Turntable chassis Development

In the cheap devices both the motor and the tonearm are assembled on a thin plastic chassis. This solution does not provide enough noise attenuation to the tonearm. It can receive noise from both the motor and the desk.

In the modified state, both the motor and the tonearm are fixed on a solid wood block. The noise can be attenuated enough this way.
See the other details about motor holder here and the arm suspension here.

The turntable is ready and operating.
More images of the turntable are here.

Details

- The solid hardwood base

On the picture at right you can see the internal parts assembled on the hardwood base.
Its full mass (including cover) is cca 18 kg.

Probably this is the most important part to reduce noise. Such a big mass with ehough strength can attenuate the mechanical noises of such a device.
Before somebody would treat this as just a flam, let's calculate a bit:

     The size of the groove is 50 micrometer

So, this is the size of the tip movement when playing loud music.
But how much can be the maximum allowed displacement of the mechanical noise?

     The best vinyls can produce cca 60 dB signal to noise ratio

This means one thousand times ratio of the amplitude (and one million times ratio of the power).
If we would like the mechanical noises to be inaudible, it must have less displacement than the noise of the vinyl.
Therefore it means that:

     The allowed displacement of the mechanical noise is less than 50 nanometers

By the way, this is about tenths of the wavelength size of the visible light. It also means that some parts invisibly small for light microscopes can produce audible noise.

But essentially the mechanical noise must be below 50 nanometers. This is the twenty-thousandth part of the millimeter.

- The legs

The mass itself is not enough to reduce noises, a flexible suspension is also necessary. There is a rubber suspension between the desk and the wooden base.
It is quite complicated because the rubber must be subjected to tensile stress to form a stable structure.

In horizontal direction this device has high compliance and therefore high attenuation factor even in lowest frequency range. Its resonance frequency is cca 4-5 Hz.
In vertical direction the material is less flexible and therefore causes less attenuation in the lowest frequency range. Its resonance frequency is cca 12-15 Hz.
However, it is not a big problem, because an additional low-cut filter can be applied in the stereo differential signal (which is caused by vertical movement of the tip).

In the result, the turntable has very good immunity against noises from the desk.

The covers are installed.

- The plastic cover

This part also induces arguments within the audiophile world. The generic judgement is that using such a cover reduces audio quality.
In such cases I usually try thinking and execute some measurements if possible. IMHO, this way I could contradict this belief too. However, it is based on some truth, see the details below.

What happens without plastic cover?

The vinyl has large enough area to receive vibrations from the air - like a microphone does. Fortunately, it also has large enough mass, but in the lower frquency range this sensitivity can have annoying level. This effect can produce a feedback to the audio signal which can result in a low frequency oscillation in worst case. At least it means low quality of your turntable.
The quality of the platter and mat are also determines this sensitivity.
The resulting movement is mainly vertical, which means the stereo difference signal in the output.
This effect works only if the tip is on the vinyl. The rotation is not necessary, so it can be measured easily with stopped platter.

If you close the cover down...

The cover also receives vibration from the air, but it is passed over the box, instead of the vinyl. Under the covcer the sound pressure gets significantly lower, therefore the vinyl receives less.
So the main difference is that the vibration passes over the box, instead of the vinyl.

Let's see two different cases:

Cheap "eggshell" turntables:
In this case the mass of the box is small, in addition it has some resonances, so it can receive those vibrations easily.
The result can be worse than without cover.
Good quality, audiophile turntables:
Such a turntable probably has higher mass, therefore its box attenuates these vibrations before it could reach the head.
The mass of the box is orders higher than the mass of the vinyl, and the sound pressure gets lower under the cover, so the resulting noise level can be significantly lower with closed cover. If this is not the case, then your turntable has low quality.

I deduced that somebody (probably having an "eggshell" turntable) determined that it is better without cover, and this information got accepted by the community. However it is true in that case, it cannot be generalized this way because the result highly depends on the mechanical parameters.

The reflected waves

It is also possible that the waves produced by the tip and the vinyl can be reflected by the cover and received as an unwanted noise. However it is true, but the environmental noise (especially if you listen high volume music) has significantly larger level, so the waves of the tip can be ignored in all cases. The noise attenuation by the cover is significantly more important than the reflection.

Measurement results

This effect can be measured easily, and I will provide results soon. :-)