DIY Turntable Control Calculator

[Psychobiker]

Hi Guys,

I just thought I'd knock up a quick calculator for a turntable motor control system. You can go as deep as you want into topologies (From simple LM317-style regulators up to full-blown Buck/Boost/Flyback circuits) and if you need assistance on that, please email me at : liam(at)liammartin.com.

This would be a closed loop (ie. with feedback) system, so it'll try to keep the speed as constant as possible with respect to load.
The overall accuracy will depend primarily on the % of allowed variation around the reference voltage...although most switching regulators are accurate to within a few percent these days.

However, this is simply a calculator for the 'handy-to-know' ratios - contained herein are :

Platter vs Drive wheel RPM
Motor RPM + Drive wheel vs Platter RPM
DC-DC Controller Feedback Resistances based on the above.

http://liammartin.com/Misc/TTCalc/Turntable_Driver_Calc.xls

If you spot a hole in the calculations, please let me know!
I hope this is of help.

Liam

[Mark Kelly]

Your platter speed to pulley speed ratio is wrong.

The correct ratio for rubber belts is (platter diameter + belt thickness) / (pulley diameter + belt thickness).

For non rubber belts the belt thickness term on top and bottom must be multiplied by twice the Poisson's ratio of the material. For rubber and other incommpressible materials the Poisson's ratio is 0.5, hence the short version above.

[Psychobiker]

Cheers for that! I will make amends ASAP. It's a couple of MM, if that...so can it be declared negligible?

[Psychobiker]

Updated to reflect Mark Kelly's suggestions, however I will assume a non-compressible material - if you want to go into compressible belt media, so be it - and I will pop it in the calculator if enough people ask for it! Hope it's OK!

L

[Psychobiker]

Something simple to calculate weights required/exerted on the tonearm.
VERY simple, though! No circular torque is taken into account.

L

http://liammartin.com/Misc/TTCalc/Arm_Calculator.xls

[cbdb]

The correct ratio for rubber belts is (platter diameter + belt thickness) / (pulley diameter + belt thickness).

Maybe I am missing something, but this dosnt make sense to me. Draw a picture with 2 pulleys and a very thick belt. The only thing that matters is the inside surface of the belt (the surface touching the pulleys). The thickness dosnt matter.

[Psychobiker]

Originally Posted by cbdb

Maybe I am missing something, but this dosnt make sense to me. Draw a picture with 2 pulleys and a very thick belt. The only thing that matters is the inside surface of the belt (the surface touching the pulleys). The thickness dosnt matter.

I thought this too.

[Jacques Merde]

Originally Posted by Psychobiker

I thought this too.

The effective radius of each pulley or platter actually occurs at somewhere near the belt centerline.

[dnewma04]

Originally Posted by cbdb

Maybe I am missing something, but this dosnt make sense to me. Draw a picture with 2 pulleys and a very thick belt. The only thing that matters is the inside surface of the belt (the surface touching the pulleys). The thickness dosnt matter.

You are missing something, Mr. Kelly is dead on with his explanation.

[Nanook]

after doing some thinking (which is not always my way...), I've given up on belt drives when designing a drive system, I'm sticking with either idler drive or rim drive. I've looked at various systems and ran my choice and basic design past a mechanical engineer whose opinion I trust.

As Mark Kelly points out, the modulus of elasticity is quite high for rubber (regardless of thickness but at smaller thicknesses the forces required to deform it is smaller) if using a rubber belt at room temperature. This implies that in small enough diameters to allow use in belt drive systems the modulus of elasticity does need to be considered. I think this is one of the reasons why many have reported the benefits of changing belt materials--- belts do vibrate and can introduce "smearing" of the sound. Even some rather expensive tables have benefitted from a change in belt materials or in motor or drive type upgrades (like the Teres Verus upgrade)

Perhaps a different belt material could be used. I have used high strength nylon upholstery thread to very good effect. Difficult to get the correct length and tension required to drive a platter, but stable once done. I assume a good quality silk thread would also provide the same, and perhaps unwaxed dental floss. Something that may be considered is what some have used. 1/4" cassette tape, or 1/2" video tape or mylar (as per the Redpoint and Teres designs) with suitable pulleys.

Take care all...