Plinth isolation: longish - diyAudio
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Old 18th January 2006, 09:07 PM   #1
pixpop is offline pixpop  United States
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Default Plinth isolation: longish

OK, so I'm going ahead with building a DD turntable (See previous thread about "Why is DD bad?") by taking the motor and platter from a mass market (Sansui) table and mounting them in a new plinth. I then want to use this as a platform for testing tonearm designs. I'll probably also buy an RB250 to use as a reference arm.

First question: Isolation between armbase and platter/motor

Initially, I thought to make the plinth large, so I would be readily able to experiment with different arms. But to allow room behind the platter, for a possible linear tracker, and room to the side, for a conventional arm, I ended up with a plinth about 16 x 17 inches. It's a bit unweildy.

What about building a smaller (round?) plinth just for the turntable, and mounting the arm on a completely separate block of something. This would give me ultimate flexibility, and now the plinth doesn't have to be much bigger than the platter. Of course, I would need to find a way to locate the arm relative to the platter so that the geometry is correct for whatever arm I'm using. But assuming I can manage that, is this a good idea, or will it lead to misery?

Second question: Isolation between motor and plinth.

My motor is mounted on a steel plate, about 1/16 inch thick, and about 4 x 5 inches. The motor axis is offset from the center of this plate by about 1/2 inch. The bottom of the bearing, sits on this plate.

Should I be attempting to couple this plate to the plinth, or is it better to isolate it. In the original design, it was coupled to the (plastic) plinth, but the plinth was isolated from the shelf beneath by rubber feet. The way the motor is constructed, any acoustic energy in the spindle is coupled into the plate. In the original table. the plate was suspended at 3 points under the top surface of the plinth. I'm planning instead to sit it directly on the 'floor' of the plinth.

I could even just sit the motor directly on the shelf, or on some feet of its own, but I think it will be more convenient to have it attached to the plinth.

Do I even need a plinth?

Should I continue to isolate the whole thing (Plinth and motor) from the supporting shelf, with rubber feet, or should I couple it?

I'm planning to situate the turntable on a heavy shelf attached to an inside wall of my room. I may be able to put a stone (e.g. slate) countertop on this shelf. The shelf will be mounted to the wall, but not directly coupled to the floor. Speakers will be under the shelf, sitting on the floor.

I'm planning to build the plinth up from pieces of 1/2 in thick maple, glued together. It will take 3 layers to get enough height to enclose the motor and control electronics.
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Old 19th January 2006, 08:54 PM   #2
pixpop is offline pixpop  United States
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Well, I guess my question was too complicated. Let me try to simplify it:

I'm considering a TT design in which there is no mechanical connection between the arm and the rest of the turntable.

The arm is simply attached to a block of something and sits on the same surface that the turntable sits on. There will be some means of positioning the arm so it has the correct geometrical relationship to the TT, but is not directly attached.

Anything inherently wrong with this idea?
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Old 19th January 2006, 09:41 PM   #3
Zen Mod is offline Zen Mod  Serbia
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Quote:
Originally posted by pixpop
Well, I guess my question was too complicated. Let me try to simplify it:

I'm considering a TT design in which there is no mechanical connection between the arm and the rest of the turntable.

The arm is simply attached to a block of something and sits on the same surface that the turntable sits on. There will be some means of positioning the arm so it has the correct geometrical relationship to the TT, but is not directly attached.

Anything inherently wrong with this idea?

if both-TT and arm base are rigidly placed on same base,nothing wrong with this approach;
in any case ,TT must not have any sort of suspension
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Old 20th January 2006, 06:35 PM   #4
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Nothing wrong with stand alone tonearm. Some essential points:
- it shall be placed on the same rigid platform with the plinth without any compliant element in between;
- arm base shall be massive/stable enough for not to be moved accidentally while playing with tonearm adjustment;
- easy leveling/hight adjustable.
You should ask yourself first, does it worth a mess? Generally, conventional armboard is easier to implement. I made my armbase for the air tracker stand alone, because I was concerned about precise leveling of both platter and arm, which is very critical for air bearing.
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Old 20th January 2006, 08:12 PM   #5
nghiep is offline nghiep  United States
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I put my direct drive Technics SP-25 inside a wooden salad bowl. I extended the spindle long enough to accept a VPI arries acrylic platter. I put the salad bowl on top of a cocobolo lead loaded plinth then install the tonearm on the plinth. It's a lot of work but then I got the best of both world, acrylic platter with steady speed without side load of the belt drive.
40 grams effective mass arm?
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Old 20th January 2006, 08:37 PM   #6
pixpop is offline pixpop  United States
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Oh wow, that's wild.

I never cease to wonder at the creativity of folks desigining these DIY TTs and arms.
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