Arm resonance - the anal way

[vince666]

will follow this new thread with some healthy curiosity.

anyways, on my TT i went more to the easy way... i used the resonance calculator at vinyl engine to make sure i was in the safe range... and, yes, being a range and not an exact value only, lets you feel free to ignore what influences the situation in a minor way.

my TT (Pioneer PL720, same as PL7) has a light arm and i have a high compliance cart (Shure M97XE)... they match just nicely and in fact i have no tracking issues at all.

but it will be nice to learn about this matter more in depth.

 

[NeverSatisfied]

For brain exorcise, fine.

For reality? Test record. No matter how good the “math” is, at the end of the day, it’s the actual performance that matters.

Anyone care to recommend a good test record or records?
Seems like a worthy investment since I now have many different types of turntables.

 

[8991XJ]

And Low compliance is 0-10
Medium compliance is 11-20
and High compliance is over 20 in general But with compliance this value in cu (not the times ten to the minus sixth power for an actual engineering value) is for a 10Hz test. The Japanese frequently use a 100Hz result and there doesn't seem to be an accurate conversion.

And we do the same with tonearms
Low mass up to 10
Medium mass up to 20
High mass over 20 in general

Low compliance wants a high mass arm
High compliance wants a low mass arm
Medium seems to be the way to go. Lots of that available.

Again nothing written in stone here

 

[J!m]

Anyone care to recommend a good test record or records?
Seems like a worthy investment since I now have many different types of turntables.

I’ll look at what I have later today for you. The good old ones are harder to find but I suspect eBay is a good source. And they usually have “low mileage” so that’s nice.

 

[NeverSatisfied]

I’ll look at what I have later today for you. The good old ones are harder to find but I suspect eBay is a good source. And they usually have “low mileage” so that’s nice.

Thanks Jim.

 

[J!m]

The first one is the main one I use. It does a good job of showing exactly what’s happening.

the last one is just selected songs to highlight the ability of a system; not a test record per se.

I recommend the first one if you have none.

 

[NeverSatisfied]

View attachment 59157
View attachment 59158
View attachment 59159
The first one is the main one I use. It does a good job of showing exactly what’s happening.

the last one is just selected songs to highlight the ability of a system; not a test record per se.

I recommend the first one if you have none.

Thanks, I will start looking. The hunt is on

 

[Bob Boyer]

OK, guys...here it goes!

The goal of this thread is to choose a cartridge to match an arm from a complete newbie standpoint.

To make things "easier", let's go with:
- tonearm is SME 309 (thanks Bob Boyer)
- looking at two cartidges; Ortofon 2M Black and audio-technica VM760SLC.

....which leads to

PREDICAMENT no. 2
I can make an educated guess about the effective mass/moment of inertia of the cartridge + mounting screws (with SME 309 the headshell is a part of the arm) with a higher accuracy than simply assuming this mass (with dimension of zero) is concentrated on the tip of the stylus...which leads to

PREDICAMENT no. 3
The effective mass, as stated by SME...based on what position of the counterweight?!...

And just to throw another little variable into the mix, my SME has detachable headshells. So there are apparently different iterations of that tonearm.

 

[Bob Boyer]

...“it’s the journey not the the destination”. At least that’s what I keep telling myself, lol.

Indeed , the journey to a lighter wallet.

 

[J!m]

Married.

“driver carries no cash”

 

[borchee]

Thanks for your input, guys!

Perhaps I should have asked specific questions?!

Is simply adding mass (of the cartridge, screws, shims, shells...), instead of their effective masses, to tonearm's effective mass (mixing apples and oranges) negligible or not?

Pointer: Take into account that the cartridge isn't a point-mass. It has dimensions bigger than zero, hence the center of mass (which doesn't correspond with the effective lenght!) and the moment of inertia around vertical axis going through the center of mass have a value. The bigger the effective lenght, the less important it gets - my arm's effective lenght is 40 mm and the Ortofon 2M Black has a lenght of 27,5 mm!

Math and phisycs are merely tools and like with every tool...it's useless, if you can't work it (feeding the formula with wrong data in our case).

Slap me with numbers/proof/facts - how the damn thing sounds is of absolutely no interest.

 

[J!m]

Welp, changing the mass of the cartridge (with heavier screws, for example) does change the frequency of resonance.

Take a ruler, taped to the edge of the desk and pluck it- it resonates at a frequency.

Add some pennies to the end of it and pluck it again- the frequency changes.

Weather you need more, or less weight, at a specific effective length, depends on where you are in relation to "ideal" frequency range before you started.

I currently sell a Rega belt test kit, with six different belts to see how they effect performance/sound of the Rega (and clones). I had considered a cartridge screw kit, with screws of various materials that have differing mass. It can help; it can also hurt... With everything... "depends".

 

[borchee]

Yes...but I'm not changing anything - as stated in post #1, I deliberately chose a specific arm and a specific cartridge so that we have a common basis for discussion.

 

[J!m]

Okay, then unless you change the cartridge position in the arm, or perhaps the weight of the counterweight (which changes it's distance from the pivot), there will be no changes.

 

[borchee]

Yes, but...no! I'm not changing anything physically (effective lenght, positon of counterweight/effective mass of tonerm with counterweight). It is all exactly as stated by SME and Ortofon/audio-technica. Just looking at the effective mass of the cartridge which doesn't equal the mass of same.

