Sorry about the first pic URL. Just paste the whole thing into your browser. |
Doug, I'll take that remark as a compliment? (unless informed otherwise.) Actually, this thread was partly inspired by the beautiful stylus pics you posted, plus people keep asking me how to do this. I know you adjust your SRA a lot, and I wish I could too, but the SME V was apparently designed in the "set it and forget it" days. I keep mine at 1.4 degrees which seems to work well for most recordings, but if anyone out there has an xtra Tri Planar for sale cheap, I'd be interested ;~))
Dan, I don't blame you for asking. It's quite simple really, and just a matter of geometry as you will see. Unfortunately, it would be a lot easier to draw it for you, but I'll try and explain it in words first, and maybe later I'll have time to make a diagram and post it. So please leave a post to let me know if the following makes sense to you:
A typical 9 inch TA measures 23 cm (or 230 mm) from the vertical bearing to the stylus tip. Imagine now that you lift your TA with the finger lift, but instead of setting it on the record, you just keep lifting it (magically of course) up and over the back of the turntable base, and continue under and up through the base to the armrest again.
OK? So you've just made a circle with the stylus tip; so far so good? The length of this circle's circumference (in millimeters) is PI times the diameter of the circle. The radius of the circle is 230 mm (the TA length) so the diameter is 460 mm. PI is 3.1416, so PI x 460 = 1445 mm (the length of the circumference traced by the stylus).
There are 360 degrees in a circle. So each (pie-shaped) degree of our circle would contain one three-hundred-and-sixty-eth of that 1445 mm circumference. Divide 1445 mm by 360 = 4 mm.
So if you raise the stylus end, OR THE BEARING END (which is what we need to do to create SRA at the stylus end) you are going to tilt the tonearm AND THE STYLUS (which is attached to it) exactly one degree!
If you have a cool tonearm like Doug's Tri Planar, you just dial up however many degrees (times 4mm) you want. If you're stuck with something less accomodating, I've found the best way is to use a stack of automotive feeler gauges to achieve the height you want. |
Ketchup, you are quite right, this is an excellent way to check azimuth. There is just one leeeetle setup problem you need to solve first:
If you just place the mirror on the platter, you'll never get a scope in there to look at the image because the platter is in the way. Here are two ways to overcome the problem:
1.) If your tonearm has the vertical bearing axis perpendicular to the headshell offset, then all you need to do is shim the mirror up on something about 3/16" thick like a piece of balsa wood or foam-core. Just high enough to let you rest the end of the scope on the platter and be able to see the stylus head-on.
2.) You could cantilever the mirror a half inch or so off the edge of the platter (taping it securely to the platter, AND taping the platter to the TT base so it can't rotate!) and this will also allow you to look at the stylus head-on.
Be very careful though! You might want to use a piece of masking tape to restrict horizontal movement of the tonearm. |
Dan, architecture (what I do to support my audio addiction) is all about geometry, so it comes quite naturally ;~)) |
If you have a spherical stylus or even an elliptical sytlus, then the actual SRA is not too critical because whether the stylus is vertical, or leaning (less than a degree) slightly forward or backward, the very rounded tip is going to fit in the groove more or less the same way in all cases.
Such is not so for (the more common today) "micro-ridge" type styli, pioneered by Shibata and van den Hul. These styli are shaped a bit like a spade shovel, broad across the groove and thinner front to back. Their long edges touch the groove on each side, in a line running more or less from top to bottom.
The cutter head is VERY spade-shaped -- a chisel actually. It is not vertical either -- never has been, even with 78s. Why? Because if it were vertical, the cuttings it produces would jam between the head and the groove and ruin the master. Instead it is tilted forward (at the top, just like you'd do with a wood chisel) so that the cuttings keep "peeling" off and out of the way.
Therefore the "wiggles" the cutter cuts into the groove are all "leaning" forward as well as wiggling side to side.
If you have a "cutter-shaped" stylus (micro-ridge) and you want it to make the best contact or "fit" with the groove, it has to be tilted forward so that its edges match the forward-leaning groove wiggles. This angle of tilt is called the Stylus Rake Angle, or SRA. It is usually between 1 and 2 degrees for most recordings. But as Doug already pointed out, there is no actual standard, and cutting lath operators have some leeway in how they want to adjust the head for a particular recording, or perhaps use with certain brands of blank masters.
This is why I recommend adjusting the SRA to just under 1.5 degrees. Then, if you have a tonearm that's easy to raise or lower, you can "fine tune" the SRA for a given record -- increasing it for a thicker record, for example, or decreasing it if your ears tell you that maybe the lathe operator set the cutter closer to 1 degree.
A "vertical" setting would never do for a micro-ridge type stylus, because the edges of the "spade" would be straight up-and-down, and would actually scrape across the tilted wiggles instead of fitting into them. |
Greg, actually, the chisel, or spade-shaped cutter, can only move in one plane. It's a plane that lies across the two "spade" edges. It moves diagonally (up left or right or down left or right) but remains constrained on that plane. It's movement is created by two electromagnetic drivers on each upper corner of the cutter blade. It can cut a signal (wiggly undulations) into either groove wall while cutting a smooth groove wall (no signal) on the other side, Or it can cut signals, even different signals, into both sides of the groove at the same time.
