Thorens 124 Plinth - diyAudio
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Old 29th February 2008, 09:09 PM   #1
Tee-Rex is offline Tee-Rex  Israel
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Default Thorens 124 Plinth

Hello there,

Haven't posted here in a while...
Lot's have changed in the past year, and among some other things I've managed to bump into a TD124 + SME3012 in a nice wooden box witha dust cover.
As much as I like the box, I think the table will benefit from a new plinth. I'm planning a heavy plinth, not using the rubber mushrooms but instead tightly coupling the chassis to the plinth using the 4 threaded bolts on the bottom of the TT.
This will be a 2 tonearm affair, so it's going to be BIG and HEAVY.
However, I've been giving this some thought lately, and was thinking this:

Putting the chassis on a nice heavy plinth is a nice way of reducing vibrations, but not very efficient. And why? (this is my intuition speaking here so let me know if I'm talking nonsense
): the source of vibrations is the motor. It is very efficiently decoupled from the chassis with the rubber grommets (I checked this with a stethoscope). So the path of the noise is through the motor pulley, to the (short belt), to the stepped wheel, to the idler, to the platter and finally to the record. From the platter also goes to the main bearing, only then to the chassis and to the armboard and arm. This kind of makes me think that damping the chassis will not yield the best results. I think I should concentrate on the source of the noise, the motor, to get better results.

Soooo...., how to damp the motor vibrations? At first I thought I'll couple it to the heavy plinth. But that looks like a bad idea. After the motor has been so well decoupled from the chassis, this will only add a direct path for the noise to get back to the chassis.
Then I thought make a plinth wihtin a plinth. The outer one for the TT chassis, and a smaller one, but still heavy, decoupled from the one enveloping it, for the motor. This is an interesting idea, because when you think of it, I can totally seperate the motor from the chassis (take out the rubber grommets and the 3 shafts that hold it to the chassis). However, when I think in terms of a path for the vibrations to escape, I kind of feel that with a heavy sub plinth, the vibrations will want to escape through the easiest path - which is still the rotor, and then to the belt etc. It's an interesting project to try, but I'm not sure that's the way I want to head...
I'm kind of thinking using friction in order to damp the vibrations by turning the energy to heat. The only thing I came up with was somehoe bolting some "spikes" to the motor chassis, and let those spikes rest inside a small sand box underneath the motor. I know it sounds strange, but maybe it will do the job???
Another thought was adding some damping material INSIDE the motor chassis (using the space between the stator and the two aluminum caps on the top and bottom of the motor.

Am I making sense here? Has someone built a plinth which somehow deals directly with motor damping?

Any inputs appreciated (just be gentle...)

-T
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Old 29th February 2008, 10:09 PM   #2
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The 124 motor is not so great indeed. Not even the Papst if you are lucky enough to have it.

One way to reduce vibrations is to feed it with a very pure sinewave from a dedicated PS. Maybe reducing the voltage will also help.

These tweaks work wonders for the Garrards but i never really tried them on the Thorens.

Something else, which should have an effect and is pretty easy is to damp the vibrations of the main bearing.

What about replacing the motor altogether?
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Old 29th February 2008, 11:10 PM   #3
Tee-Rex is offline Tee-Rex  Israel
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Well, reducing the voltage will end up reducing the torque as well, no? I feel it may impair the idler drive benefits. Perhaps there's a sweet spot that allows both enough torque AND less vibrations... maybe worth trying!

I think the Shindo plinths somehow bolt the main bearing to the chassis (couldn't find any infornmation about this. Not sure how much it will gain.

Trying another motor is something I was contemplating... but I'd really like to keep it "original", if you know what I mean..., maybe if I get a second one.

-T
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Old 1st March 2008, 12:58 AM   #4
jlsem is offline jlsem  United States
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In this plinth (http://www.diyaudio.com/forums/showt...84#post1399484) I coupled the bottom of the bearing housing with a threaded rod to a magnesium plate screwed to the bottom of the plinth. The user has the option of then coupling that plate to the tonearm mount with another threaded rod. The motor is pretty well isolated from the assembly as it is but you could maybe damp the vibration a little by firmly attaching a block of magnesium like this: http://cgi.ebay.com/Pure-MAGNESIUM-I...QcmdZViewItem.

John
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Old 1st March 2008, 07:10 AM   #5
Tee-Rex is offline Tee-Rex  Israel
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Those are beautiful, John, thanks!
Do you have a picture that shows the actual coupling of the bearing? I mean the threaded rod, how it's attached to the bearing and to the magnesium plate..

btw - why magnesium? just wondering..

-T
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Old 1st March 2008, 07:57 AM   #6
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I once owned a Trio L07 which had an adjustable threaded spike under the main bearing. Could this idea originally belong to them?
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Old 1st March 2008, 09:15 AM   #7
Tee-Rex is offline Tee-Rex  Israel
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Trio L07... I want one..

Anyway - I don't know who originated this (Ikind of came up with it myself)

Was the spike connected to anything below?
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Old 1st March 2008, 10:37 AM   #8
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Quote:
Originally posted by Tee-Rex
Trio L07... I want one..

Anyway - I don't know who originated this (Ikind of came up with it myself)

Was the spike connected to anything below?

I guess you want it because you've never heard one.

From what i remeber the spike was threaded in the chassis and was accessible from the bottom of the plinth. It allowed for varying degree of support to the bottom of the bearing well. The sound was so off-putting that i never got motivated to play with this. Not to mention it weighed almost 30kg which made the bottom a bit inaccessble.
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Old 1st March 2008, 02:37 PM   #9
jlsem is offline jlsem  United States
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Quote:
I once owned a Trio L07 which had an adjustable threaded spike under the main bearing. Could this idea originally belong to them?
Yes, I stole the idea from them. I do not have, nor have ever had, an original idea.
Quote:
btw - why magnesium? just wondering..
Magnesium damps vibration very well, better than any other metal.

John
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