Ditch the tonearm counterbalance?

[Conrad Hoffman]

Forgive me if this topic has been discussed before; I just haven't been able to read everything here. Concerning tonearms, the unipivot diy designs seem to have a lot going for them, but from a fundamentals standpoint, suffer from flaws that almost all arm designs suffer from. The counterweight is on the opposite side of the arm than the cartridge, so all manner of forces get applied across the arm. IMO, that's inherently bad. For a long time a lightweight headshell was considered good, but only because a small increase in weight required a proportionately large increase in the counterweight. For a while, compliant counterweight attachments were used, making the situation even more complex. IMO, the whole counter-balance design needs to be rethought.

Consider a scheme where almost all the mass required for the desired resonant frequency is placed directly above the cartridge, say a small brass plate. Now, make the tonearm out of a flat piece of compliant material. I saw some carbon fiber strip at work, about 1mm thick and 10mm wide, very light, that looked promising. Solidly mount the strip on a vertical pivot pin, with a provision to bend it upwards, say with a setscrew. This provides the tracking force set point; with the right material the force should remain nearly constant for the worst case record warp. No counter balance is used, only spring force. There will be a friction issue with the pivot pin due to torque in the vertical plane, but IMO that's manageable, even if small precision bearings have to be used.

I've described this using the whole arm as the counterbalancing spring, but it could also be done with a stiff tube and shorter flexure pivot at the far end. Using the whole arm results in simpler construction and maybe improved damping. The attachment to the pivot could be adjustable for vertical height, allowing tracking angle adjustment.

What you end up with is a much less complex situation regarding masses, forces, and unwanted resonances, which hopefully leads to better sound. Has anything like this been discussed before, and does anybody see any obvious reasons not to try it? Or is my description so confusing that I need to work up a sketch?

[harwoodspark]

No sketch needed, I understand exactly what you mean but do you not think that a thin strip of whatever you intend to use as the arm would not be susceptible to torsional/ rotational movement? Si.

[Conrad Hoffman]

Hmmm... Quite possible. I'll have to start bending things to see what the sensitivities are. I can eliminate any tendency towards torsion by putting the right design flexure pivot at the far end, and using a more rigid arm, but the idea of using the arm itself was appealing. It occurs to me that the horizontal pivot can also be a flexure, if the spring constant can be kept low enough. I've done a fair amount of work with thin shim-stock flexure pivots, and am surprised they aren't used more.

[SY]

IIRC, this was done some years ago by NAD. You might search around a bit.

[Conrad Hoffman]

I can find some references to the 510 tonearm, and one not-too-clear photo. Very interesting. Reports of sound quality vary, not all great, but I think the design path still has merit, especially with the materials available today, combined with more diy interest. If you mentioned building your own tonearm in 1984, the laughter would have been deafening.

[Nanook]

the NAD does use a counterweight, just a very heavy one close to the pivot.

without attaching an anchor to a non-moving part, "stressing" the arm will have no to little effect, other than perhaps VTA. An alternative would be to use magnets to counteract the weight of the arm/cartridge, and then add small reference weights to the end of the arm for tracking force.

Unless I'm not understanding your explaination correctly

[tubenut]

I am probably not understanding this too well but afaik most cartridges weight more then their recommended VTF, you kind of need a counterweight I think...

WRT NAD, it sounded far better then its successor, it was just to radical for customers to understand that a flimsy flat piece of PCB could work better then a more conventional tube and I think NAD lost interest in convincing people.

[Conrad Hoffman]

Tubenut- yes, you're right, the VTF will be less than the cartridge weight, but there are two things involved. You need a static VTF within a certain range; I propose using the springiness of the arm itself by bending up the attachment point, but you also need a certain effective mass to resonate with the stiffness of the cantilever somewhere below the audio range. With too little mass, the assembly could actually resonate within the audio band, and that would be a disaster! Exactly what this frequency should be isn't entirely clear to me, but it's probably in the 10 Hz range. That mass is going to be more than the mass of the cartridge. IMO, you should also have some amount of damping because you don't want the resonance to have much Q. I'm starting to get serious about building up a prototype, as my old Sony direct drive turntable is getting gummed up, and it looks like a pita to take the arm mechanism apart to clean it. Or maybe I just need an excuse