For the turntable curious; The Korf Blog

Discussion in 'Vinyl Nutjob World: Turntable and Related Gear' started by k3oxkjo, Apr 28, 2020.

  1. k3oxkjo

    k3oxkjo Friend

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    I would like to recommend this to anyone who wants to expand their turntable/tonearm/cartridge consciousness:

    http://korfaudio.com/blog

    You have theory backed by some practical vibration measurements, some of which seem to upend some long held beliefs, especially about tonearm tube materials and associated resonances, headshell resonances, proper azimuth adjustment and the old-as-the-hills tonearm mass/cartridge compliance calculations, their derivation and practical effects. Again, with measurements.

    I found this fascinating since I have been messing with Vinyl since high school, back when both fish and cars had fins and tilapia hadn't yet been invented. I have no connection with the blog except for reading and (trying) to learn and buying one of his own design headshells.

    Best, Kevin
     
  2. k3oxkjo

    k3oxkjo Friend

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    Mr. Korf just posted a summery of the latest conclusions about tonearm/cartridge relationships. According to Korf:

    1
    Carlson's formula of a low frequency resonance does not describe the measured low frequency behaviour of the cartridge/tonearm interaction
    2
    Modern cartridges (meaning all those built in the last 60 years or so) have too much suspension damping and non-linearity for the resonances to dominate
    3
    The frequency of the observed motion is determined largely by the frequency of the excitation
    4
    The cartridge/tonearm system acts as a lowpass filter for vibrations picked up by the stylus
    5
    Too low an effective mass for a given compliance (or too low a compliance for a given effective mass) results in low frequency attenuation and excessive tonearm motion.
    6
    Too high an effective mass for a given compliance (or too high a compliance for a given effective mass) results in "ringing"—a small resonant peak—that is largely benign and barely registers in the measurements

    Of course, reading the series of blog posts is the best thing, but this summery is enlightening for "people in a hurry".

    Best, Kevin
     
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  3. k3oxkjo

    k3oxkjo Friend

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    Azimuth adjustment summery.

    The best way to adjust azimuth is so that the stylus is perpendicular to the plane of the record. Measuring inter-channel crosstalk does not necessarily insure this (unless the cartridge is perfectly built by the manufacturer). If the cartridge stylus is not properly aligned with the generator from the factory, aligning the stylus for verticality will result in degraded crosstalk, but if the stylus is out of alignment and the generator properly aligned, it will result in increased distortion, increased FR difference between channels and worse tracking. This effect gets worse as the stylus profile gets less "conical". Most quality carts today have some kind of advanced profile.

    An aside, one of the reasons that some people prefer conical stylus profile may be its looser alignment requirements for acceptable performance.

    If you want to measure azimuth rather than eyeball, the way to do this is to adjust for equal FR and distortion in both channels to the degree you can.

    Best, Kevin
     
  4. Serious

    Serious Inquisitive Frequency Response Plot

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    As I mentioned in the other thread, I spent some time setting up cartridges with a test record and REW after reading the Korf blog. I still think that this is the proper way to do it and likely the only way that guarantees good channel balance. Still, I want to add some comments:
    It may not accurately predict the exact frequency, but I do think the general idea isn't that bad. There is a system resonance, although there's also the high-pass behaviour and I think it's likely that suspension stiffness doesn't really affect the resonance frequency. However with an added mass I did see a lowering of the system resonance frequency.

    Here's a measurement I took of a modded DL103 with a shiabata stylus on a boron cantilever. Green is as is on the VPI JMW 10.5 metal arm, blue is with an added 4.0g mass on top of the headshell directly above the cartridge (with the VTF adjusted to remain the same 1.75g). Note that this close to unsmoothed data (1/48th RTA in REW, 1M FFT length, 7 averages), so try to ignore the noise. Other cartridge FR measurements show more bass and treble extension and I always seem to get a 10kHz peak. I think all of that is attributable to my test record. I may try to compensate for the record's FR in the future, since I measured some cartridges that I have proper measurements of, but so far I left it uncompensated. There definitely shouldn't be such a large drop off past 20kHz. It's a 7" record, so we'd expect somewhat less treble extension, but not such a sudden drop.
    DL103SA added mass.jpg
    The FR is an average of the left and right channel mono (lateral) and out of phase (vertical) pink noise responses.

