The importance of VTA, SRA and Azimuth - pics
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Lucky are those whose stylus geometry is such that SRA can be set by microscope... Such is not the case with the A90.
This picture is with the recommended tracking force of 2.3g and the arm parallel to the record surface; it's supposed to give us a VTA of 23 degrees - I measure close to 26 (TA2). Notice also how radically the stylus is mounted to the cantilever - nothing that would resemble a vertical stylus to the surface; moreover, the angle between the stylus and the cantilever is 93.8 degrees (TA1). TA0 represents the angle of the rear edge of the stylus to the record surface, but I am not sure how relevant this is - the Replicant 100 is too radical and probably asymmetrical to make any assumptions:
This picture is with the recommended tracking force of 2.3g and the arm parallel to the record surface; it's supposed to give us a VTA of 23 degrees - I measure close to 26 (TA2). Notice also how radically the stylus is mounted to the cantilever - nothing that would resemble a vertical stylus to the surface; moreover, the angle between the stylus and the cantilever is 93.8 degrees (TA1). TA0 represents the angle of the rear edge of the stylus to the record surface, but I am not sure how relevant this is - the Replicant 100 is too radical and probably asymmetrical to make any assumptions:
Mine was slightly over 26 degrees at 2.3g, but it seems to vary somewhat from cart to cart. This is my second one, the first was at about 25.5.
Regarding the SRA, unless there's yet another angle at the very tip which is too small to see with a USB side microscope, I had always assumed the right side of the tip was the actual angle of concern. It seems to conform with tweaking by listening, then checking the microscope.
I took several reference shots while breaking in the second A90. This was taken at 8 hrs and without cleaning the shank first.
It *seems* rather easy to adjust visually.
--Bill
Gents, would you mind describing the USB 'scope and software you used to take these pics and measure the angles? Thanks!
Actually, I made the same assumption as well, so I used the microscope to set vertical (TA0 above) - and magnified with various tools - then based on the effective length of the arm and trigonometry I calculated SRA; the result:
I have had numerous listening tests thereafter, up and down, and no other adjustment is as accurate as the mathematically calculated, using Reference Recordings records. The results are actually easily verifiable with Diana Krall's sibilance Live From Paris on ORG. I am sure you have seen this thread...
I'm using a Dino-Scope AD413T with the plastic nose piece removed (to get closer, at the expense of smoother lighting). Also using an outrageously priced MS36B semi-articulating stand to hold the scope in a stable enough position to get useful shots. PCGEARS had the best price on both last I looked.
The measuring software comes with it, though it takes a little getting used to.
--Bill
Mine is the 3013T. I am going to get te 413T or higher because I need the higher resolution. I think I have the same articulating stand, and I also remove the nose piece, thus rely on flashlights - overall, it's an arduous process.
I wouldn't have a clue as to calculating it mathematically for an ET2. Yes, I've seen the thread, but somehow I missed the date ranges that had the pictures and explanations. Will have to revisit that.
I spend some time in making sure my measurement and interpretation of the visible angle are correct and match a point in my SRA gauge (adjustment on the ET2) that are centered in a repeatable range. I'm usually extremely close by using 91.5 as a target angle, and then tweak individually on the gauge for older vs 180/200gm records. You get better at it with numerous tries.
There's a freeware software called 'crosshair' (very old) which puts a full screen crosshair on your display. I use it as a guide to tilting the scope so that the record surface is exactly level and I don't need to concern myself with the horizontal position of the measurement angle on the Dino software. Once that is known and stable, it's pretty easy to line up the vertical part with the edge of the stylus. Not easy, but pretty easy.
--Bill
It sure is. And it can be quite frustrating if you happen to be a little shakey that night. I don't lose the LED's by removing the nose on the 413T, but however you light it, a good back light really helps with detail. Mine broke so I've been doing the most recent shots with front lighting only, which really robs detail.
I'd really recommend going higher res than the 413T if it exists. The 413T is just ok for an Ortofon-size stylus, but others are quite small and there just isn't enough res to see the ridge detail sharply enough to conclude an angle. Also, the AM series has a bigger nose than the AD series and really gets in the way.
If you run into to something more hi-res, let us know. I'm certainly looking forward to an update.
--Bill
So the general idea to using the math approach goes as follows - keep in mind it's really still an approximation, as explained later on:
- Raising the arm to get a specific SRA is really trigonometry
- You start with the arm perfectly parallel to the record surface, and the cartridge already aligned
- You adjust VTF [edit: or raise/lower arm] until the stylus is perfectly perpendicular to the record; you really need to use a microscope to assert this as best as possiblest, as we have done above. This step assumes you can get a perpendicular stylus within the recommended tracking range. At this point you have a perfectly parallel arm and 0 degree SRA.
