Cartridge Compliance

the thin end of the wedge
Bebé Tonto

Post by Bebé Tonto » 03 Nov 2010 16:34

BigE wrote:sreten triggered this by suggesting that there is a "dramatic" difference in moving the counterweight
Depends on the mass and the geometry of the counterweight. You can change effective mass by changing the counterweight to a heavier and narrower one. A heavier CW would sit closer to the tonearm pivot, reducing the effective tonearm mass!

Guest

Post by Guest » 03 Nov 2010 18:57

Bebé Tonto wrote: A heavier CW would sit closer to the tonearm pivot, reducing the effective tonearm mass!
Hi Bebé Tonto. Indeed ! Counterweight = Counterintuitive :wink:

Heavier counterweight = lighter effective tonearm mass !

Bebé Tonto

Post by Bebé Tonto » 03 Nov 2010 21:47

Bebé Tonto wrote: A heavier CW would sit closer to the tonearm pivot, reducing the effective tonearm mass!
Hi Bebé Tonto. Indeed ! Counterweight = Counterintuitive :wink:

Heavier counterweight = lighter effective tonearm mass !
Funny, isn't it?

Guest

Post by Guest » 03 Nov 2010 22:49

Hi,

Yes but the counterweight mass makes no sensible difference, as
long as its ballpark for the arm and fairly near the pivot points.

The only case it does is parallel tracking arms where it is purposefully
moved away from the pivot to reduce the ratio of vertical to horizontal
mass, in this case a small counterweight does effect (vertical) mass.

For heavier counterweights for heavier cartridges the heavier cartridge
massively dominates the increase in effective mass. You cannot in any
sensible sense reduce the effective mass of a normal arm by using a
heavier counterweight, its very nearly pointless.

(Ignoring of course any counterweight decoupling, another can of
worms, but can IMO work well tuned to the suspension, not the
arm / cartridge frequency, and this can make it easier to set.)

rgds, sreten.
sreten triggered this by suggesting that there is a
"dramatic" difference in moving the counterweight
That was not me, my position is as above.

starterwiz
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Post by starterwiz » 03 Nov 2010 23:27

I've been using a SeriesIII for 30 years, and have gone through about 20 different tips. I like having such a light arm (even 'tho it resembles HornbyOO's version of the lunar lander) 'cause it's easy to add mass just by moving the CW back. I've not yet had a tip that I can't keep between 8 and 14Hz. I've never tried a really low compliance cart, but I would like to give it a shot some day.
Interestingly, I tried adding some weight to the back end of my SeriesII for some of the heavier cartridges, and found it added too much mass for the high compliance ones. Perhaps more weight closer to the pivot is the answer.

Bebé Tonto

Post by Bebé Tonto » 04 Nov 2010 01:48

sreten wrote:You cannot in any
sensible sense reduce the effective mass of a normal arm by using a
heavier counterweight, its very nearly pointless.
My dear friend... YES YOU CAN and it's not pointless. You are 100% right in saying that the headshell/cartridge mass is the dominating mass on the system, but you can reduce it significantly by using a heavier counterweight. Case in point, the Lenco L75 tonearm:

http://web.archive.org/web/200705250314 ... 5924715046

Quoting from the (very good) article:
How can it be so low?? One main good reason: lowering the mass of the headshell lowered Ih, approx from 800 000 to 500 000 but the main consequence was to change the balance of the arm: now the counterweight is much closer to the pivot : from 40mm to 30mm and therefore Icw changes from 168 000 to 95 000.

Cheers,
Flavio.

Guest

Post by Guest » 04 Nov 2010 09:02

Hi Bebé Tonto. When I did the same calc of fitting a Rega counteweight to a Lenco 75 arm, I reckon that equates to reduction of effective tonearm mass of only 0.8g. There's also a (smaller) contribution from relocating the CW for rebalancing following headshell mods.

Which, as I posted, in total is large enough to be notable, but pretty small as an overall percentage of the Lenco 70 effective mass. For practical purposes it can stil just about be ignored. Inconveniently !

Likewise, if one doubles CW mass on a stock RB250, tonearm mass might reduce by c 1.1g. Again, that's notable, but not very large in the scheme of things. One can't do much useful with it !

The circumstance in which counterweight mass/location might be more influencial is the other end of the spectrum, an ultra-light arm and light CW far from the pivot at balance. I think this would be an unusual configuration, an exception. For most tonerms, the CW doesn't contribute enough to overall effective mass for reasonable variation to be significant. As I see it !

BigE
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Post by BigE » 04 Nov 2010 17:22

starterwiz wrote:I've been using a SeriesIII for 30 years, and have gone through about 20 different tips. I like having such a light arm (even 'tho it resembles HornbyOO's version of the lunar lander) 'cause it's easy to add mass just by moving the CW back. I've not yet had a tip that I can't keep between 8 and 14Hz. I've never tried a really low compliance cart, but I would like to give it a shot some day.
Interestingly, I tried adding some weight to the back end of my SeriesII for some of the heavier cartridges, and found it added too much mass for the high compliance ones. Perhaps more weight closer to the pivot is the answer.
The Denon DL103D is giving me fits in my Series IIIs. If I don't add about 2.4 gms to the headshell, it does not sound right. Since I don't know the static compliance of this cartridge, I assumed that it would be
similar to the 301mkII. I could be mistaken.

