TheKiller - the turntable

Immediately following the tonearm-project the build of an matching turntables is on the agenda. It would be sufficient to build a basic stand for the arm and place it alongside the MyTechnics, but I preferred a decent matching turntable to which the arm can be fitted solidly and permanently.

Analogue to the arm the turntable shall be named "TheKiller" also.

Due to the larger dimensions of the arm the turntable too requires a bigger build.

Again a Technics motor and platter functions as donor, which will be placed on a slate plinth. I ordered the slate plinth via ebay. It measures 40x50x3cm, with chamfered edges and rims.

The extra height of the arm asks for a additional plinth of roundabout 28mm height between the slate plinth and the motor/platter. It will be made from MDF and orientates at the design of the Technics SP-10, SP-15, or SP-25.

Here some pics found on the net. Each two SP-10s, SP.15s and SP-25s.

Plinth after SP-15 or SP-25
Plinth after SP-15 or SP-25

A thick MDF board will be mounted below the slate plinth. It will offer mounting space for subassemblies like the power supply.

A CAD sketch gives a first impression.

I like the slightly curved contour of the SP-15 and SP-25 more than the easier to manufacture straight shape of the SP-10.

Through the beautiful summer the project remained untouched. I also have had intensive second thoughts about the concept, especially on the point of vibration isolation/attenuation. The player will be positioned on a fairly flexible, swinging wooden planks-floor.
Since I have a second plate of slate, of course a structure similar to the above first pic came to my mind. The two plates of slate are separated by damping elements. Such elements of Sorbothane or similar elastomers have good internal damping, but only in a higher frequency range >50 Hz.
For low-frequency oscillations of the rack or of the lower plate of slate the dampers act as a rigid connection and transfers them almost 1:1 to the top plate. Particularly the tonearm pickup combination is concerned, as it has its own low-frequency resonance point, which should preferably settle between 8Hz and 12Hz. To not exite this point of resonance the spring/damper system must act effectively here already. If the stimulus shall be decreased to the half (-6 dB)  a one octave lower cutoff frequency of the damper, 4 Hz to 6 Hz, is required. My goal would be to arrive at below 3 Hz and to reduce excitation to 1/3 to 1/4.

After intensive research of various systems only the classical wound tension spring remained as a effective, yet very cost effective solution. The useage of four to six springs makes sense, since the mass of slate, including the drive and tonearm is distributed fairly homogeneously.
The springs can therefore be distributed fairly evenly around the geometric center of the plate. So the total weight will be evenly divided between the springs. For the calculation of the individual spring the total weight can be divided by the number of springs.
This now enables us to calculate the resonant frequency and the excursion- and weight-reserves

Out of many calculations, the VD-234G from Gutekunst turned out optimal. This spring is made from 2mm stainless steel wire 1.4310, measuring 34mm in outside diameter and the untensioned length Lo 78,1mm. The turns number is  3.5 and has a spring rate c of 1221N / m. Note: Parameter R of the datasheet
defines the value of R in N/mm instead of N/m.

 

The calaculations are simple.
The estimated total weight of about 22kg rests on four springs, each spring then loaded with 5.5kg. Mutiplied by the vaue of the gravitational acceleration

g = 9.81m/s² each spring is tensioned with a force of 54N.

With the spring constant one calculates the distance s2 by which the spring
is compressed to: s2 = F/c, where s2 = 54/1.221 [N]/[N/mm] = 44,2mm.

The compressed length of the spring is L2 = Lo-s2 = 78,1-44,2mm ~ 34mm.

The smallest length of the spring is Ln = 14,4mm.

Unter dynamic conditions though calculate with only Lndyn = 16,1mm.

A immersion-reserve of L2-Lndyn of 34-16,1mm = 17,9mm results.

That means that the spring can rebound uo to ~44mm and immerse up to ~18mm around the point of equilibrium.

The resonant frequency is calculated as F = 1/2pi x sqrt(c/m)

F = 1/6.28 x sqrt(1221/5.5) [1/s] = 2,37Hz

The maximum spring force at Ln is Fn = 77,7N.

Here too a dynamic value Fndyn = 75,7N is given for practical calculation.

This corresponds to a weight force of 7,72kg, or 30,9kg for four springs.

So there´s a good 9kg weight reserve to the estimated 22kg.

We have to keep in mind the fact that a spring tends to bend away sideways when it is compressed heavily. Also lateral deflection and tilt the of the contact surfaces can lead to buckling. In such cases, a guide by external sleeves and/or inner mandrels should be provided.

Since a spring provides for no damping, other means need to be applied.

I'm thinking of foam plug located within the spring, or a fluid damping in form of a paddle attached to the slate, which dips into a highly viscous oil filled trough.

under construction