    We also see multiple sidebands in both cases. I'm not entirely sure why that is. The record isn't centered perfectly and as such will show some pitch fluctuations and the VPI Classic doesn't have the most stable speed, either, but I think both aren't to blame for the whole effect.

    It probably depends on the arm, the platform the turntable sits on and the turntable feets (small vibrations of the floor will excite this resonance) how strong the resonance is in the measurement. The VPI JMW arm may be less damped in its motion than the arms Korf tests. I'm sure it also shows much stronger midrange armwand resonances than some of his designs.
     
    Last edited: May 3, 2020
  5. Serious

    Serious Inquisitive Frequency Response Plot

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    Regarding azimuth I found that both methods (equal crosstalk and equal HF extension/equal distortion) result in close to the same angle. At least with the 4 cartridges I have measured so far the tolerances between generator and stylus were much, much closer than the tolerances between the generator and the cartridge body. Of course 4 cartridges isn't large enough of a sample size to generalize from, but I think it's an indication.
    I also found it odd that Korf conveniently posted a non RIAA-corrected FR here: http://korfaudio.com/blog38
    That will definitely hide some of the differences in the much larger total FR error. You can see that in his 'properly adjusted' graph there's easily a 1dB channel imbalance from 1kHz to 15kHz, which I think is likely more important than the differences past 20kHz. A 1dB channel imbalance is easily audible from my experience. In my measurements I aim for a level difference of less than 0.2dB before it gets insignificant (or at least hard to adjust).
    In my measurements I also found that it's often just not possible to the get the ultrasonic range to match perfectly, no matter which way azimuth was set. These aren't new cartridges and I think it's possible that this is from uneven wear on the stylus, but that's just a guess. I'd also expect better channel matching to be more important than equal distortion and equal ultrasonic FR, so setting it by measuring crosstalk is easier and more reliable to achieve that (In order to get good channel matching for both mono and out of phase signals you have to have matching crosstalk.).

    Getting equal distortion will also depend on the anti-skating as @deniall83 mentioned, so setting it by azimuth when the anti skating isn't set properly will result in improper azimuth, I found. Easier to match FR first and then set anti skate by looking at the distortion.

    So basically IME: Set azimuth by matching FR curves, use crosstalk to guide you. Then set anti-skate to roughly match distortion. Better to have a 1-2dB THD difference than having a 0.5dB channel imbalance.
    EDIT: Anti skate seems to mostly affect higher order harmonics while azimuth affects mostly 2nd order, so ideally you should look at distortion before setting anti skate aswell. Sometimes 2nd may be lower and 3rd may be higher for one channel, which I think is fine. And different frequencies will have different distortion channel matching (similar to crosstalk).
     
    Last edited: May 3, 2020
  6. Biodegraded

    Biodegraded Friend

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    This is interesting, but there's an error in his calculations. He doesn't seem to be recognizing that the Denon's compliance is specified at 100 Hz, not 10 Hz as the other carts are and as the Carlson formula assumes, so when doing the calculations to compare them, the Denon's 5 um/mN should actually be higher by around 1.5-2x (I've seen an assertion that the Denon's compliance is even higher at 12, but no backup was given).

    If the Denon's compliance was in fact around 9 um/mN, the Carlson formula would give a resonant frequency of around 9 Hz (not the 12 Hz that he calculates), pretty close to where its big peak appears on his accelerometer graphs.

    This is not to say he's incorrect about modern stereo carts being more damped and less linear than rigorous adherence to the old compliance formula warrants - but it might partly explain the height of the peaks that he's seeing from the Denon (system resonant frequency coinciding with excitation frequency) vs the other carts.

    Edit: Also the spec for the Shure cart he's using, 25 um/mN, is actually the static compliance (see here). To get the equivalent dynamic figure at 10 Hz, should purportedly be halved - putting it between the Denon and the Ortofon.
     
    Last edited: May 3, 2020
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  7. k3oxkjo

    k3oxkjo Friend

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    I think the point that he was making was not that the graphs he drew using the specified cartridge's compliance were gospel, but that the DL-103 which is, by anyone's standards, low compliance (as you would expect due to its high recommended 2,5 gram downforce) and the Shure M97xE, a high compliance model (with its rated 1.25 gram optimum downforce) measured essentially identical as far as the frequency of the measured peaks. The formula would of course infer that the resonant peaks frequencies should be significantly different, but that is not what he measured.