I wish I could find a decent diagram somewhere to post. If not, I'll try and whip something up and post it. |
Greg, yes you simply stated it backwards. The cutter head stays in one plane but moves in two directions.
Imagine you have a nickel on your desk. Imagine that's the cutter head looking from the front. You put your finger on the nickel. You push it up and down, left and right. But it still remains in one plane -- the plane of the table. |
Greg (and anybody else who seriously wants to know how this all works,) check out this page: YOU ASKED FOR IT !!
http://aardvarkmastering.com/westrex.htm
Note: until you get down to picture no. 9, you are only shown one coil, but pic. 9 and 10 clearly show both coils (at 45 deg. to the horizontal, 90 deg. to each other) each with a pin or "link" that is soldered to the torque tube. The cutter head or stylus is installed in pic. no. 11.
At the bottom of this page (I finally found one) is a line diagram of a Westrex cutting head:
http://aardvarkmastering.com/history.htm
Any questions? |
Readster, basically, you're on the money or you're not. HOWEVER, it's perfectly valid to take the position that if you could get kinda, sorta, close to the money, you could live with that.
Here's how I'd go about doing that:
First, let me say that of a few different cartridges I've inspected (plus what I'm learning from folks who have taken the time to check using instruments) it appears that all cartridges with micro-ridge styli are built with SRAs in the following range:
At one end of the range, the stylus is vertical in the groove when the cartridge (top surface) is parallel to the record/platter. At the other end of the range, the SRA is around 1.5 deg. when the cartridge top surface is parallel to the platter.
In other words, if you got your cartridge parallel to the platter (which generally means your headshell and tonearm too) then you know this much: the SRA is somewhere between zero and 1.5 (+/-) degrees.
Let's say from this level position, you arbitrarily apply 1/2 degree more, or about 2 - 3 mm. at the tonearm post. So now you know that the SRA is somewhere between 0.5 degree and 2 degree (depending how much or little SRA was built into the cartridge initially) but of course you don't know which end of this range your particular stylus is at.
However you can tell by listening, IF you know what to listen for. Unfortunately, this kind of "informed listening" ability is most usually gained by my making SRA adjustments up and down from a correctly set SRA.
But basically it goes something like this (assuming everything else is set up correctly -- load, VTF, ASF, overhang, tangency, etc.):
Listen first to bass. Write down what you think you hear. Is there enough? If yes, how does it sound -- crisp with slam and attack or sort of rounded and/or muddy. If there's not enough or it sounds thin, then you have too much SRA. Reduce it until you have good clean bass without bloat. If it sounds bloated and muddy after you obtain good bass response, then you've gone too far, back off a bit.
Then (or if bass was OK to start with) listen to highs and midrange. If it sounds glarey or grainey or harsh in any way, it usually means (unless it's something else in your system) the you have too much SRA. Usually you won't have very good bass either, which confirms it's the SRA and not some component or cable. Reduce the SRA until the highs are silky. Still not enough bass, reduce the SRA a bit more, it won't affect the highs that much.
Were talking about tenths of a millimmeter here. So get yourself a set of automotive feeler gauges to help determine how much change you apply to the tonearm post (unless you have a TA that provides for easy adjustment of SRA/VTA)
Once you've got good tonal balance, see how you feel about image/soundstage. This is very cartridge/arm/TT dependent. But as anyone with a SRA-adjustable tonearm will tell you, there is one magic (SRA) spot with every record where the audio hologram just snaps into focus. It's very obvious and thrilling to hear. |
Greg - good pictures! I think the high-mag one shows pretty conclusively that the groove undulations slant -- in this case toward the bottom of the picture (from bottom to top of the groove.) |
Greg -- what a terrific website (you were referring to the article on Vinyl Microscopy?) My favorite part:
The animal (below) is a young silverfish with its extended cercae overlapping the closing bars of the final movement of Bruckner's third symphony. I mention this as it is not possible to tell from the photograph. There are few places in a Bruckner symphony where the sound of a diamond stylus colliding with a dead silverfish would be less welcome. The cadaver was removed by brushing with a carbon fibre brush. |
SRA is very easy to match to the cutter head rake angle. True, there is/was no absolute standard, but the range of variation is very small, and so even if you don't have a tonearm with on-the-fly adjustable arm height, you can usually find a happy setting that fits most record grooves at least 90% perfectly if not 100% all the time. The trick once again is to find the absolute vertical point described at the beginning of this thread and then go from there.
VTA is another matter altogether. It is the angle of travel described by the cutter head as it moves up and down and depends on the design of cutter's torque tube (something like a cartridge cantilever.) There is some variation here also, but the big (and impossible to match) variation is among cartridges themselves. Cantilever length is all over the place among different cartridges, so it always struck me as folly to even attempt to match cutter VTA. A mismatch wil only create forth order harmonic distortion (if I remember my reading correctly) which is virtually inaudible, and so any attempt to "dial in" VTA by ear would be futile anyway. That's why I get upset when I hear the term "VTA adjustment" in any context. Call it whatever, but what's really going on is SRA adjustment.
