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Turntable Motor control

G

Geordie

pfm Member
Hi everyone - I have been a member for a couple of months, but first post in the D.I.Y. section - so be gentle with me....

I am building a D.I.Y. turntable..have only got a 2nd hand RB250 (which I am going to rewire) to date, but my main worry at the moment is that I have NO IDEA of how to control the speed of the deck. I would like to 'switch' from 33 1/3 to 45 if possible. From looking in books and on the internet it seems that a DC motor would be better to use?? Is this right??

Seen a DC16V motor from rapid electronics which looks OK but don't know how to control the speed.....because its a project, I don't want to buy a project speed controller @ £75 but like the look of the SE II at £300. Would love to be able to make someting like this (with LED display!!)

Can anyone help with / schematics / kits / point me in the right direction etc.. I would be most grateful!!

Happy new year to you all!
 
Turntable motors and speed control, dont get me started on that one!
Wether you choose an AC or a DC motor you will need a fair amount of electronic knowledge (something I dont possess in any way shape or form) to get it right.
DC control units are notoriously difficult to get right, the main problem being thermal drift. To bring a DC motor down to a speed usable for a TT requires relatively low voltage, a motor that runs at (say) 20 to 30v for its maximum rated speed will probably need only about 5v. A very small change in voltage at these levels affects the speed significantly. If you want touch button speed change you have the same problems only twice! Uncle Ants (Needles and Spins) sell Project motors, pulleys and power supplies as spare parts.
If you go for a AC synchronous motor and you want to change the speed you need to change the mains frequency, easiest way to do this would be to buy a Hercules power supply, I think they are about 2-300 quid.
What type of drive are you using? If belt drive then its relatively straight forwards but if (like me) you want to use Idler Drive then AC synchronous motors have a bit of a problem in that the take up is a bit to harsh, this is because they produce maximum torque at start up thus putting the drive train under a fair bit of strain. Ideally the motor needs to have a soft start, but this means varying the frequency,not the voltage.
Asynchronous AC motors (as used by Garrard 301/401 for example) have further issues. They are very inefficient, only about 10% efficient in fact. Its a common misconception that the Garrard motors are powerful, high torque motors, there not, a premotec from Scotland's finest has more torque. What makes a asynchronous motor like the Garrard so suitable for TT use is a high inertia due to the flywheel effect of the rotor. the eddy current system helps matters even more, smoothing the motor further. The outer rotor Papst AC motors make good TT motors but many of them come with noisy roller bearings, not good. Michell went DC as Papst stopped making the AC motors.
My advice, for what its worth, is to use a AC synchronous motor to develop your initial design, then refine it from there.
 
Thanks Paul for your reply....but I didn't understand most of what you said.

I know that I want a belt drive TT but thats about it. I am looking for an 'idiots guide' with parts and layouts so I can learn and understand more about electronics. (I might even do some maths evening classes...if it would help!!) This might be too much to ask but I really want to get into DIY audio...and maybe in a few years time, I will be in a position to have proper input into this forum...??
 
Geordie said:
.......so I can learn and understand more about electronics.....
Click to expand...


Well, building a TT in itself is not going to teach you much about electronics. Better off getting hold of (say) a Rega Planar 2 or 3 and build a AC power supply for it.
Do a forum search for a DIY Armageddon for a gentle introduction, this is a very simple AC PS for the standard Rega motor. There are also details on the forum for a couple of other power supplies for AC motors, Les W's TAPS and I think the Norton Air Supply is on here somewhere.
After that you could move onto DC motors, not sure if there is much info on this forum about them, but there's plenty on DIY Audio.

A second, and perhaps better alternative would be to get hold of a cheapo Naim NAC 42 preamp and mod that, huge amount of info on this forum and plenty of people to hold your hand. Build a simple PS first, then split the tracks and dual supply it, then........


Paul.
 
Thanks Paul - a really good idea, and a way into getting to know others and talking to others on the forum......I'll give it a go!!!....... ;)
 
I have a circuit for controlling a DC motor with tacho-generator, if that is any help. The drawback of this approach is that suitable motors are costly - I got a quote for over £100 for one.

Simple DC motors suffer drift problems, but can be OK.

AC motors are simplest to use; especially the versions that run more or less straight from the mains. The drawback is that direct connection to the mains means single speed.

What design and parts are you planning to use for platter, bearing, sub-chassis and suspension (if any)?
 
My platter is going to be made from solid beech -12-15mm thick. Its going to sit on a solid beech plinth...the whole thing will sit on granite. As for the bearings and spindle ???? I haven't got that far yet.

