Matching cartridges to Jelco tonearms - useful info

Discussion in 'Audio Hardware' started by Benzion, Jul 16, 2017 at 5:02 PM.

  1. Benzion

    Benzion Forum Resident Thread Starter

    Location:
    Brooklyn, NY
    I just ordered a Jelco SA-370H tonearm for my upcoming Rek-O-Kut Rondine idler TT, and was trying to match some carts to the arm, using the following formula from Galen Carol Audio:

    "Resonant Frequency = 1000/[6.28*square root (M*C)]. Where M is the mass of the arm and cartridge and C is the compliance of the cartridge. As an example, if we had an arm/cartridge with a combined mass of 14g, and a cartridge with a compliance of 20, the resonant frequency would be 9.535" (Hz).

    Now, the effective mass for the 370H that I found by scouring VinylEngine and other resources is given as 21 g.

    As I was doing the formula, adding the weight of the cart to the given weight of the arm, my resonant frequency kept falling slightly below 9 Hz, when it should be, ideally, between 9 Hz - 11 Hz. And it was driving me crazy, thinking the Denon DL-110 I just purchased and the AT7V I am about to get are both no match for the 370H.

    More digging ensued, and, finally I found some useful info, albeit for the 750D, but I'm sure the system/logic is the same, as both are by Jelco. In short - they give the effective mass of the arm of 20-21... including the cartridge! And the generic weight of the cartridge they use is 6.5 g. So, you need to subtract that given weight of 6.5 g from 20/21 and substitute the weight of your cartridge into the formula.

    Having done that, my DL-110 is now a perfect match: 1000/[6.28*square root (19.3*14)]=9.697 Hz, where:

    Effective mass of the arm: 21 g - 6.5 g (generic weight of cart) = 14.5 g.
    Weight of Denon DL-110: 4.8 g, (so 14.5 + 4.8 = 19.3)
    Compliance: 8 in Japanese units, conservatively multiplied by 1.75 to get 14 in Western units

    Side note on compliance: I've read that this figure of 1.75 isn't universal for all carts, you can use between 1.6 - 1.8 for calculations, with lower ratio for higher compliance carts
    100Hz cartridge compliance

    And, finally, here's a photo of value chart by Jelco themselves. You can easily spot mathematical inconsistencies, so, just use your own logic and understanding of the concept, and plug the numbers you have, to get an answer within a fair range of accuracy:

    [​IMG]

    Notice how effective mass goes down with heavier counterweights and with longer arms.


    And finally, the formula is obviously useful for ANY arm, not just Jelco's, as long as you know your arm's effective mass, and weight/compliance metrics of your cart
     
    Last edited: Jul 16, 2017 at 5:16 PM
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  2. Doc Diego

    Doc Diego Member

    Location:
    Nevada
    Does connector in the Jelco chart equal headshell?
     
  3. blakep

    blakep Forum Resident

    I would take the info you posted with a very large grain of salt. There's conflicting info all over the net on the effective mass of the Jelco arms and I can tell you that I have an email from Jelco (from 2009 just before I purchased my 750D) stating that effective mass of the 750D is 20 grams.

    In light of the fact that the standard headshell which comes with any of the 750 derivative arms and the SA-250 weighs about 13 grams with any normal mounting hardware and leads, the likelihood of these arms having an effective mass of 13 is pretty much physically impossible.

    Not to worry, though: I can tell you from about 10 years of experience with the 750D that regardless of what its actual effective mass is (I'd wager in the 18-20 range), it is actually a pretty flexible arm and will work very well with low compliance or medium compliance designs, particularly with the fluid damping.
     
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  4. blakep

    blakep Forum Resident

    No.
     
  5. missan

    missan Forum Resident

    Location:
    Stockholm
    Yes, with a large, very large grain of salt. There is no possible way these figures are correct.
     
  6. Benzion

    Benzion Forum Resident Thread Starter

    Location:
    Brooklyn, NY
    The 20 g effective mass is right - this is what they quote when asked. They give it including the stock headshell. This notion is empirically supported by the disparate info you find on the net, plus people's experience.