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Post by BigE » 04 Nov 2010 17:24

sreten wrote:
sreten triggered this by suggesting that there is a
"dramatic" difference in moving the counterweight
That was not me...
Sorry, I'll try to find the triggering quote.

Guest

Post by Guest » 04 Nov 2010 19:25

BigE wrote:The Denon DL103D is giving me fits in my Series IIIs. If I don't add about 2.4 gms to the headshell, it does not sound right. Since I don't know the static compliance of this cartridge, I assumed that it would be
similar to the 301mkII. I could be mistaken.
Hi BigE. Yes, DL103D static compliance could be anything i suppose, no reason to think damping is high, as the DL301 appears to be. By default, perhaps 18cu@static is a good fit ? That predicts trackability to be 80um@300Hz for a VTF of 1.65g. And damping factor of 0.05, which is more 'normal' range. In which case, is the resonant f getting a bit high ?

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Post by BigE » 05 Nov 2010 15:57

Yes, that is a good fit, and a great explanation as to why adding 3 grams of brass between headshell and cartridge sounds better -- the resonance I get at that point is 9.1 Hz

I use 0.16 mm lateral, 300 Hz, 5 grams eff mass, 12 grams cartridge and fixings, 12 dynamic compliance and 18 static to get those numbers.

It's all becoming clearer now....

Thank you ld! Your help is MOST appreciated.

At this point, I'm tracking a 1.70 grams. I could go lighter.

Guest

Post by Guest » 06 Nov 2010 00:01

Bebé Tonto wrote:
sreten wrote:You cannot in any
sensible sense reduce the effective mass of a normal arm by using a
heavier counterweight, its very nearly pointless.
My dear friend... YES YOU CAN and it's not pointless. You are 100% right in saying that the headshell/cartridge mass is the dominating mass on the system, but you can reduce it significantly by using a heavier counterweight. Case in point, the Lenco L75 tonearm:

http://web.archive.org/web/200705250314 ... 5924715046

Quoting from the (very good) article:
How can it be so low?? One main good reason: lowering the mass of the headshell lowered Ih, approx from 800 000 to 500 000 but the main consequence was to change the balance of the arm: now the counterweight is much closer to the pivot : from 40mm to 30mm and therefore Icw changes from 168 000 to 95 000.

Cheers,
Flavio.

Hi,

I don't care, once you factor in the effective mass of the cartridge as
well as the arms effective mass, unless the arm has a pointlessly
long rear stub*, and a pointlessly lower weight counterweight, the
option of a higher mass counterweight makes very little difference.

Note the example first lowers the headshell mass (fair enough)
but ignores cartridge mass, and then went on to describe the effect
of the counterweight for that obscure case, its simply not typical.

Note the effect of effective mass is proportional to the square root.

(Ignoring the decoupling of counterweights in many arms).

rgds, sreten.

*Say it has a long rear stub for high mass cartridges, in that
case your already stuffed for total effective mass, but here
the effective mass change is maximum for fitting a heavier
counterwieght, but not the % change or the real change.

Guest

Post by Guest » 06 Nov 2010 00:33

Hi,

One could argue what is the best effective mass of an arm ? IMO
its around twice that of the cartridge practically, a lot lower is not
good IMO, it means an inevitable lack of rigidity compared to a
higher mass arm, and personally I do not like pointlesly high
mass MC's, the extra mass could be used effectively in the arm.

rgds, sreten.

Guest

Post by Guest » 06 Nov 2010 10:18

sreten wrote:One could argue what is the best effective mass of an arm ?


I've been playing about with a math model of the tonearm as a transmission line. More or less the same maths obtained during cantilever TL modelling a few months back. Transverse wave propagation in a hollow tube etc .

Then the answer to the question mostly depends on characteristic mechanical impedance of the arm tube versus 'lumped' equivalent impedance of the headshell mass. And the way in which armtube mass affects mechanical impedance (rigidity etc). The simplest general answer is headshell mass is best if the armtube is high impedance (rigid). But higher impedance armtubes naturally have higher mass, as a rule. So that roughly tallies with sreten's point. Otherwise it's 'how do you prefer your poison ?' in terms of tuning armtube transverse vibrations, resonances, and amplitudes thereof.

And some further interesting results drop out, not least that mechanical impedance of tonearm tube is frequency dependant, proportional to sqrt(f). I think that defines the 'perfect' form of tonearm damping, but it seems really hard to physically realise. But this is in danger of wombling well OT ! I hope to post another thread once I have thoughts coherently ordered on this topic.

BigE
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Post by BigE » 06 Nov 2010 22:59

sreten wrote:Hi,

One could argue what is the best effective mass of an arm ? IMO
its around twice that of the cartridge practically, a lot lower is not
good IMO, it means an inevitable lack of rigidity compared to a
higher mass arm, and personally I do not like pointlesly high
mass MC's, the extra mass could be used effectively in the arm.

rgds, sreten.
How do you separate the mass of the headshell from the effective mass of the arm? Changing a headshell is a common device to change effective mass. Do you consider the OEM headshell as the one defining the effective mass of the arm? And how is that "ideal" chosen?

I fear we're on a slippery slope here.