Saturday, September 1, 2012

Know your angles. (new toy)

I finally picked up a milling machine that will fit into my already cramped workspace.  It was a good deal.... you know used only to drive to church by a little old lady in Pasedena.  Gotta love craigslist.

After my return from the land of walter, the field coil cartridge moved to the top of the heap and this thing showed up to empower me.  Life has kept me from actually using it but yesterday I woke up with a purpose... i wanted to make a stylus!

OK I made a scale model of a stylus and unlike cars or planes, this model went in the other direction.

It seems a lot of people have the notion that the new way to set SRA is by using a USB microscope.  Well I have seen some sincere abuse of this and came up with the observation that if you do not have the lens perfectly centered perpendicular to the contact facets, your measurements will be off.

Before going public with this I needed to actually prove it to myself and hence the split "slagle" contour above.  The split was key so I could see through the contact edges and make sure the camera allignment was correct for my test.

that small bit of light tells me I am in the right position and the lack of uniformity of the gap tells me not to quit my day job...oops  nevermind.  Now lets provide some measurements of this beast.

I placed my contact line at 90 degrees and measured the angles on either side.  Guess what... I really need to pay more attention to my day job.  Beyond that, I also realized how difficult preciison measuring is and I am actually content with the results of both my machining and measurement errors.

Here is the rub.  What happens if you are even a bit off center?  I shifted the camera to the left a bit to see what happens to the "slagle" profile. You can see the degree that I am off center by viewing into the side of the model.

Now the total shift may seem extreme and it was an inch or so but remember that we are dealing with a total dimension of only a few thousandths of an inch any shift of the camera is extreme.

I did my best to keep the SRA at a perfect 90 degrees but now notice that due to the camera being off center, the SRA line is now appears to be 87 degrees even though it is still actually 90 degrees.

I chose the symmetrical "slagle" stylus profile for this to make it easy but even with asymmetrical designs the point remains.  Unless you have both the left and right contact points of the stylus in in a straight line to the center point of the lens, any measurements made from the image will be incorrect.  Furthermore,  the image capture plane must be at exactly 90 degrees to the center line or things can get wonky again.

Personally,  I am happy setting the VTA by ear so I enjoy the music and believe introducing a flawed measurement system puts us back into the dark ages of audio.


  1. Hi Dave, I really enjoyed this commentary. Perhaps you will find interesting an article written in 2004 by Geoff Husband. See


  2. But if Wally measures it for you, it must be perfectly accurate right??

  3. "But if Wally measures it for you, it must be perfectly accurate right??"

    That's cold -R.

    But, true.

  4. Dave, this post is ludicrous on its face and your claim that measurements with a microscope is a "return to the dark ages" is hysterical and ridiculous. Using a microscope demonstrates and cantilever/stylus assemblies can be wildly off the mark from 87-94 degrees with the arm parallel to the record surface. Therefore your claim to be able to set SRA "by ear" is foolish. The microscope at the very least gets you close enough to be able to "set it by ear" but not having a reasonably close starting point guarantees failure if the manufacturing is off as is often the case even with expensive cartridges. And yes, if you don't do it correctly the microscope will give off base results. But that's true of any measurement so your point is even more ridiculous.

    1. Not to speak for the author, but I think the point was to articulate the difficulty of the measurement process in and of itself, not to say that SRA is irrelevant.

      Now, back to your tonearm jacked up 4 cm in the rear...

    2. yes... wut he said.

      I have yet to see anyone mention the issue of parallax (thanks robert) when it comes to microsope setup, however I have seen measured results and claims of mis-alignment that obviously weren't photographed from the correct angle.


  5. Mr. "Anonymous" missed the entire point about parallax. Even a mind-numbed robot should see that. I know I did.

    If I get the thrust of the posting, it was to show that inherent flaws occur when the control process itself is flawed. Using a microscope without a way to insure that it is correctly aligned is an inherent flaw. Maybe another microscope is needed to align the first? Hmm...