What's the Deal with SRA?
...and what does it have to do with a garden hoe?
In high performance analog playback, azimuth and stylus rake angle ("SRA") are the two setup parameters most likely to have a major impact on the playback performance, yet they are the two that are usually implemented improperly and imperfectly. This article focuses on what the ideal SRA is and why achieving it is so critical to playback performance.
When we playback our records with our fine-contact profile stylii (think Shibata, Geiger, microridge, line contact, etc.) we want the angle of the stylus contact edge to be at the same angle the lathe's cutter head was at when the lacquer was cut in order to keep distortions to an absolute minimum. Why? Because the turntable is a transcription instrument, so the closer we get it to read the grooves in the same manner in which they were cut by the lathe, we will enjoy lower playback distortions and higher playback quality.
THE LATHE AND LACQUER DEFINE THE IDEAL SRA RANGE
Setting the ideal SRA at home starts in the cutting room with the record lacquer on the lathe. The lathe's cutter head drops down into the soft lacquer and is modulated by the musical signal, cutting the spiraled groove that will later be pressed onto our LPs. The angle at which the cutter head's sharp edges meet the lacquer MUST be greater than 90 degrees in order to remove the "chip" that is cut out of the lacquer. Cutter heads of 90 degrees or less will result in some of that soft chip being pressed back into the freshly cut grooves - not good.
So we know the cutter head needs to be greater than 90 degrees, but how much more?
Imagine using a garden hoe in loose soil. When you drag the hoe through the soil you would notice that the hoe begins to dive into the ground as you draw it forward. It does this because the hoe's blade is at an angle greater than 90 degrees to the ground and when forward motion of the hoe's blade is met with the friction of the soil a downward vector force is created on the hoe, causing it to dive down.
If a lathe's cutter head is at too steep of an angle against the lacquer, this same downward vector force causes instability in the cutting process. This instability causes cutting distortions which will end up on our LPs. This cutting instability can be avoided by keeping the cutter head over, but barely over, 90 degrees. The greater the angle, the greater the cutting instability.
We can imagine that aiming for a 91 degree setting may not allow enough room for setup error and would not provide the engineer with enough confidence that the chip will always be kept away from the freshly cut groove. We also know from doing some calculations that anywhere over 94-95 degrees causes downward forces to increase to the point where distortions become an issue in the cutting process. By inference then, 92-93 degrees would be the ideal cutting angle and the ideal SRA for us to aim at.
But perhaps the inferential approach isn't enough?
A MEASURED STUDY
In 1981, Audio Magazine ran an article by Jon Risch and Bruce Maier entitled "More than One Tracking Angle". In this definitive analysis on VTA and stylus rake angle, the authors arrived at the conclusion that 92 degrees is the ideal stylus rake angle for most records. There is no need to go over the details in this space when you can read the article:
GIMME THE BOTTOM LINE
The bottom line is that audiophiles should aim for a 92 degree SRA on the average thickness LP. This setting won't necessarily be perfect for every LP, but it will put you in the best strike zone to get most of your LPs at optimal playback performance. Once we know we have set our SRA for 92 degrees, we don't even bother with further adjustments by ear since the result of adjustments by listening will only apply to the records you are currently playing. Also, on many tonearm designs, adjustments in SRA will impact vertical tracking force and even azimuth. This is why the WallyTools approach has you set SRA as one of the first parameters to dial in. So set it at 92 and forget it!
SO...HOW DO I GET 92 DEGREES SRA ON MY SETUP?
Those details will be left to the instruction manual for the WallyVTA which are coming soon. In the meantime, suffice it to say that there are only two ways of finding 92 degrees on your cartridge: either by performing intermodulation distortion tests using specialized equipment and the underlying assumption that your other setup parameters are perfectly correct so they do not influence the IM distortion results OR by using a microscope, a thoughtful setup apparatus and some basic drafting software.
We are currently working on an affordable and easy to implement at-home solution with the WallyVTA for using an inexpensive microscope to do this for yourself. Stay tuned!
HEY, YOU'RE WRONG! SRA WILL CHANGE WHEN THE STYLUS MEETS A SPINNING RECORD GROOVE!
Yes, this is true! The drag upon the stylus WILL cause a force that pulls the cartridge downward and therefore DECREASE the SRA you measure under static conditions. How much SRA changes under dynamic conditions varies by factors such as groove modulation, cantilever length and cartridge compliance. Wally Malewicz has measured this effect and have found between 0.2 and 0.5 degrees reduction in SRA occurs under dynamic conditions. This is why you will find in the WallyVTA instructions that we suggest aiming for 92.2 to 92.5. We'll walk you through it!
The WallyVTA has two additional uses beyond the ability to measure for perfect SRA.
It gives you a REFERENCE point for the cartridge - a perfectly level top surface of your cartridge - from which you can then determine whether your stylus mount is acceptable or needs to be returned to the manufacturer for repair or replacement.
Unfortunately, we have found far too many cartridges costing well over $5000 that have SRA at 87, 88, 95, 96, 97 degrees when measured at nominal VTF and a level cartridge top surface. Stylii mounted at these angles are unacceptable since many tonearms will never be able to raise or lower enough to achieve the ideal 92 degrees before bottoming out or coming out of the arm base.
The WallyVTA allows you to quickly and easily move your cartridge from arm to arm without having to put it under the microscope each time to confirm ideal SRA.