Side 'c' is your arm that must be raised by 'a' units from parallel to get a resulting 'A' angle of whatever SRA offset you want - in the picture above, I configured A=2 degrees [edit: well, not exactly - this is for illustration purposes; see below for the true desired angle of 1.8 degrees]. Before you raise the arm, 'c' and 'b' obviously coincide and you have a "collapsed triangle" (new term
) - that's because your arm is parallel to the surface. To calculate side 'a' - the distance you have to raise your arm - you need to know 'b'; however, 'b' is your arm's effective length!So next you need to calculate your arm's effective length. With a linear-tracking arm like the ET-2, it's obviously a piece of cake. With a pivot, you need to know your arm's two null points and the pivot-to-spindle distance, and then the following formula can be used (also borrowed from the A90 thread):
(OC^2 + NP1*NP2)^0.5
where NPx are the null points, and OC is the pivot-to-spindle distance.
Finally, you plug in the desired 'A' and calculated 'b' into this calculator - where I obviously borrowed the pictured from - and you have 'a' which will give you SRA offset ~= 'A', but not quite exactly 'A', because, since the arm is raised:
- Side 'b' effectively shortens a bit
- VTF is inherently affected, which will affect final SRA [edit: Fremer claims here that, in general, for a 9 inch arm, a 1g VTF change will result in a ~1 degree change in SRA]
So this is why the calculated angle 'A' is an approximation, but I believe a very good starting point. In my case, the A90 still tracks within the recommend VTF, slightly above 2.1g, and I feel good about it. [edit: because I 'lost' 0.2g of VTF while adjusting SRA - which means the final resulting angle will be higher than calculated - my true desired value for angle 'A' in the calculations above was actually 1.8 degrees, considering Fremer's claim; the shortening of side 'b' barely affects final calculations]
Perhaps my system's resolution doesn't enable me to hear improvements outside the calculated range, or perhaps my expectation bias kicks in, but I believe any audible errors would be in the calculations (including the desired resulting angle 'A'), not the approach.
[edit: also see this related thread on setting SRA with the Fritz Gyger stylus on the A90]
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There's a problem with step #3 as it pertains to the A90. The two that I have had come no where close to arm parallel for 90 degrees at recommended VTF of 2.3g. To get there, you would need to drop VTF down to the 1.7-1.8g range, or hold 2.3g and significantly tail-up the arm. It only gets worse from that point as the cartridge breaks in. That was why I sometimes wondered if there isn't another hidden tip angle that we can't see with the current level of magnification. But listening tests don't bear that out.
So what do you fudge (or how) to accommodate this kind of variability?
The rest of your post is excellent but definitely makes my head spin.
--Bill
This is not an issue at all. I tried to keep my post as simple as possible, but in that case, all you have to do is compensate for this in the final calculation:
- You start again with a perfectly parallel arm, and constant VTF
- You lower the arm by X units to get a vertical stylus
- Using the same trigonometry formula, side 'b' and now side a'=X (a' sits now below 'b'), you calculate the "negative" angle of "error" (i.e. the angle that's formed now below side 'b'), A'. Don't forget that now side 'b' has lengthened a bit and VTF has increased, so A' is yet again another approximation, but still very accurate
- You return the arm to the parallel position and then you base your subsequent calculations on a desired SRA angle of A-A' - so if you want a final SRA ~= 2 degrees, you base your calculations on 2-A'. Done.
In the end, it's really simple trigonometry that yields very good approximations because of the caveats mentioned before (side 'b' and VTF being affected during the process). I found empirically that the best sound was within +-0.5mm of arm adjustment from the calculated value, and we are splitting hairs at that point...
Therefore, this approach offers a predictable mapping of desired SRA and arm adjustment, and does not pontificate that 2 degrees SRA is really ideal. The real trick is in getting a good reference point: a vertical stylus within the recommended tracking range; and, in my view, getting the final SRA still within the recommended VTF.
So this is how I got the A90 to actually track within the recommended range...
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How do you deal with the different thicknesses of each record ? IME not all 180 gram, 200 gram and standard weight albums are the same thickness in each weight category. All this effort to set VTA yet it probably won't be correct for the next record. I prefer adjusting VTA for every record I play. How do I do this easily and conveniently ? I use a Graham Supreme arm with the built in VTA bubble level. Whatever record I play, I get it dead level and adjust by ear from there if necessary....all within 30 seconds of starting the album side.
Yeah that's fine, if you are comfortable tuning by ear. Again, the above is simply mapping _desired_ SRA to arm travel up and down. Each record, as you said, will have a different SRA requirement, and it's partly related to weight (i.e. thickness) as well as to actually how the record was cut (what angle was used) - that, you cannot possibly know unless you start calling the factory. So yes, tuning by ear is still very valid. Mathematically, you can still easily factor in the thickness of the record to the calculations above, if you: a) first, measure the thickness at the spindle hole; b) raise/lower the arm accordingly in your attempt to achieve perfectly parallel relationship. As I mentioned before, I used a Reference Recordings (Quality Pressings) LP for my final adjustments. There is no denying it's an endless cat-and-mouse game, and it all depends how anal you want to be. Personally, I set it and forget it...
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