Setting VTA/SRA by ear
Based on some recent email discussions, I wanted to revive this thread a bit... I have read numerous claims on the net how to best set VTA/SRA by ear, and the one that's been floating around on audiogon for quite a while now is listening for harmonics either leading or trailing the fundamental, as an indication of SRA being off in either direction. I think this is entirely misleading: the very definition of a harmonic is that it cannot possibly exist w/o its fundamental, so for it to lead or trail its fundamental is meaningless to me.
There are some very informative posts in this thread, primarily #7 from Myles (pointing to ET2 arm's manual), and #35/#37 from DonH50. After reviewing this thread again and other articles on the net, I wanted to point out the following:
SRA affects Intermodulation Distortion (IMD), and is thus optimally set with an oscilloscope and/or spectrum analyzer and two test tones in a test track - and Don says up above that they were best able to do that with test tones; but who really has one of those analyzers and what exactly is the SRA angle of any test record used with it to be used as a reference point? These issues render such advanced methods useless for us mere mortals. So listening tests and USB microscopes is what's left. Microscopes have been discussed elsewhere here and TAS has an excellent article on its site and current issue by Mr. Jennings; so what about adjusting by ear?
The ET2 manual includes IMD tests in pages 51-54; moreover, Fremer says the following at
http://www.analogplanet.com/content...icroscope-set-92-degree-stylus-rake-angle-sra
The more accurately the playback stylus replicates the cutter stylus angle, the less IM (Intermodulation Distortion) is produced, resulting in smoother yet far more detailed sound. Setting SRA by “ear” usually meant going from brighter, harder sound when set too high, to duller, thicker and less focused sound when set too low.
IMD as a function of VTA/SRA goes back decades; I found this article very interesting as well
http://www.hi-fiworld.co.uk/vinyl-lp/37-technology/73-vertical-tracking-angle.html
Because the practical subjective consequence of vertical tracking angle errors on sound reproduction are really of most significance I don't intend to dwell too long upon measurement technicalities, but a few consequences of incorrect vertical tracking angle are presented for illustrative purposes. Classic in measurement terms is the waveform to be seen in Fig 5. The upper trace alternately shows rounded and sharp peaks of significant second harmonic distortion and the spectrum below shows the harmonic structure. From the same disc can be seen significant intermodulation distortion in Fig 6 due to this cause. Lower trace shows two signals, one of 4kHz and one of 200Hz from the disc; above can be seen the 4kHz component filtered out. The change in level is due to presence of distortion sidebands from amplitude modulation which, in more straightforward terms, means that the 4kHz tone is now accompanied by other tones of significant level that weren't there in the first place and wouldn't be there either but for a vertical tracking angle of just over 30°.
See also post #19 here
http://forums.stevehoffman.tv/threads/how-to-set-vta-from-first-principles.151366/
The definition of IMD is (from wikipedia) is:
Intermodulation or
intermodulation distortion (
IMD) is the
amplitude modulation of
signals containing two or more different
frequencies in a system with nonlinearities.
The intermodulation between each frequency component will form additional signals at frequencies that are not just at harmonic frequencies (integer multiples) of either, but also at the sum and difference frequencies of the original frequencies and at multiples of those sum and difference frequencies.
The key here is, additional harmonics at the "sum" and "difference" of the fundamentals - basically,
IMD introduces EXTRA unwanted harmonics beyond those occurring naturally.
This is why the Analog Productions Test LP
http://www.analogueproductions.com/index.cfm?do=detail&Title_ID=35532 (and others like it, e.g. the CBS and DIN test discs referenced in the articles I quoted) have a special track for VTA/SRA:
Track 9 VTA adjust
This is an IEC intermodulation distortion (IMD) test signal; 60Hz & 4kHz 4:1 ratio.
Using an IMD tester, adjust VTA by raising or lowering the tonearm for minimum distortion.
Notice, it includes just two frequencies so that one can see the extra sum and difference harmonics on a scope or spectrum analyzer, and make sure they are minimized or eliminated. But again, who has such equipment and no one knows the SRA of those test record (thus even these types of tests are not meaningful enough).
Therefore, to me, the bottom line is that we should be listening for the minimum amount of intermodulation distortion - minimal or no additional unwanted harmonics - when adjusting VTA/SRA, and hopefully for every LP if we can.