turntable design...separating the arm from the table..

[plexi]

I'd be interested in your opinions if you've successfully either totally separated the arm onto it's own 'pod' or somehow disconnected it vibrationally from the plinth.

I am building a hardwood plinth and noticed great improvement when I loosened the arm connection by not as tighly securing it to it's arm base and slipping some thin leather undeneath so no I am contemplating totally separating the arm onto a giant aluminum pod..But i would also consider some method to disconnect it vibrationally while still technically having it mounted on the plinth....Any cool trick or ideas or lessend you've learned in this aspect of turntable design/building?

[bluetomgold]

Any cool trick or ideas or lessend you've learned in this aspect of turntable design/building?

Grommets like SME used? Or just leave the armboard unscrewed!

[plexi]

yes , so you think it's not necessary to totally separate the arm? I would rather do something like you suggest....I wonder why i've never seen anyone insert a thin piece rubber some other similar material under the armboard? or is this done and i have just not seen it?

[bluetomgold]

yes , so you think it's not necessary to totally separate the arm? I would rather do something like you suggest....I wonder why i've never seen anyone insert a thin piece rubber some other similar material under the armboard? or is this done and i have just not seen it?

Transcriptors/Michell used to use a piece of foam under the armboard on the Hydraulic Reference etc. It was then simply glued in place. They also favoured unipivot arms so very minimal coupling in this area. The rubber V blocks on a Lenco achieve the same sort of thing, possibly part of the reason why a stock lenco is so much quieter than e.g. a Garrard SP25.

I posted somewhere else that I discovered by chance that the Technics SL110 sounds much better with the arm board unscrewed...to my ears at least.

I haven't experimented with separate arm pods, it could work, but on the other hand coupling the arm to a decent mass might be part of the problem, rather than the solution. I'm sure others will chime in.

Well worth experimenting, anyway. Maybe you've cracked it already with the loose fixing and leather!

[haydn]

Plexi, have you looked in the Gallery?

For example, Ian's Stanley (http://www.lencoheaven.net/forum/index.php?topic=108.0, \Jon's Candy http://www.lencoheaven.net/forum/index.php?topic=118.0 and Graeme's plinth -http://www.lencoheaven.net/forum/index.php?topic=2079.0 all meet the idea of separation of the tonearm from the motor base. There are others  as well.

I seem recall Paul (awty) used a piece of kangaroo leather on a mat to great effect but the search facility isn't finding anything so can't give you a link to look at. The principles are identical for an armboard.

[plexi]

thanks for the input. so leather has been used to 'isolate' the arm in some ways..this is encouraging. I may go that direction...What I have in mind is similar but scaled down to those last two links you put up....I'm thinking rout out a 4x4 section where the arm will mount, rout it down to near the bottom of the top plate (in my case that's all I use it's 3" thick) and then put some leather or rubber on the bottom and put a 4/4 block in that rout so it's not connected to the top or sides of plinth and only at bottom (where the leather is). the 3 other side to the 'pocket' maybe lined with a bit leather and/or leave an 1/16" airgap.......What do you think?
So it will look almost stock but 75% separate.....I think that will be easier for me than the separate arm pod although If I had a shop access and more time I'd probably go that direction....



but my recent experiments tell me if one uses an 'active' plinth (i.e. hardwood) and not 'inert' stacked ply, then it is ESSENTIAL to separate the arm.....this is my belief at this point.....stacked massive ply probably can get away with mounting the arm on the same plinth but this is not the case with more active plinths that have more resonation going on and are lesser mass to deal with it....So my ideal plinth is medium mass with a semidetached arm... a compromise to these larger more separate plinth/arm sections

[ACHiPo]

I'm currently running an arm pod, and while it sounds pretty good, I think it will sound much better when everything is solidly connected with the appropriate damping between the vibration sources and the arm, especially since the turntable still is suspended and the arm is basically rigid--not ideal for extracting accurate information from the groove.

[plexi]

yes you've got a different table. especially since it's still on springs.....the one i'm building is PTP hard mounted and it's on hardwood....It is about the total package I guess....old wisdom suggests sprungloaded top plates like an arm on the same board, but once you hard mount the top plate then that doesn't apply, is what I understand....

[m.french]

With my Flinston build, I tried separating the soapstone armboard from the plinth via a slip mat of leather.  I had perceptible rumble issue, and it drove me crazy.  IT kept up for a couple of months, despite my efforts to 'tune" the noise out.
Finally, I removed the leather slip mat, and had the armboard firmly on the plinth base, and the noise went away.
There is a lot to be said for the AR TT theory of having the motor, bearing, and tonearm all firmly joined.  The self-induced noises on a firmly attached basis help to cancel; whereas, the separated armboard amplifies any new noise influences, as they are all new influences to the pickup, and all rightly amplified.

