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Tonearm / Catridge matching?

I have a potentially stupid question about tonearm in general, and linear tracking arm in particular. I am using the Rockport 6000 tonearm, and in his reviews, Michael Fremer had repeatedly stated that the 6000 sounded excellent, but a little weak in the bass. He stated that “ The 6000’s relatively low vertical effective mass put its resonance frequency above the ideal 8-12 Hz region with moving coil cartridges of average weight and typically low compliance, causing the bass to begin rolling off prematurely”. He also stated that Rockport had solved this problem in the 7000, and the arm on the Sirius III, by using heavier armtubes.
Now, if the objective is to make the armtube heavier (so it has a higher effective mass), and assuming the armtube is already rigid enough on the 6000, can I simply add a little lead spacer between the cartridge and the headshell to increase the effective mass and achieve similar result. If this method is workable here, does it also apply to fix pivot arms. Am I missing something here?

Thanks in advance

Michael
thekong
10 responses Add your response
dougdeacon
3,338 posts
 
Pittsflyer,

What made you decide your woofer pumping is related to your arm/cartridge resonance frequency? For that to be true, there would have to be some source of energy at or near 8.5Hz to excite your arm/cartridge into resonance. What energy source do you suspect?

I can't think of any myself. Except for the rare 8.5Hz warp, few if any LP's contain modulations below 16 Hz or so. Footfalls and other floorborne vibrations are typically said to fall well below 6 Hz.

I'm not conviced your tonearm has anything to do with the woofer pumping. I suspect a variation of the infamous Grado dance. The solution, like it or not, may be to choose a cartridge with a properly damped suspension.
pittsflyer
8 posts
 
Thanks for the replies. I'd prefer not to replace either the arm or the cartridge, but the inescapable fact is, the combination produces a ridiculous amount of woofer pumping. The resulting IM distortion is so unpleasant that I inserted a couple of capacitors after the turntable to roll off the lowest frequencies. The problem, however, is a mechanical one, and I hope to solve it as such.
kel34
64 posts
 
What about a Grado Reference on a RB300 is it a good match and can any of the above tweaks improve things?
nsgarch's avatar
nsgarch
2,524 posts
 
Pittsflyer: according to the interactive table at cartridgedb.com, your Robin tonearm (effective mass 11gm.) combined with your Grado cartridge and mounting hardware (6.5 gm + ~1.0 gm. = 7.5 gm.) with a compliance of 20 x 10-6 cm/dyne, gives you a system resonance of 8 hertz. Quite acceptable without any alterations to arm or cartridge.
pittsflyer
8 posts
 
While we have some expertise on the line, I'm trying to go the opposite direction. I have a Grado Reference Sonata on a Graham Robin with a resonant frequency of 8.5Hz. Two questions come up: Will reducing the tonearm mass help reduce the substantial infrasonic signal, and if so, does anyone manufacture a very lightweight headshell?
dougdeacon
3,338 posts
 
Increasing effective mass will in theory cause greater problems when tracking warped records. The higher inertia of the arm in the vertical plane will cause a greater swing in effective VTF at the top and bottom of warps. How much this will effect you in reality depends on the warp tracking capabilities of your rig, and of course on your records! If your Rockport has vacuum hold-down then it's probably a non-issue.

Adding the weight on top of the headshell may be better, as Raul suggested. Not only will your cartridge/headshell interface be unaffected, but your VTA/SRA will too. If you put a shim between headshell and cartridge, you'll have to raise your arm by a similar thickness to compensate.

Read Twl's HIFI Mod thread for a very thorough discussion of the positive effects of increased eff. mass on bass and dynamics. His mod was designed for certain pivoted arms, but the principles are the same in many respects.
thekong OP
203 posts
 
Hi, Thom, Raul, and Nsgarch,

Thanks for all the information. I am using a VDH rebuilt Koetsu Rosewood with my Rockport 6000. Actually I am not feeling that the combination has any bass deficiency at this point, but I, as most of you guys always look for ways to improve the system.

In his review, MF had stated more than once that he liked the 6000 very much but had to give it up because of the bass. That got me thinking, if the only objective is to increase the mass, it should not be that hard to do. It may be hard for an end user to decrease the mass of an arm, but adding mass should be easy. I just want to check with all the experts here before doing anything stupid

I will probably get one of those test LP to see what the resonance frequency of my system is right now. So, other than changing the material interface between the cartridge and the headshell, is there any potential down side with this “fix”?

Thanks in advance

Michael
nsgarch's avatar
nsgarch
2,524 posts
 
There is a very easy to use interactive arm/cartridge matching table at:

www.cartridgedb.com

.
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rauliruegas
13,052 posts
 
Dear Michael: I don't know with which cartridge are you working in your Rockport tonearm.

My advise is that first than all you have to know which is the frecuency resonance between your tonearm/cartridge combo, if the resonance frecuency is between 8-12Hz you don't have to worried about. The formula is:

f=1/2pi x square root: 1/mc, where pi= 3.1416, m is the mass of the system= tonearm effective mass+cartridge weight, and c= cartridge compliance.

I don't like to put anything between the cartridge and the headshell. I think is better to add mass on top the headshell like Tom advise.

Regards and ejoy the music.
Raul.
thom_at_galibier_design's avatar
thom_at_galibier_design
729 posts
 
Hi Michael,

No, you are not missing anything.

Bear in mind when you add weight in the form of a shim between cartridge and headshell, you are changing the material interface - sometimes for the better ... other times for the worse. To try to isolate the variables, start by adding mass (a couple of grams, and compensate for your tracking force change, of course) to the top of the headshell.

When you're adding so little mass as a proportion of the arm's effective mass, you're not changing it's mass distribution significantly. Frank Schroeder for example makes two different cartridge carriers (see the close-up photos on my website, if you're curious). One is aluminum, and the other (brass) is 5 grams heavier ... for adjusting the arm to different cartridge requirements.

Enjoy the journey.

Cheers,
Thom @ Galibier Design
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