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DIY Tonearm Counterweight

AV Cables

jls001

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A fortnight ago met up with fellow forumer mpw to see if a Denon DL103 cartridge can be made to work on his Technics SL 23 turntable. The effective mass of his arm is 12 grams. So at least in theory if there is enough counterweight mass, it should be possible to balance the 8.5 grams mass of the DL103. From the outset, I suspected that the counterweight mass will not suffice so I brought some extra weights with me to tie/attach to the counterweight. Turns out his arm does not have enough counterweight mass to properly balance the DL103. So the extra weights came in handy. After much misadventure including a broken headshell wire that had to be re-soldered, the combination played nicely, aided by the rubber band that held the extra weights to the counterweight. But it was obvious that larger mass was needed.

Also, a huge hum developed while we were fooling around with his setup. Much troubleshooting ensued. Finally it was decided we will try making a heavier counterweight for his arm, and change his tonearm cables and replace the 2-pin power connector. So I ended up bringing home his turntable and his LehmannAudio Blackcube II SE phono stage for the above work.

So for the first time in my life, I made use of the formula for a cylinder that one learns in high school.

Mass of Cylinder = Density of Material x Volume of Cylindrical body
= ?V
= ? x (?r^2h)

Here's the drawing of the "cylinder":




There are two cylindrical cutouts here - one for the counterweight stub, and the other for the grub screw that will tighten the counterweight to the stub. So these two cylindrical cutouts get subtracted from the actual mass. So mass
= ?(V - V1 - V2)

So I worked backwards with radius of the holes for stub and grub screw, as well as h already defined, with target weight of 168 grams (a la the super heavy Jelco counterweight), with only the outer radius kept as unknown.

This is the outcome:




I made one for my Origin Live tonearm as I need to use two counterweights to balance the DL103 when using a super heavy DIY headshell spacer.

Here they are side by side. The inner radius of the two designs differ as stubs are of different diameters, but target weight for both is same (168g):




After chrome plating (the right most weight):

The others are (L to R): stock counterweight of Technics SL 23 (106 g), stock counterweight of Origin Live Silver Mark IIIa (106 g), heavy counterweight of Silver Mark IIIa (136 g), and the new kid on the block (168 g).




Here it is mounted on the Technics SL 23 arm:



There is enough stub length for even heavier cartridges. There is about 10 mm length left on the stub after balancing a DL103 with a 9+ gram spacer, using a headshell borrowed from a Technics SL 1210. The spacer is needed for two purposes - for correct VTA and to increase effective mass of the tonearm for the low compliance cartridge.

Next step: to measure the resonance freq of the combination and calculate the new effective mass.

It is playing well and seems to be a bit more energetic.
 

jls001

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Measured a couple of parameters:

1) trackability is 100 microns, which, BTW, is damned good. The test record can measure from 40 to 100 microns and the arm-cartridge combination sailed through all groove sizes (100 micron groove is very loud). The last time I tested this on another arm-cartridge combo, it managed only till 80. At 90, it could no longer track coherently.

2) resonance frequency is 8 Hz. Now that the resonance frequency is known the effective mass can be calculated. Using the arm resonance calculator here, it says that for the Denon DL 103 whose published compliance must be multiplied by a factor of 1.5 to 2 as the usual Japanese practice is to state compliance at 100 Hz unlike the European practice of stating it at 10 Hz, the calculated effective mass is 39.5 grams (when conservatively using a multiplication factor of 2). That's an astoundingly high effective mass, and well above my expectations. I am beginning to have doubts about the way effective mass is calculated. I calculated it manually too (using the formula, resonant freq = 1000/[6.28 x sqrt(mass x compliance)] where mass is measured in grams and compliance in cm/dyne), and the numbers matched. Any thoughts? More importantly, assuming the calculations are fine, is there any harm being done to the cartridge by using such high effective mass? FWIW, the combo is sounding rather nice.
 

omishra

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Joshua,

Thanks for sharing this information. Great Sci-Fi effort for Hi-Fi. :clapping:
 

Hiten

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As with most audio things effective mass is little debatable topic. Some people say while playing records lateral effective mass which is dynamic in nature is also important. I no longer have link to that article. It had formulas and other stuff.
is there any harm being done to the cartridge by using such high effective mass? FWIW,
For a X setup of arm-tt-pre-speakers if resonance fq. is below 10hz (you have acheived 8hz) there are no issues I guess. Harmonics will be there but at half the decible levels from original.
By theory more heavier arm than necessary will have tendency to not move creating tracking issues due to inertia. So if arm+cartridge is tracking well across the vinyl and channel balance is good then for low compliance cartridge 39 gms is OK I think. If I remember Continuum turntable's Cobra arm is 1 Kg. I may be wrong though.
Regards
 

jls001

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So if arm+cartridge is tracking well across the vinyl and channel balance is good then for low compliance cartridge 39 gms is OK I think. If I remember Continuum turntable's Cobra arm is 1 Kg. I may be wrong though.
Regards

May be I will fashion a lighter spacer and see if I can get it to resonate at 10 Hz. That will make the effective mass a more believable 25 grams.

Regarding the Cobra, 1 kg could be the mass of the whole arm. Arms do weigh 600-700 grams. If they are more elaborate and bulky like the Cobra's "hood', may be they'll weigh more.
 

jls001

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I replaced the 9.11g spacer with a 5.97g spacer and the resonance is still 8 Hz. Will have to re-do the math to get it to 10 Hz.
 

jls001

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Tried a very heavy spacer (~11 grams) and the resonance degrades to about 7 - 8 Hz. So lighter is definitely the way to go. Something well below 6 grams ought to do (since 6 grams also produced 8 Hz resonance) but I will have to experiment further to find out what is the best weight for arriving at 10 Hz resonance.
 

jls001

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I tried another spacer weighing about 3.5 grams and the resonance moved away from 8 Hz towards 9 Hz but it is still 8.x Hz. So I have concluded that this exercise of trying to push the resonance frequency to 10 Hz is not worth the trouble. I will stick to the original 9.11 gram spacer and be done with it. As it is, 8 Hz is not a bad resonance freq. So this concludes this exercise for now. Or at least till some new brainwaves hit me :).
 
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mpw

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many thanks josh.

really appreciate the time at my home the other day where i got a peek at the DL 103 and ofcourse the knowledge gain and the detailing of all that you could do to the SL23.

My DL103 is on the way to Mumbai and so is a new technics 1210 headshell.

Hopefully we will be able to put it all together in a nice manner.

best regards
mpw
 

denom

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Glad that I played a small part in this 'experiment' as well :D

BTW, have 'repaired' the busted head shell :cool:
 
M

mpw

Guest
Josh,

many thanks for having me over last evening and for all the work on the tonearm of my SL 23.

I learnt also from the brief comparo between a CD 6003 and the high and aero CD player / pre / DAC ofcourse to see and hear those lovely TT;s you have.

The GR rresearch speakers have come good too.

Appreciate the time hugely.

regards,
mpw
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