I have decided to build an armageddon clone but will be using either a rega P2/P3 motor. I can't get hold of a LP12 motor.........hope the rega one will be OK???? The DC motor is beyond me I think (at the moment)
 
Look up the floppy drive motor on
http://www.audioorigami.co.uk/Archive/FloppyDIYMotor01.htm

If you want to build a supply for a rega motor, I've never seen one, but I guess it's an a.c. synchronous motor. One supply is a wein (or wien) bridge with an amplifier. There are circuits around on the net.

As for bearing, a guy did one with a valve and valve guide from an engine.
You should be able to find it with a little help from google.

diyaudio.com is a great resource for diy turntable building, it's probably where you'll find the valve and guide bearing and lots more good stuff.
 
Geordie - also research driving low-voltage dc motors using current feedback, which makes for fair regulation under load at low motor speeds - much better than a simple voltage-regulated supply IME. I might have a couple of circuits to dig-up and post later.

a guy did one with a valve and valve guide from an engine.
Click to expand...
I was convinced that page is a nice joke. Thse parts are not ideal for a turntable bearing - they have comparatively huge clearances cold to enable the things to run hot...
 
Geordie said:
My platter is going to be made from solid beech -12-15mm thick. Its going to sit on a solid beech plinth...the whole thing will sit on granite. As for the bearings and spindle ???? I haven't got that far yet.

I have decided to build an armageddon clone but will be using either a rega P2/P3 motor. I can't get hold of a LP12 motor.........hope the rega one will be OK???? The DC motor is beyond me I think (at the moment)
Click to expand...

The LP12 motor is just a basic synchronous motor with two phase connections - for years it was a cheap Philips item. Philips sold the motor business, and it now trades using the Primotec brand.

If you use a natural material like beech, the density will not be quite uniform, so even careful machining does not result in the centre of mass, the centre of gyration and the bearing axis coinciding. How are you going to balance the platter?
 
For a simple start to DC Motor control, e-bay has a few possibilities which could get you started (be warned though, some may not work due to continuous 'hunting' to keep the correct speed):

e-bay (just as an example of what is out there)

Good motors to try to find are made by Escap & Maxon, the latter is used in the Teres turntable which started as a web DIY project. Have a look online for this or go to their page:

http://www.teresaudio.com/project/index.html

...and when you're ready to spend a lot of time reading & learning use this, it's a thread on motor control for turntables (it's been a great help to me):

www.diyaudio.com

A good basic start for a bearing would be a Rega that you can find on e-bay for reasonable money...simple, well-made, proven & easy to implement...or go magnetic which I'm playing with just now....

Have fun,

J
 
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PigletsDad

I have decided to use beech as the platter as I already have a slab of it...didn't realise it may not be balanced.....don't want to go the compressed ply route....can you suggest anything else??

RS motor looks great...thanks.
 
JimmyB

Thankyou for your input.....your right, lots of reading. I am going to begin with an AC motor..see how it works before I try the DC route.

I am still hunting around for a bearing though.........
 
{engineering head on]
I don't think it is necessarily foolish to use a valve and guide from a car as the basis. You'd need a hardened steel ball for the base and you couldn't get the right precision out of the box for the reasons stated. If I were engineering this I would use 2 undersize bronze bushes pressed into a housing and ream them to fit. However this is a tough old job and you need engineering facilities, ideally a lathe. Not easy compared to buying a bearing for a Garrard, Thorens or similar. Interesting job though. If you can't bring yourself to do the engineering have a chat with someone like Martin Bastin.
 
Still wondering about an alternative platter to solid beech. Even though it will be a project...is it a lot less hassle to buy one?.......defeates the object don't you think?
 
Geordie said:
PigletsDad

I have decided to use beech as the platter as I already have a slab of it...didn't realise it may not be balanced.....don't want to go the compressed ply route....can you suggest anything else??

RS motor looks great...thanks.
Click to expand...

What about perspex? There are lots of people on e-bay who sell it in decent thicknesses.
 
As far as the bearing is concerned you could go for the self lubricating bearing from the Michell Orbe or Gyro Deck. Was one on e-bay a while ago, went for approx £40 if I recall correctly. A very good starting point I would suggest. You would have to make the platter high mass though. Maybe your beech one with machined holes underneath on the periphery for lead weights?
Would also allow balancing?
 
My DIY(ish) TT uses belt drive round the outside of the platter. Looks a bit ugly, but works well.

I used a O-ring based belt for while. It was terrible - you could hear a warble every time the joint went over the drive pulley. It is really hard to get an acceptable joint with the same properties as the rest of the belt. The guy who built the TT eventually sourced a long enough proper belt, and things have improved enormously since.

Unless you have a solution for making really neat joints, don't waste time on O-ring belts.
 
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