LENCO LOVERS :: View topic - How to reduce L75's tonearm's dynamic mass

François

Hi folks,

We will now talk about a topic that may concern many of you: the perfect(or best) match between cartridge and tonearm.
This match is mostly determined by the effective mass of the arm and the cartridge compliance.
And in the case of the Lenco's arm, due to its very high effective mass, we are quite limited in our choice of cartridges.
I found the other day this website: http://www.cartridgedatabase.com/ that clearly shows that with an effective mass of 23g., 99% of the "best" MM cartridges don't match the Lenco's arm .
So, what is the solution?
As you can't change the compliance of a given cartridge, you have to work on the arm.
-"So change the arm!!!" you will tell me...
Yes, of course, but too easy, and it doesn't match with the philosophy of my project : keeping everything closest to its original aspect...
So, I have to lower the effective mass of the arm!!!
From what I understood, the effective mass is not a static value, but a dynamic one : it has to do with inertia and therefore, it has to do with mass and distance of these masses from pivot.
What influences more inertia is not so much the mass but its distance as in the calculation we do: mass X distance2 (how to make the little 2 for square?)(from the pivot in this case).
I don't know if is clear because it is difficult to write mathematical formulas.

So let's get back to our arm: to lower this, I would propose:
- Make a shorter arm stupid isn't it?
- Make a lighter headshell and cartridge coupling system: I did it but all I could save were 2 miserable grams...
- Use a heavier counterweight and thus shorten its distance from the pivot...
How does it look on the paper : the counterweight is 106 grams and to get a perfect balance with a 5 grams cartridge, its centre is at 40mm from the pivot : 106X40X40=169 600

I put a 133grams counterweight(Rega's), set the balance and measure the distance : 30mm. So we have now 133X30X30=119 700 that, compared to the previous result is approx. 30% less.
Will this method also lower the effective mass by 30%?
Since I have tried the "soft spring system" to measure the arm effective mass and I am not convinced (must have done something wrong), I will leave for the moment this part of the research to someone who has more confidence with this technique.

So here is the formula of the momentum of inertia: I = md², where "m" stands for the mass of the counterweight or headshell+cartridge, and "d" for the distance from the pivot in the case of a tonearm.

And here are the results !!!

First, a few statements:
- it is obvious that the VTF device has to be banned!!!
- do everything you can to have less mass as possible at the end of the main arm's pipe. I did that removing the plastic plug in the bayonette ( as pictured elsewhere) and in the headshell, and removing the cartridge coupling device + removing some metal all around. Here is the result:

Almost a piece of Emmentaler cheese...sure, Burgdorf is on the Emmen river Wink

So, since now, it has been fun, pictures and all, but yet be prepared for a serious headache : MATHS Confused

First of all, a list of all the terms, symbols and formulas that will be used.

I= momentum of inertia=M×d²
M= mass of the considered part of the arm
d= distance to the pivot

After some research and dialogue with my father and girlfriend, both mechanical engineers, we have concluded that the "effective mass" is the sum of all the momenti of inertia of the arm divided by the "pivot to stylus distance"².
m= « effective mass » or dynamic mass = (Σ Ii) / l²
l = distance from pivot to stylus = 230mm.
l²=52 900

I have named, weighted and calculated the momentum of inertia of all the parts of the tonearm :

cw: counterweight
r: rear part of the arm
p: main arm’s pipe
h: headshell+bayonette+collar+cartridge’s coupling device

Mcw =103g. Icw = 95 000
Mr =25g. Ir = 33 800
Mp =9,3g. Ip = 76 000
Mh =12.3g. Ih = 500 000

Now, lets calculate Σ Ii = Icw + Ir + Ip+ Ih = 704 800

(Σ Ii) / l² = 704 800/52 900 = 13.32

m = 13.32g! Shocked

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.

Note that the "effective mass" of the arm without the VTF weight is around 20.5g.

Does all this make sense to anyone, or is it to nice to be true??...

Of course, the "shortest" the counterweight is, the lowest its momentum of inertia will be, and YES we need to ignore the mass of the cartridge...
How would it be then that arms' manufacturers provide "effective mass" data...with which cartridge did they get their figures?
Just consider cartridges' weights going from 3g. (Ortofon Super OM 30 without the inner weight) to 8.5g./9.7g. (Denon DL 103), only to give two examples...
Anyway, the weight of the cartridge and it's accessories is then reintroduced when you calculate the resonance of the arm+cartridge system :
159 / sqrt ((eff. mass + cart weight + fastener weight) x (compliance))

Just an "addendum" or an "errata corrige" to what written previously:

-About the cartridge in the measurement of the effective mass:
I have made calculations this morning including a cartridge (Pickering XV15-625E) and its screws for a total weight of 7g.
Obviously, the position of the counterweight had to be changed to maintain the proper balance and as a consequence, its momentum of inertia increased a lot.
The total momentum of inertia increased dramatically and the effective mass of the system reached 20g.
I have then subtracted the weight of the cartridge+screws and got: 13g.

