Adding mass to the Zeta counterweight


Here’s a question directed at the very small group of LP/turntable/pickup arm/phono cartridge designers/retailers/installers/enthusiasts who can make sense of it!

I just acquired a Zeta arm in great shape, which is however missing the thinner of the two weights included in it’s counterweight. For most users, a garden-variety steel washer would provide an easy solution, as that is all the missing stock disc is. And I may do just that. But for my assembled record player components, I need to do more. The need arrises from the fact that:

1- The arm is going on a Townshend Rock table, which requires the mounting of two "outriggers" and a tubular "paddle" (the mass of which totals 11 grams) to the headshell of the Zeta. The mass thus added to the front end of the arm requires comparable mass be added to the counterweight, as the Zeta’s counterweight, even positioned on the arm as far back as possible, cannot balance the arm with the added mass on the headshell. Adding mass to the counterweight raises the effective mass of the arm, which is 16 grams. Effective mass introduces the consideration of cartridge compliance and cartridge/arm resonant figures, which brings up the second complicating factor in my particular situation:

2- The cartridge is a London, with compliance figures of 10 vertical and 15 lateral. The differing vertical and lateral compliance figures results in differing resonant figures. I calculated those figures as 7 lateral and 8 vertical, a little lower than optimum. Luckily, the damping provided by the trough system of the Rock table mitigates those figures somewhat. But it occurred to me that as long as I am adding mass to the counterweight (I have no choice---there is no way around it, even though it lowers even further the resonance figures), perhaps it could be done in such a manner as to add as little as possible. I was thinking of sticking the little lead weights (around 3/16" x 3/8" x 1/8" thick) found at bait & tackle shops onto the backside of the counterweight (the lead weights have double-sided tape on them), placed in a vertical line top to bottom. I believe that will keep the mass more in the vertical plane, less in the lateral, which will be best in relation to the compliance of the cartridge.

Thoughts, anyone?


bdp24

Adding mass to the counterweight reduces effective mass. In this case getting the weight as close to the pivot as possible will minimize moment (MOI) and give better transient response, tracking.  Nice arm.

Regards, 

Ah yes, I had forgotten that adding mass to the counterweight will allow it to be positioned closer to the arms bearings, thereby automatically lowering the arms effective mass. Elegant, and counter-intuitive (no pun intended)! Thanks fleib.

The question remains of whether or not adding mass in the vertical plane only, rather than in the lateral as well, will make any difference? I'm thinking of the after-market counterweights for Rega arms (by Michell and Groovetracer, amongst others), which place their mass both below the arm bearings and further out from them in the lateral plane, "longhorn" style. If I did the opposite, adding the mass not laterally but vertically only, right on the bearings vertical axis (running in a line from top to bottom on the back of the stock counterweight) will help better match the low compliance of the cartridge in that plane with higher mass in the same plane, leaving the lateral mass of the arm more-or-less the same, which matches well with the cartridges higher lateral compliance.

An underhung (as it were) counterweight lowers the center of gravity and gives a mechanical advantage to a static balanced system, in the vertical plane. Adding mass to the horizontal bearings has no consequence on VTF and can provide an extra measure of bass authority, although examples cited aren't heavy enough horizontally to be of consequence.

The Moerch DP8 seems to be a good example of mass added to the horizontal plane. 

The Michell and Groovetracer counterweights are underhung and wide, for the reason you state fleib. And you're right, adding mass below the bearings should provide an improvement as the Zeta is a dynamically balanced arm---hadn't thought of that, either. Thanks again!

I had a dream last night, in which I saw the counterweight with a brass bolt sticking out the back (through one of the holes in its back plate, if you remember what a Zeta cw looks like), at the bottom of the circle. A number of brass nuts were threaded onto it, for fine-tuning the mass. They probably aren't heavy enough to add much, but the lead fishing weight should. I'll attach them on the underside of the cw, and that oughta do it.

Experiment my sticking blobs of clay on/under/around the arm.
Zeta arm is fantastic.  My friend has it.  Thanks anything with ease.

**I had a dream last night, in which I saw the counterweight with a brass bolt sticking out the back (through one of the holes in its back plate, if you remember what a Zeta cw looks like), at the bottom of the circle. A number of brass nuts were threaded onto it, for fine-tuning the mass.**

Didn't see this post before now, you had a dream about my old Zeta?

It was mounted on a Studietto.  That's exactly right.  Put the bolt on the bottom and add mass until you get the proper weight with the counterweight in the most forward position possible.  Tracking/tracing is improved with eff mass minimized. With no azimuth adjustment you'll have to shim, if necessary.  

fleib

All right, I'll give it a try! I just noticed in that same post I misspoke. I meant to say "And you're right, adding mass below the bearings should provide an improvement as the Zeta is NOT a dynamically balanced arm". Thanks for all the ideas fleib!---Eric.