    I have just purchased an NOS Jelco SA-370H tonearm - the direct predecessor to the 750D. The effective mass I found for it is 21 grams with headshell on, and the stock headshell on it weighs 8 gr, giving the weight of the tube of about 14 gr.

    You are correct that the numbers in the chart don't make much sense, I also noted inconsistencies. Based on my calculations for the 370H, it seems the figures they quoted in the chart for effective mass are for the tube without headshell but with cartridge, which sounds kinda dumb, but if you take that assumption as base - than it starts to make sense.

    I'm not sure what the "connectors" quoted in the chart are, but they seem to be around 3.5 gr, on average. So, if you subtract that same 3.5 to 4 grams, it should give you about 10-11 gram for the tube itself.

    Now, with the 750D's quoted combined effective mass of 20 gram, knowing the stock shell is around 10 gr, without it the tube should be about 10 gr - just as figured above. Again, these numbers are estimates, basically reverse mathematics with largely estimated variables, but it's the best I've got.

    It would have been a lot simpler if Jelco had just bothered to include the effective mass in the published specs. I mean, as audiophiles, we are supposed to take the time to compute the resonant frequency for the tonearm/cartridge pairing - why would the people who know it best withhold such vital metrics from us?

    I sent an inquiry to Shure yesterday about compliance of their M97HE cartridge, which I bought NOS. Got the answer today: "sorry, we don't measure dynamic compliance of cartridges". - How 'bout them apples?

    It appears we take the metrics and values sometimes even more seriously than the manufacturers themselves. Jelco won't give effective mass, Rega won't give wow and flutter, nor care for VTA adjustment, Shure (and Shelter) couldn't give two sh..ts about compliance of MM carts. Grado doesn't publish compliance for wood-body carts, but were nice enough to reply to my email with the answer (Shelter wasn't).

    I think we are being somewhat discouraged from relying too much on the numbers, and, who knows - maybe it's kinda good advice?
     
  7. vinylkid58

    vinylkid58 Forum Resident

    Location:
    Victoria, B.C.
    You're aware that the 370H doesn't come with a 5-pin plug at the mounting pillar, just wires.

    jeff
     
  8. Benzion

    Benzion Forum Resident Thread Starter

    Location:
    Brooklyn, NY
    Yes, I was told it could be wired directly into the RCA's in the back.
     
  9. Rolltide

    Rolltide Forum Resident

    Location:
    Vallejo, CA
    I think the best part is how many "Okay, I emailed Jelco and this is what they said" threads exist on the matter, each with different information supposedly straight from the horses mouth. I think Jelco is a great company, but their inability to supply the single most important bit of data on their tonearms is a bit silly at this point.
     
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  10. Benzion

    Benzion Forum Resident Thread Starter

    Location:
    Brooklyn, NY
    We're in agreement here.
     
  11. McLover

    McLover Forum Resident

    Location:
    East TN
    Jelco does so much OEM work, I think their lack of end user support is more business to business in origin. Many a huge OEM manufacturer lacks in the end user service and support mindset, Asus is a major example of such businesses as well as MSI in the computer world.
     
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  12. Benzion

    Benzion Forum Resident Thread Starter

    Location:
    Brooklyn, NY
    LOL, this gets better! Below, please find the reply to me from Jelco:


    tsukamoto <tsukamoto@jelco-ichikawa.co.jp>
    1:14 AM (6 hours ago)
    [​IMG]
    [​IMG]
    [​IMG]
    to me
    [​IMG]







    Dear ,



    Thanks for your inquiry.

    Effective mass for SA-750D is 13.48 without cartridge but with head shell.



    Best regards,



    Tomoyuki Tsukamoto

    Ichikawa Jewel Co., Ltd.
     
  13. action pact

    action pact ^^ Sandy Warner, "The Exotica Girl"

    That is consistent with the above chart.

    I'd still like to know how the effective mass goes down with the heavier counterweight though.
     