If this doesn't make sense,.... I just finished my morning coffee, and, not yet feeling its wonderful influence on my awareness and life.

[h-j-hill]

A couple of thoughts:

With the arm on some isolating/resilient component disconnected from the platter, any
environmental disturbance (and that's a lot at extremely low frequencies) will facilitate
movement of the stylus/cartridge/arm relative to the platter; and that will contribute to
the signal from the cartridge.

The stylus/cartridge/arm/arm-board assembly on a resilient mount will have a natural
frequency of oscillation (indeed, potentially six of them; one for each degree of freedom
of movement {3 translational, 3 rotational}).  The stylus/cartridge/arm assembly inevitably
has natural frequencies of oscillation due to the pivoted assembly resting on the stylus-
cantilever which is mounted in a compliant material.  There is a risk of coupling between
oscillations (small though they may be) due to compliant support at the stylus and compliant
support at the arm-board.

[Nick]

Mine has a separate arm pod fixed with two aluminium tubes to the main plinth. As Haydn said, it's worth browsing through the gallery if you want to see lots of different and imaginative ideas.

[bluetomgold]

There is a lot to be said for the AR TT theory of having the motor, bearing, and tonearm all firmly joined.  

On paper it certainly makes sense.

Interestingly though the tonearm in the AR-XA isn't really rigidly coupled to the subchassis, the horizontal bearing (which is essentially where the arm meets the subchassis in this design) is a shaft / well design a bit like a turntable main bearing. Gravity holds it in place.

Besides, Vilchur talks about minimising "unwanted relative motion between pickup cartridge and record", but no attempt was made to couple the record rigidly to the platter. At the end of the day if you're going to use a felt or rubber mat, this is probably the greatest potential source of "unwanted relative motion". A great design for sure, but I'm not convinced that it's this theory that makes it work!

[timelog]

Few ideas ..In current protyping project lenco plinth springs are moved from bottom to under steelplate ( floating on springs ) and tonearm is tighly installed to steelplate to keep platter-tonearm connection as solid as possible. Steelplate area around tonearm is damped with "Blu-Tack". Also steelplate area between motor/drive and tonearm has separate 3-4 damping "walls" made from "Blu-Tack" where idea is that plate can resonate ( consume its energy ), but less is conducted to tonearm. Damping material is used only little compared to steelplate area (150-200g).

[decanterlime]

I like this experimental practice with the blue tac, however I think Lenco provided 4 black rubber grommets for the chassis to sit on which MAY do the same thing. I think the title of this thread.."separate arm from the table" I will use an acronym SAFTT.
My main TT is a SAFTT version and sits on a solid glulam plinth bespoked by Neilb a former LH member. (painted by yours truly!)
The TT sits on 12mm thick rubber pencil erasers one in each corner. This rubber is soft, unlike the stock grommets.
The arm is a Rega R200 and it glides over the record devoid of any motor interference thus no "polluting noise."
If you look at those prior mentioned Lenco grommets they are a hard rubber and very hard wearing. They are of an unusual rubber perhaps harder than road tyre rubber?
That was the secret why Lencos sounded so good even with their so so arms attached to the chassis me thinks.And also coupled with vibrating absorbing non metal V blocks.
Tom mentioned the Garrard SP25 being noisey...the explanation of this is I think the arm glides over the motor area; the motor being directly below the line of travel. Thus EMF noise.
It certainly is obvious with similar BSR idler drive TTs that this is the case.
Soon I will making an Idler BSR UA10 SAFTT to compare with my Lenco SAFTT. However the BSR SAFTTs will be just mounted on a motor board which the Rega arm will be attached to instead of a glulam design.
And I reckon the BSR may be caterpulted up there with other good SAFTT   TT's?
I will see!
Mark

[bluetomgold]

Tom mentioned the Garrard SP25 being noisey...the explanation of this is I think the arm glides over the motor area; the motor being directly below the line of travel. Thus EMF noise.

No... I'm talking about mechanical rumble.

Hum aside, the main bearing and drive system are inherently noisy on the Garrard decks, but I think the arm decoupling (v-blocks) on the L75 (etc) helps with whatever rumble a stock Lenco makes (in the absence of a decent plinth to "soak" it away). Certainly the L70 I had rumbled much more than the L75. Maybe just a coincidence, but I think not. Similarly IME a saggy counterweight stub (as opposed to one which has been tightened up with thread) seems to result in less rumble (and better sound to my ears) in a standard plinth. Decoupling at and around the arm certainly changes the way a turntable behaves and performs.