Greg, adjusting arm height (in the range we're talking about here) has no effect on stylus overhang. That's function of the position of the tonearm base relative to the platter spindle -- with fine adjustment available via slotted headshell mounting holes, or in the case of SME, a sliding tonearm base. |
Greg, maybe you (or Doug or I) can find some microscopic groove photos that will show this rake or slant in the groove undulations clearly. I know Last used to show such photos with their products. I'll see what I can find.
Neil
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If you read Mr. vdH's notes (the Q and A part) on tonearm setup on his website, you'll run across a statement where he flatly recommends raising the back of the tonearm about 6 - 8 mm (from level, or from parallel to the platter), I think it says.
This is actually about right for his vdH cartridges which (most of the time, but not all!) are manufactured with the stylus perpendicular to the top of the cartridge body when the cantilever is deflected under normal VTF.
Following his instructions gives you 1.5 to 2 degrees SRA probably 80% of the time for vdH cartridges (though I still think it's best to check with a scope to make sure.) My point is, that even if you just rely on his recommendation and don't check first, you should still be close enough to the correct SRA to find it by ear without much further up or down adjustment.
Greg's assertion that all manufacturers mount their styli so they have proper SRA when the cartridge is parallel to the platter would be really nice if true, but unfortunately the variations are pretty wide. Even among the products of a single manufacturer. My guess its that it's a difficult parameter to control tightly in real-life production. Also there are three ways I know of for attaching the diamond to the cantilever, so most makers are probably thrilled just to get that pesky diamond stuck to that little boron rod!!
The one thing I've never seen (at least in my economically limited cartridge auditioning experience) is a cartridge which requires the tonearm post be lowered from level (as in "ass dragger") to achieve correct SRA. Anyone who thinks their setup sounds best this way really needs to do the investigation I described at the beginning of this thread. I'm not saying it couldn't happen (never say never) but if I were a tonearm designer, I'd be upset to see my tonearm used this way because it creates a less stable mechanical assembly (relative to gravity) as the vertical pivot center gets closer and closer to, or even lower than!, the stylus' position in the groove. |
Andrew, I only mentioned the business about setting new carts' VTF at the high end of the range because it is necessary to do that in order to get the cantilever/coil assembly to "line up" squarely in the magnetic flux field (maximum output).
Later on, after about 100 hours of being compressed under record playing conditions, when the rubber suspension "limbers up", a somewhat lighter VTF will be sufficient to achieve that same proper mechanical alignment of the coils in the flux field.
Again, there is no magic involved here. In fact, as far as I'm concerned (here we go!!) the only criteria for the correct VTF setting is this alignment of coils in the magnetic flux. If you have that right, and the SRA and AS are on the nose, there should be no tracking problems (the most common reason for fiddling with the VTF).
If for some (other) reason your cartridge still won't track, look elsewhere, especially for possible compliance vs. eff. mass mismatch with the tonearm. |
Doug's post reminded me that this word "level" is getting tossed around quite a lot. Just so it gets mentioned, the first thing to "level" is the platter. And if you have a suspended table, that means with a clamp and record in place on the platter. (Keep the tonearm in its rest, it won't make any significant difference to "level" in its rest as opposed to sitting on the middle track of the record!)
After that, the position of the tonearm relative to the platter (either parallel or nose-down) will be determined by setting it to provide the proper SRA for the stylus. |
Andrew, don't be skeptical. There is absolutely no mystery here. It's all physics and simple geometry. The most important thing to remember when approaching the matter of setting correct SRA, is to have all the other settings perfect beforehand. IMO, the proper order for setting up TT/TA/cart. is as follows:
1.) Level the platter, and then cross-check to make sure the headshell is level (across, not front to back) -- especially important if the headshell is not adjustable (like an SME V!)
2.) Install the cartridge and adjust the stylus overhang at the spindle.
3.) Adjust the cartridge yaw (rotation as seen from above) to reduce tracking error at two points across the record using one of the many cartidge alignment gauges available.
4.) Set the VTF -- at the top of its range if it's a new cartridge, less if already broken-in. Use a digital stylus gauge that reads to 1/100 gram (two decimal places.) Do not apply antiskate yet. You'll need it off to check the stylus vertical position.
5.) Then set the arm parallel to the platter. By eye is good enough. I sometimes use one of those little round bubble levels. They weigh only .15 gm, so you can put it right on top of the headshell before setting the stylus on the record, without fear of cartridge damage.
6.) Now you can begin the SRA setup process. First find where the stylus is vertical as I explained earlier -- and which was the reason for this thread ;-)) -- and then raise the tonearm post 4 or 5 mm.
7.) Now you can set the antiskate. I do it visually if I can (meaning if I have a high compliance cart. which lets me see the cantilever shift with the application of AS force) Otherwise set it by the numbers. Using a test record can also be helpful.
8.) NOW you are ready to begin listening and moving the tonearm post sligntly up or down from the +4 mm setting.
I cringe when people make absolute pronouncements, but honestly, you must follow this process exactly if you expect meaningful results. |
Excuuuuuze me, but when did this turn into a musical preferences thread? |
