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Tonearm mounting rigidity and considerations?

I posted this on AK and it has brought up some interesting discussion. I decided to post my initial post verbatim over here because I know there are some very knowledgeable folks on here and I would like your input. If you follow the links you can see some of my build thread on the custom aluminum sub armboards that will hold the interchangeable armboards that I’m machining. This subject seems to have a lot of differing opinions, I’d like to hear from the folks on here so I can learn.Thank you BillWojo---------------------------------------------------------------------------------------------- "Something I’ve been thinking a lot about over the past several weeks as I design and build new arm boards for a two arm Victor (JVC) constrained layer plinth. You can see the thread on my build progress on page 11 of this post. https://audiokarma.org/forums/index.php?threads/jvc-ql-owners-thread.909550/
There was this thread that made it clear as to how important it really is. https://audiokarma.org/forums/index.php?threads/jelco-easy-vta.916431/
Here is a quote from that:
"I use one and couldn’t be happier.
For me, the improvement in the arms sound quality alone was worth the price of admission.
Being able to adjust VTA "on the fly" is just an added bonus."
Seems the Jelco VTA attachment really helped with the rigidity of the mounting.
When I was searching for post on armboard material I found all kinds of opinions as to what sounds better as well. That’s what really made me think about what was happening.
The tonearm has to move very freely in two axis of motion all the while being fed a steady stream of vibrations. Any looseness in the tonearm bearings or the mounting will degrade the sound. Thin armboards are vibrating with the tonearm and are "coloring" the sound. Even if the bearings are adjusted properly the tonearm may be vibrating in the support collar, especially if it’s the type that is only locked into place with a single screw of some type.
My thinking is that the tonearm should have good bearings, properly adjusted and the tonearm post should be held very rigidly to the plinth. The tonearm tube should have either internal or external damping as well to quell the vibrations.
Has anyone here experimented with this idea? I need to make an armboard insert for the back position of my Victor and I think I will do away with the stock mounting collar. Instead I will make a clamp that will support the tonearm post and it will be rigidly secured to the armboard. It will fill the entire area of the armboard cutout and be secured by 3 or 4 screws per side for maximum rigidity. Material will be aluminum. The tonearm will be a Audio Technica ATP-12T using a Denon DL-103 cart. That cart should feed a lot of energy into the tonearm.
I appreciate any and all thoughts on this.
Thank you"
billwojo
8 responses Add your response
billwojo OP
397 posts
 
atmasphere, I'm trying to get my head around how a 18 lb DD precision built motor is going to excite my constrained layer plinth. It's only running at 33.33 rpm, even if the platter was out of balance by several ounces  it really wouldn't inject a lot of energy into the plinth at such a low rotational speed.
I've worked with dynamic balancing machines, balancing the rotating mass of a 502 CI big block Chevy engine that will run at 8K rpm. As you get closer to being perfectly balanced the machine will up the rpm of the rotating mass so the sensors can get a reading. At low rpm's the computer will tell you it's in perfect balance. At speed on the balancer the crank and counter bob weights are turning close to 1K rpm, kind of scary to stand in front of it.

BillWojo
atmasphere's avatar
atmasphere
11,794 posts
 
@billwojo  You have most of this right.

The missing bit is the plinth itself. If the platter is able to vibrate in a plane or differently at any rate from the base of the arm, the result will be a coloration. To this end the mounting of the platter bearing must be both dead (properly damped) and rigidly coupled to the mounting of the arm as possible. In this way if there is vibration, both mounts will be in the same plane, preventing the arm from transducing it.

This is why the armboard must be made of the same material as the rest of the plinth, and coupled to it again as rigidly as possible! Otherwise if the armboard is damped in some way that the plinth lacks, you'll get a coloration.
bukanona
174 posts
 
SAEC had extra weight AS-500E
Micro Seiki was famous for gunmetal armboards
Fidelity research B60

IMO if to use aluminium better to try to make it in conjunction with wood. 
I have experimented with MDF but for me hardwood worked better on Garrard 301 plinth. I am thinking to make like sandwich hardwood and stainless in the future just for curiosity.

slaw's avatar
slaw
21,402 posts
 
Have you considered carbon fiber?
millercarbon's avatar
millercarbon
12,531 posts
 
Could be. Point is, hard to be sure. Could just as easily be the majority of arms are screwed down simply because that's what everyone does and what everyone expects to see. Its also easiest. None of which means its the best.

My arm before the Conqueror was the Graham 2.2. The Graham uni-pivot, the Conqueror uses two pivots. Same idea, eliminate all bearing play, but using two for greater stability. Anyway point is the Graham screwed down but did not have to be screwed down. It balanced and was stable just sitting there.

All arms if you think about it are stable like this. They all use a counterweight and pivot on the center of mass. So of course they will all be stable. Duh! So this is yet another case where we can easily just try it and see.
elliottbnewcombjr
5,363 posts
 
millercarbon.

Impressive arm indeed, I suspect your arm's unique design is pertinent to your 'loose mount' experience

" Floating vertical bearings decouple the armtube from the deck, reducing coloration"
elliottbnewcombjr
5,363 posts
 
I go where Bill is going regarding stability: SOLID foundation:

I use the OEM arm boards of the deck he has. Like the deck’s 70mm thick 7 layers, the 3/4" arm boards are laminated layers, very tight fittings.

He will mess with many arms, thus his idea/project.

Cartridge tight to arm, if headshell, tight to arm, if adjustable/changable arm, tight to arm mount, Vibration free platter, weight on LP, solid mount of spinner to deck, serious fixing of arm to post, post to arm base, base to board, board to deck, terrific deck construction, solid adjustable feet to level, isolation blocks if needed to avoid room vibrations, and, dust cover off while playing.

And, check your arm’s bearings base connection, I left my long arm’s bearing ring a speck too loose to the base when I re-installed it. My Shure sounded better in the rear arm, huh, figured it out, now the long arm is back to wonderful. Shure better in long arm, and new AT MC beat the Shure for me.

I want the stylus, and nothing but the stylus, to get the vibrations.


millercarbon's avatar
millercarbon
12,531 posts
 
Your reasoning sounds really good on paper. Now here's the reality. My Origin Live Conqueror arm slip fits into the table. Its held in place by gravity. There's not much play, but neither is it a snug fit. Directions for cutting the hole had something like a 1/8" tolerance. Not fussy at all. Sounds great. 

Would it sound better to be more tightly anchored? I don't think so. Not just because of the manufacturer either. The one mod I did was to replace the OL collar with one machined by a modder who added a teflon layer to the top. So with this its even less tightly held in place. It sounds even better!

The moral of the story is not that the arm needs to be free to move around. Nor is it that the arm needs to be rigidly mounted. We don't know that either. All we know is we just don't know. 

My advice is do your proof of concept. Build one each way. Try em and see. Only way to know for sure. Too many guys waste mass time and effort trying to make everything perfect the first time. Remember, anything worth doing is worth doing poorly. 
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