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RE: Fremer's ZENITH ANGLE CORRECTION or have we all gone crazy.

Posted by dave slagle on February 11, 2021 at 09:59:51:
My take thus far on this is quite different and only a hypothesis at this point so apply salt to what follows.

I suspect that the idea that incorrect zenith simply effects phase is flawed. The Electronic measurement I use is measuring the IMD of a 60Hz+7kHz tone with a 4:1 ratio. You then filter out the 60Hz and look at the FFT of the signal and the level of the sidebands at 6940Hz and 7060hz tell you the amount of IMD. If those levels are 40dB below the 7kHz fundamental then the IMD is 1%.

This is not a new testing concept. The Ultimate Analogue Test LP has instructions and a test track for setting VTA using this exact procedure and both the CBS STR-110 and the Shure TTR-103 lay out this test for mistracking. I stumbled upon this using Analog Magic's VTA setup and quickly realized that the zenith if slightly misaligned dominated the distortion measurements and a zenith error of 1.2� assures that a null point will never be hit on the playable record surface.

In trying to use this test to set VTA, The distortion was high and I could never obtain meaningful numbers by any amount of VTA change. I saw reference to try another alignment geometry so I twisted the zenith a bit and was shocked how much the measured distortion numbers changed. After a few I then began initially setting the Zenith first by using the VTA test outlined above. After a null in distortion was located in the vicinity of where the null point should be I them proceed to using the track to set VTA as it was intended.

here are the plots of IMD distortion vs. Zenith angle for a conical and a micro-ridge.

I have used this approach for electrically setting zenith on more than 20 cartridges and once you get the feel for it, it is a quick and effective way to get repeatably good sound across the entire side of an album.

I decided I should document this more fully as I typed this so I made a forum post about more of the details and to better document the whole process. It will be linked below and to avoid any appearance of being a shill I will say that I am a dealer for analog magik and while it is entirely possible to measure IMD by other means, I find it to be a simple and reliable tool for electrically aligning zenith. I would be all for any discussions on alternative methods of easily measuring IMD with a test record and a scope.

dave