*************************************************************

In a second step, I did more mods to the headshell, taking it to a weight of 9g. It has been put on a severe diet since the beginning : a loss of almost 5g.
Here it is in all its beauty...

Then I have done again some calculations with the new value of the headshell's weight.
I have also made the calculation more accurate: instead of taking a global value for headshell+bayonette+collar, I have taken individual ones.
So, now, my new result is: m=12.60g.

*************************************************************

After a few days of brainstorming and serious headaches (thanks Richard ) I got to the conclusion that : provided that you don't use the VTF weight of the L75/78, using such a cartridge(Pickering XV-15 625E in that case, but also other brush-equipped cartridges) will lower the dynamic mass of the arm ( I will no more use this term of "effective mass" as it corresponds to a totally different concept : something about fundamental physics and electrons stuff...).
Why :
-When you calculate the dynamic mass of the arm WITH the cartridge mounted and then subtract its weight (cartridge's) you get the same result as the "nude" arm (see above in the post).
- When you use such a cartridge, you have to add more VTF for the brush and you do it shortening the counterweight/pivot distance: this reduces the momentum of inertia of the counterweight and as a consequence, the dynamic mass of the arm...
In which proportions? I still have to completely recover from these violent headaches to be able to give you precise figures, but do you really need some or do you feel satisfied just knowing it's better to use such a device ? Please, answer YES!!!

*************************************************************

It was a few days that I wanted to sum up all the steps you may follow to actually reduce the dynamic mass of a L75's tonearm.

-"Driiiiiiiiiiiiing..."...the phone...
-"Oui, allô?"
-"Allô, Pierre Lurné from Audiomeca on the phone, I call you back, maybe a little late, regarding the message you left me about the calculation of the dynamic mass of tone arms..."
Funny, isn't it?
In the end after quite a long talk (quite a nice man I must say...), he confirmed the calculations I have made and the method. We'll meet: he lives 10 km. from my workshop

So, back to the topic.
The genuine L75's tonearm has a dynamic mass of 23 g. that makes it a heavy weight arm . But it is not a fate. Following these few and easy steps, you can reduce it drastically:

-Don't use the VTAF counterweight:

-Be as light as possible at the headshell's end, at the cost to eliminate plastic plug (also at the arm's end), pins and cartridge's coupling device:

You will have to rewire the arm, but if done properly , it can only be better: think of eliminating 3 connections between the cartridge and the RCA sockets...

-Use a heavier counterweight:

To conclude: with the changes you have seen here, the dynamic mass of my arm has now reached 10,0 g. seems unbelievable, but it is true!!!

*************************************************************

Did you think that was all and that there were no more possibilities to lower this d****d dynamic mass? You have underestimated me ...

Just do that:

And, if you replace the back stud with one like this (aluminum):

...you will reach a dynamic mass of around 9g. Shocked

_________________
François.

Enough of Gillette Mach3 or Wilkinson Quattro...from now on, Ockham's razor!

Posted: Wed 23 Aug, 2006 3:31 pm Post subject:

François

Well done, some excellent answers to the perennial Lenco mass problem. Your work as usual is superb, and I appreciate the time you have put into this arm project.

I have an observation to make in the spirit of positive discussion, please do not be offended, that is not my intention.

One thing that makes low mass arms more of a problem for the designer is that the bearings must have very little horizontal friction. Once the cartridge tracking force gets much below 2gms in a featherweight arm even tiny amounts of bearing friction will play havoc with the smooth working of the ensemble.

The corollary of this is that average bearings actually need some mass at the end of the arm and a corresponding stiff cantilever in the cartridge to get the bearings moving. If the arm with average bearings has the mass reduced in an attempt to use a low vtf, high compliance cartridge, I doubt it will realise the potential of the chosen cartridge, and will probably provoke mistracking. Unfortunately, I think the 75 arm has average horizontal bearings.

This got me thinking about other Lenco 'problems' where I have racked my brains and changed one thing or another only to find out that the 75 is a deliberate design. They actually meant it to be exactly as it is!

That is not to say that modern techniques and developments cannot be applied, but it is important to know why things are as they are before starting to tweak. I have spent many happy hours tweaking the 75 arm, but when all the tweakings done it's not designed for high compliance mm cartridges.

It will produce excellent musical results with a Shure M55e, and the tweaks can help to improve the performance with this cartridge. Once you have a need for high compliance cartridges then you also need a different arm imho.

Regards

Colin

François · Posted: Wed 23 Aug, 2006 4:09 pm Post subject:

Richard · Posted: Wed 23 Aug, 2006 9:09 pm Post subject:

A wonderful and maddening topic. I agree with Colin (Hello, Colin!).
The Lenco presents us with forensic problems. Here we have a turntable with multiple unique engineering features. It is difficult to come at this thing three decades in the future, with no design or repair literature to give us clues, and try to understand the thing. In the course of my audio work, I have sometimes tried to "get into the heads" of the people who designed the products -- to understand the philiosophy of the engineering. The Lenco often frustrates this persuit. Worse, the only Lenco engineer I've managed to find says strange things... Confused

Colin impresses me with his practicality. When one has enough audio experience, it becomes obvious that one can be far too much of an audiophile. For example, there's no benefit in using a stylus that will resolve 50,000 Hz when the record only goes up to 12,000. This needle will give us stunning, vivid reproduction of groove scratches and molding defects. Sometimes, going backwards will produce the most satisfying listening: a Shure M55e may be just the ticket. And let me add that, although I love parabolic styli, that my current interest is in spending some time with a classic Stanton disco cartridge: 680SE and its .4 x .7 mil stylus.