  14. Benzion

    Benzion Forum Resident Thread Starter

    Location:
    Brooklyn, NY
    The reply from Jelco corresponds to the chart, so I tend to believe it. Then notice they don't use the whole weight of the cartridge in calculating the combined effective mass: for a crtridge weight of 6.5 g they add only 5.75 g to 13.48 g mass of the arm, to get the 19.23 figure.

    So, the figure of 20 for effective mass of the 750D that's been circulating around audiophile resources such as this forum and others is actually true, what they haven't made clear to others yet is that it's including a cartridge with a generic weight of 6.5.

    When calculating yours - substitute the cart weight for weight of your cart (my Zu Denon weighs 14 g!), and remember to subtracts .75 from it, to give it the same "weighting ratio" as in the formula.

    Then follow the same logic for other arms/weights/lengths in the chart.
     
  15. Benzion

    Benzion Forum Resident Thread Starter

    Location:
    Brooklyn, NY
    This has to do with the "proximity to the pivot point of the arm". Heavier counterweight puts more weight closer to the pivot point, thus slightly less of it on the tip of the arm - at the stylus. Hence the "effective" mass - it's not precisely the same as the combined weight of the assembly. There's some fine math working here, we just don't have the exact formulas. Guess it's their proprietary secret.
     
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  16. missan

    missan Forum Resident

    Location:
    Stockholm
    That is because the MOI at the pivot = mass x (distance mass center to center pivot)^2

    So balancing with a heavier CW will mostly give a lower MOI as the distance to the pivot is smaller, (and the distance is in square)
     
    Last edited: Jul 18, 2017 at 8:29 AM
  17. action pact

    action pact ^^ Sandy Warner, "The Exotica Girl"

    MOI = Ministry of Interior? (Math and physics is not something that I know very well!)
     
  18. Benzion

    Benzion Forum Resident Thread Starter

    Location:
    Brooklyn, NY
    Yikes, thank God it's not MOT - Ministry of Truth, or we'd be in "1984"...
     
  19. action pact

    action pact ^^ Sandy Warner, "The Exotica Girl"

    Fake news.
     
  20. missan

    missan Forum Resident

    Location:
    Stockholm
    MOI stands for 'Mass Moment of Inertia'. This is what is affecting the arm when it is going to accelerate the CW. The formula is MOI= mass x distance^2.
    It will mean that if the CW is farther from the pivot it will greatly affect the MOI.
    In turn this mean it will also affect Effective Mass at needle, as this is 'MOI at pivot' = EM x arm length^2.

    So a heavier CW will in principle always lower EM.
     
  21. 33na3rd

    33na3rd Forum Resident

    Location:
    SW Washington, USA
    So if one has different sized counterweights for their tonearm, one can change the effective mass of a given arm to best match their cartridge?
    Does this mean that there may be times it would be more beneficial to use a smaller counterweight further away from the pivot point to match an arm to a low compliance cartridge?

    This is very interesting stuff! Although it makes my head hurt a little bit this early in the morning......
     
  22. missan

    missan Forum Resident

    Location:
    Stockholm
    Yes one can match the EM a bit, even if the figures originating from the CW is quite small normally. Depending on the rigidity of the CW stub, it might sometimes still be better to keep the CW very close to the pivot. It depends.
     
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  23. 33na3rd

    33na3rd Forum Resident

    Location:
    SW Washington, USA
    Thank you!
     
  24. Rolltide

    Rolltide Forum Resident

    Location:
    Vallejo, CA
    The best way to alter effective mass is by using a different headshell, as the headshell weight is factored into the effective mass. As the chart shows, counterweights have an impact, but its relatively small, not really enough to move the dials on compliance mathcing.
     
    Bob_in_OKC, Benzion and 33na3rd like this.
  25. missan

    missan Forum Resident

    Location:
    Stockholm
    No the CW´s impact is small. The mass of the headshell or cartridge contributes with about 75% of their mass to the total EM normally.
     
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