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Building an arm to match cartridge resonant frequency.
Building an arm to match cartridge resonant frequency.
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Old 7th January 2018, 03:43 AM   #1
tribute is offline tribute  Australia
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Default Building an arm to match cartridge resonant frequency.

Hi, I am building an Uni-pivot arm & want to make sure that the arm/cartridges resonant frequency is in the recommended 8-12 Hz range.
My cartridge requires a light weight arm to achieve this, however when I measure the weight of the prototype arm - I can get 2 answers.
1) With the end weight stub in place it is 8 grams.
2) When the end weight stub is not in place I get 14 grams.

Which figure should I use.

Cheers
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Old 7th January 2018, 06:54 PM   #2
DF96 is online now DF96  England
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Which measurement corresponds to how the arm will be used? That is the figure you want. Of course, it is not the "weight" of the arm which matters but the effective moving mass. I assume that is what you mean by "weight".
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Old 7th January 2018, 07:22 PM   #3
lcsaszar is offline lcsaszar  Hungary
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You can't determine the effective mass of an arm by static measurement. It is best to use a test record with LF sweep or stepped discrete frequencies. Then add/remove weight at the cartridge end until you are in the desired range.
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Old 7th January 2018, 11:30 PM   #4
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Default Tone arm / Cartridge resonance

Hi Guys, thanks for your replied. Unfortunately I do not have the equipment to test using a test record.
However as I understand, there are several charts available on the internet (Vinyl Engine etc) that allow you to input the different data for a cartridge & arm & that would give you a good estimation of the the resonant frequency the 2 will generate.
But what is the situation with the arm weight stub, should it be on of off. I realize that several commercial arm cannot be measured with the stub removed as they are fixed in position at the factory. However the manufacturer obviously can do this in the development of an arm.
Cheers
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Old 8th January 2018, 01:34 AM   #5
PRR is offline PRR  United States
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You have to have the end-weight on to play, right? That's the only condition which matters.

If the mass were all one end, you don't even need a calculator; but a counterbalanced system is more subtle.

Here's the nub. You don't have any great choice. You are stuck with the mass of the cartridge. You must apply enough structure to support it solidly. By observation, nearly ALL arm designers go as light as possible without getting floppy. You do the same. The result will be in the right zone, unless your structure mass is more than most.

I *think* another path is to build your half-arn, needle to pivot, no counterbalance. Measure weight on needle. Add enough weight to double that (this approximately equals the counterbalanced tip-mass). Use a very sharp ruler or a dial-indicator to measure cartridge height. Set the needle down, just-touch, and self-support. The difference is the deflection. The deflection *directly* tells the resonance frequency. 8Hz to 12Hz is 0.05 to 0.17 inches deflection.

I think the real goal is to be well off of 20Hz (bumps the bass) but way above 0.55Hz (prime warp rate for 33RPM). 20Hz is about 0.03" deflection; 0.5Hz is around 30" deflection(!). Obviously we are in no danger of half-Hertz trouble (without more complication). A super-light construction could run near 0.03" deflection.... but observation says so little mass is very likely to be "flimsy", "ringy", etc.
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Old 8th January 2018, 10:20 AM   #6
billshurv is online now billshurv  United Kingdom
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Building an arm to match cartridge resonant frequency.
Provided a straight tube the calculator that luckydog did is fairly accurate. But as has been said, cartridge and headshell are the major contributors to effective mass.

0.55Hz is not the warp rate BTW its the eccentricity. Warp is around 8Hz. My personal view is that 12-13Hz is the ideal for resonance, but this can be hard to meet. All depends on cartridge. What are you designing for?

Should also note that damping is your friend.
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Old 8th January 2018, 01:24 PM   #7
lcsaszar is offline lcsaszar  Hungary
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I found a good explanation and calculating method on Vinylengine:

Of Effective Mass and Moment of Inertia


@admins: please remove if citing it here breaks forum rules
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Old 8th January 2018, 03:02 PM   #8
billshurv is online now billshurv  United Kingdom
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Building an arm to match cartridge resonant frequency.
yes the quote there is from user luckythedog, who does post here now.
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Old 9th January 2018, 04:01 AM   #9
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Thanks guys, That calculator chart by Lucky Dog is just what I needed.
It does however not answer my original question, but I think I can work around that by using a Titanium Tube for the end weight stub. That adds a very insignificant weight the the assembly & allow's the arm to fall within the ballpark fugues with it filled or without it fitted.
Cheers for all your help
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Old 9th January 2018, 09:06 AM   #10
DF96 is online now DF96  England
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Your original question was answered at least twice. We also queried whether you were asking the right question. Just for the record, can you confirm that you are fully aware of the distinction between arm weight and arm moving mass?
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