    I will let Mr. Korf speak for himself here:

    "Looks like the low frequency behaviour of the cartridge/tonearm combination is shaped more by Newton's third law than by the compliance resonance. Modern cartridges (meaning all those built in the last 60 years or so) have too much suspension damping and non-linearity for the resonances to dominate."

    BTW, I have both cartridges here (as it happens) and just lowering the cartridges to the record it's clear the Denon suspension is quite stiff and the Shure much softer.

    BTW2, I am not suggesting that Korf's conclusions are gospel either, but I do find his methodology backed by measurements to be fairly persuasive.

    Best, Kevin
     
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  8. Biodegraded

    Biodegraded Friend

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    Yes, I too like the accelerometer measurements, and I'm not trying to claim his overall conclusion is wrong - but it'd be better received, and he might have gotten some more insights, if he'd quoted comparable specs.
     
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  9. k3oxkjo

    k3oxkjo Friend

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    Actually, Korf agrees with you in the case of accurate factory cartridge alignment. Assuming this accuracy and that the tonearm's headshell alignment is parallel to the record (and the record perpendicular to gravity :)), there would be no need for azimuth adjustment. Of course, this assumes a lot of accuracy along the line. But if the stylus were in true alignment with the generator and body, you could adjust for crosstalk and be done, the rest would also be optimized.

    But if the generator/stylus/body/headshell relationship is not accurate, you have decisions to make. I would opt for better FR/distortion (which more affects tonality) over crosstalk/channel imbalance (which more affects soundfield) up to a point, but I would not argue that as an absolute. All of these problems point to a more fundamental problem where they occur.

    Ultimately, my view is that the stylus is where the rubber meets the road and that the most important thing is to insure equal contact of the stylus on the walls of the groove, if that's not true you have a compromise whatever else you do.

    I also believe that, rather than (or in addition to) lavishing cartridges with exotic stylus shapes, cantilever materials and exotic cart body materials, it would be better for a given price point to lavish more time on the bench to make sure that the stylus/cantilever/generator/body alignment was as perfect as possible. That would certainly make it easier to get the optimum performance a given design is capable. Then, simple adjustment for crosstalk (which has the benefit of being easier to measure than FR/distortion) would be sufficient. Maybe the alignment tolerances are an important spec we need to know :) (Don't hold your breath...)

    Best, Kevin
     
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  10. k3oxkjo

    k3oxkjo Friend

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    Maybe you should be upset with Denon for not making standard compliance specs available :).

    But whatever the Denon's compliance at 10 hz is, it's much lower than the Shure for sure (groan), so I don't think the Denon's exact number affects his conclusions or would have actually provided much further insights, IMHO.

    I guess that you could argue that that the cartridge's compliance all should have been measured rather than just accepting the manufacturer's figures (and to correct for any suspension changes with time), but I am willing to accept he went far enough down the rabbit hole as it is.

    Best, Kevin
     
  11. purr1n

    purr1n Finding his inner redneck

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    All good stuff that you posted from Korf, but if we learned anything from Korf, and also that little ditty I posed in azimuth years ago, it all comes down to measure measure measure. Eyeballing doesn’t work including in many expensive carts. The tolerances are shit for these carts and for many the mark up is obsene. But really, that slightly warped record is going to be of way more concern.

    BTW, the DL103 works on a helluva lot more arms than the "specs" and vinyl wizard knowledge would suggest. Lots of bullshit and misinformation out there because vinyl is somewhat of a black art these days and weirdo audiophiles love making up weirdo stuff without understanding the real reasons in the first place.

    Sometimes it’s best to just try it. And if it works and sounds good, then it’s good. That’s how normal people did it 30 years ago before it became mystic arts.
     
    Last edited: May 3, 2020
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  12. k3oxkjo

    k3oxkjo Friend

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    You are exactly right and I have felt this way for a long time. I bought an Infinity Black Widow years ago because of its very low effective mass for high compliance carts that I was afraid I wasn't doing justice to. It never made much improvement. Anecdotal evidence I guess, but still...

    Best, Kevin
     

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