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back to TD150 project page

Making a better Thorens TD150.  stage 3: upgrading parts

1)

above: the standard TD150 frame

2)

Above: the proposed subchassis frame design.  If the ballast hanger idea works, weight distribution will be equally distributed between the three springs and center of gravity of the oscillating mass would be lowered.

3)

above: real parts less springs, less any counter weights.  Taking weight measurements at each spring location.

4)

5)

Total weight loads: ( 1 + 2 + 3) 190 ozs. or 11 .875 lbs

Average weight load is 63.3 oz each spring before counter weights are added.

In a preliminary test, the original Thorens spring will not quite carry the above load.  The spring nearest the tonearm coil binds  while the other two are still within a working range of their compression.  No point in attempting to balance the load until a workable spring rate is found.

Data for the standard Thorens spring:

6) Standard_spg.jpg (27609 bytes) click on thumbnail for full size image

Type: conical compression, ground ends, open
wire alloy: unknown but it is steel and isn't rusty after 30 years of use.
No. of working coils: 11
mean diameter at small end: .602 inches
mean diameter at large end: .906 inches
avg mean diameter: .754
wire diameter: .059 inches
free length: 1.935
spring index: 13,,,?
static deflection : .5 to .7 inches in the standard setup (see image # 1) (varies from spring to spring)
spring rate: ?
natural frequency: estimated 4 to 6 hz 

More notes: 

The three springs carry the tonearm, which has a natural resonance between its suspended cantilever at the cartridge (carries the stylus)  and the effective tonearm mass of around 8 to 12 hz,  This is an industry standard but may vary outside this range if arm and cartridge are not well matched.

Footfall resonance is thought to be approximately 3 hz but will not be an issue as the turntable will be mounted from the wall.  

Even more notes about springs:

Spring stiffness is expressed in terms of how much force  is needed to compress a spring one inch.  If a spring is rated at 4 lbs, this means that it takes 4 lbs of pressure to deflect the spring one inch.

Static deflection is the amount of distance the spring is compressed when under a normal load and at rest.

Natural frequency is the number of oscillations a spring will go through under a given load.  Some formulas for calculating natural frequency:

NF = natural frequency in hz
K = spring stiffness
DST = Static Deflection
hz = cycles per second

NF = 3.13 SQRT K/load

or

NF = 3.13 / sqrt DST (in inches)

or

NF = 5 sqrt 1/DST (in cm)

The natural frequency of the sprung suspension should exist between foot fall  and the cart/arm natural frequency.  If foot fall is approximately 3 hz and the low end of cart/arm frequency is 8 hz, then the target for spring frequency is 4 - 6 hz.