true for maybe 90% of cartridges but on the other hand I've came across some cartridges that are incredible sensitive to VTA/SRA. Even a smaller change than a degree makes an important difference. No change in readings but very audible. Extremely sharp edges of micro ridge tip at 3 micrometers might be the reason, I don't know. If we rule out those outliers I believe we can conclude that VTA/SRA is not a degree sensitive adjustment.

BTW I'm long aware any change on VTA/SRA also changes azimuth, zenith and sometimes overhang. I always check others and realign them if necessary when I change VTA/SRA. My finding is not polluted by changes on azimuth and zenith. I'll keep calling zenith for convenience.

 

[Calle_jr]

Check this one @Calle_jr
It's a very informative article about VTA/SRA.

Thanks!
I've seen that article and thanks for reminding me!
They tested a DL-303 with a modified elliptical stylus.
It shows a change of +/-2dB distorsion when changing VMA between 10 and 30° (!!!) for a 1.85+3.15kHz double tone track (DIN 45 542).

If we would go for more normal VTA adjustments, say +/-2° (which is still a lot), it would mean distortion variations of less than +/-1dB.

 

[gwernaffield]

would all off this depend if the pressings are exactly the same thickness and the same groove width , which they never are, and perfectly Flat would this not give different readings if they are all different in thickness more then + - 2% ,

 

[Calle_jr]

If I use the current "6 turns CW" (the green curve) as "0 VTA" and lower the whole tonearm by some 10mm, it may show correlation. I.e. it may be that I was too far from "correct VTA/SRA" in the above measurements and that the distorsion level differences sort of flattens out. Maybe.

I moved the tonearm down by 5mm and did the same 3-measurement procedure (0 turns, 6 turns CW and 6 turns CCW).
I named the "0 VTA" as still being when the tonearm is level.






It actually shows convergence.
If I lower VTA by 9.6mm compared to a level tonearm, the distorsion will be at minimum. For that change, the tonearm angle will be about 2°, which is quite logic since the VTA=23° for the AT33PTG/II cartridge.

So, by doing a 3-measurement procedure with a 1kHz vertical cut track, make an FFT and study the 2nd harmonic, it seems we will be guided in what direction and how much the cartridge should be angled.

I agree it is not optimal to do this by changing the tonearm height for two reasons:
1. It affects the VTF (and hence parallel HTF) quite a lot.
2. It affects the waveform flanks.
But so far what I've seen and what I expect, the influence of this is very small for a few degrees VTA change. It is however important to adjust the VTF after the VTA is set.

would all off this depend if the pressings are exactly the same thickness and the same groove width , which they never are, and perfectly Flat would this not give different readings if they are all different in thickness more then + - 2% ,

In my opinion, the influence from difference in LP thickness and groove width is very very small compared to the variations in VMA and cartridge VTA.