 

[J!m]

Well, if we have two identical (geometrically) cartridges with different weights, the heavier one has a higher effective mass than the lighter one.

Putting a "number" on it may prove to be a challenge, but not impossible.

Since effective mass is, um, effected greater, the further you get from the pivot, a small change at the headshell (which is far from the pivot), will have a greater effect that a large change of the counter weight, which is close to the pivot. Hence the idea to offer screws of differing weight. Change one at a time and have zero positional change, but effective mass can be increased or decreased. (This can be why a "junky" headshell sounds good- it changes the effective mass, for the better in some cases; for the worse in others)

Changing the "shape" of the counterweight changes things too, so for sake of discussion we will assume typical "round" weights of equal dimension, but varying density, for this exercise.

Also, since moving the cartridge further from the pivot changes the length, it can therefore change the effective mass more than changing weight can. (I think the formula is a square of the length but it's from memory so...)

 

[Makymak]

Although I don't understand the question clearly I will try to answer.

A specific tonearm has a specific effective mass and this effective mass is determined by the center of mass and it's distance from the pivot.

A cartridge does not have a specific effective mass by it's own since it's not hunged to a pivot. It adds mass to the effective mass of the tonearm but the way it does isn't just an addition. To see what is the effective mass of the tonearm + cart you have to estimate the new center of mass and it's new distance from the pivot. Quite hard to do as a cart isn't a point with 0 dimensions (as borchee said) but it has dimensions. And to make things better (haha!) it's weight isn't homogeneous and equally distributed across it's whole body.

Take an egg and give it a haircut (as we say here!)...

 

[borchee]

All excellent points, J!m!

Here's mine...

We want to buy one of the cartridges mentioned (let's stick with Ortofon). Prior to doing so, we want to verify that it is a good match for the SME - or any! arm for that matter - by filling data into the formula or a calculator (Vinylengine for example).

Now:
- the only things we know about the arm (SME 309) are effective mass (we'll disregard the uknown counterweight-position and go with it) and effective lenght
- we know the compliance
- we know the value of PI...to the eleventh decimal - ridicilous, but which one matters...let's go with the usual 3.14
- we know the mass of the cartridge, but not the effective one. Adding cartridge's mass to arm's effective mass is false in principle and that's a fact.
- to make life easier, let's say the mass of screws is zero.

The only thing that is unclear is the value of the cartridge's effective mass and at which point can it be neglected/substituted with mass.
We are changing absolutely nothing, but trying to calculate cartridge's effective mass more accurately to se if it makes a significant difference in resonant frequency. As we don't know cartridge's mass moment of inertia, nor where the center of mass is, we are screwed in generall. But we know cartridge's dimension and mass, so we can make an "educated" estimate - it is still inacurate, but the principle is correct.

I'll do some math and post an example in the next days - it'll take some until I'm ready to present it in a manner it can be understood (hopefully).

Edit: @Makymak, exactly! Only...of course the cartidge has an effective mass of its own - it rotates around the pivot as well as around the vertical axis going through the mass center. Every body with mass bigger than zero has. Arm's effective mass is nothing but a sum of effective masses of every part.

 

[Makymak]

Of course the cartridge has effective mass. It depends on the position of the pivot and the force applied in conjunction with it's center of mass, if talking about circular movement (our case). On straight movement, it's mass is also the effective. When I said a cart hasn't an effective mass of it's own, I meant when it isn't attached to the arm. Falsely wrighten by me.

To be more accurate, the effective mass changes in accordance with what forces and where they are applied.

Edit: don't forget that the pivot is a kind of force applied to the arm...

 

[NeverSatisfied]

All excellent points, J!m!

Here's mine...

We want to buy one of the cartridges mentioned (let's stick with Ortofon). Prior to doing so, we want to verify that it is a good match for the SME - or any! arm for that matter - by filling data into the formula or a calculator (Vinylengine for example).

Now:
- the only things we know about the arm (SME 309) are effective mass (we'll disregard the uknown counterweight-position and go with it) and effective lenght
- we know the compliance
- we know the value of PI...to the eleventh decimal - ridicilous, but which one matters...let's go with the usual 3.14
- we know the mass of the cartridge, but not the effective one. Adding cartridge's mass to arm's effective mass is false in principle and that's a fact.
- to make life easier, let's say the mass of screws is zero.

The only thing that is unclear is the value of the cartridge's effective mass and at which point can it be neglected/substituted with mass.
We are changing absolutely nothing, but trying to calculate cartridge's effective mass more accurately to se if it makes a significant difference in resonant frequency. As we don't know cartridge's mass moment of inertia, nor where the center of mass is, we are screwed in generall. But we know cartridge's dimension and mass, so we can make an "educated" estimate - it is still inacurate, but the principle is correct.

I'll do some math and post an example in the next days - it'll take some until I'm ready to present it in a manner it can be understood (hopefully).

Edit: @Makymak, exactly! Only...of course the cartidge has an effective mass of its own - it rotates around the pivot as well as around the vertical axis going through the mass center. Every body with mass bigger than zero has. Arm's effective mass is nothing but a sum of effective masses of every part.

I think this is what Micro Seiki was trying to address with this tone arm design. Although they don’t really provide any data on how to adjust it.