All this mass and resonance stuff is enough to drive us totally crazy. And I'm terrible at math anyway. So, let's see how one can bring some sanity and bliss to the Lenco tonearm, especially by going in via the back door:

- The Shure V15 IV and V, as well as others with Shure's damped brushes (such as the M97) were intended to eliminate all this awful mathematics. The damped brush tames the tonearm. The resonance is damped at the front of the arm, and that's that. The arm simply has to have low enough mass to be able to follow the stylus as it, in turn, follows record warps. This isn't so hard. Stylus resonance problems vanish.

It's a pity that Shure copped out of the whole issue, but they're like that!

- Use a viscous-damped tonearm (Decca, Morch). Resonance is damped at the arm pivot area. No more resonance problems. Damping fluid/paddles must be light enough to allow the stylus to follow warps without bottoming. Considering the awful quality of late stereo records (super-thin plastic -- always warped), we'll have to select a lighter grade of fluid than was probably originally intended. And again, the compliance should be within reason. Often, problems are easily visible by just watching the stylus closely as it plays a record.

- Use a less-compliant cartridge with an appropriate stylus. I've gone two opposite ways with this:

a. Stanton, uniquely, made a heavy-tracking parabolic -- a disco-type stylus/cartridge. The sound was incredibly good, and it could make a klutzy record player (Garrard changer!!) sound better than it had any right to. Wear was reduced by the parabolic shape, too. Unfortunately, styli like this are long-gone.

b. Use a fatter stylus, such as a larger-radius elliptical (see above). This is not as bad as it seems: for one thing, record wear is reduced because the bearing area is spread out over a larger ellipse. In fact, I'm convinced that one of the worst things you can do to a record is to play it with a .2 x .7 stylus -- the audiophile standard before parabolics were introduced.

Note that the Stanton/Pickering brush does not provide Shure's type of damping. Its taming of arm resonance is too slight to matter.

- Make a viscous-damping assembly for the Lenco tonearm (I'm getting sick already).

I use a similar solution for cable capacitance with my favorite tonearms, which have high-capacitance wire: just don't use conventional magnetic cartridges. This is an entire story in itself, so I won't start!
_________________
Richard Steinfeld

François · Posted: Thu 24 Aug, 2006 6:55 am Post subject:

François · Posted: Fri 25 Aug, 2006 8:46 am Post subject:

François · Posted: Fri 03 Nov, 2006 4:04 pm Post subject:

Hi all,
I post here this little reflexion about the dynamic mass of arm and the limits of its calculation formula.
As I was fitting a new bridge on a violin, listening to Bach's Sonata & Partita, I started thinking about the good matching of the Denon DL103 with the L59's tonearm.
To check this, let's do the calculation of the dynamic mass of the L59's tonearm according to the above mentioned formula.
-Headshell+cartridge: 36g.
-Length of the arm: 230mm.
So we get in the end a dynamic mass somewhere around 5,6g Shocked

Do you believe it Question

I don't

_________________
François.

Enough of Gillette Mach3 or Wilkinson Quattro...from now on, Ockham's razor!

Damián · Posted: Mon 07 May, 2007 5:29 pm Post subject:

Richard · Posted: Mon 07 May, 2007 7:54 pm Post subject:

Damián asked:

"Richard wrote:
Stanton, uniquely, made a heavy-tracking parabolic -- a disco-type stylus/cartridge. The sound was incredibly good, and it could make a klutzy record player (Garrard changer!!) sound better than it had any right to. Wear was reduced by the parabolic shape, too. Unfortunately, styli like this are long-gone.

"I hadn't heard about this. Do you remember what the model was, Richard?"

680SL. It's on the technical specification list that I'm including with the Stanton Professional Catalogs that I'm offering in the Flea Market.

It tracks at 2-5 grams. It's like a 680EL, but with the high-performance Stereohedron stylus tip. I forget the stylus number.
_________________
Richard Steinfeld

james_texas · Membre Joined: 24 Jan 2008 Posts: 2 Location: Texas, USA

Also to consider is lateral dynamic mass.
Horizontal stylus movement may have a different resonant frequency than vertical, particularly if you use a hanging weight for anti-skating.
If you must use a weight for anti-skating, it's better to use a heavy weight with string attached near the arm pivot, for reasons of lower dynamic mass.
Of course, a better anti-skating design would use a high compliance spring.

I have a derivation of tonearm/cartridge/effective mass resonance equation from first principles if anyone is interested. Agrees with using moments of inertia.