Can I use a computer PSU with a floppy/DVD motor drive TT?

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morepower4me

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#1
Hello all,

After reading all this great info on floppy/DVD drive powered turntables, I am curious whether I can power the motor with its original source - the computers PSU.

Are there any inherent problems with this I am not considering?

Also, I was planning on controlling the platter RPM via a potentiometer on the DC motor rather than fine tuning pulley size. Can that be done?

Please excuse the 'newb' questions, I am just trying to do this on the cheap, and have it come out better than the equivalently priced Sony or AT off the shelf models. Any suggestions are VERY welcome!!
 
#2
Floppy drive motors are designed to run at a fixed speed, and in the ones I've investigated, have a servo to compare pulses from their rotor with pulses from a crystal. I don't see that any potentiometer is going to allow speed to be varied while that servo is working - you'd need a variable division digital divider following the crystal.

If you can use the motor rotating at its original speed, it could be powered from the computer.
 
#3
Please excuse the newbie response, but could the motor be removed from the electronics that control its speed (crystal and such) and just wired directly from the PSU to the potentiometer then to the motor?

Wouldn't it then work like most any other DC motor? Or is the crystal and accompanying bits what make it useful as a TT drive?

I have searched, I just can't find many details regarding this.

Thanks!!
 
#4
Floppy drive motors do not work like 'ordinary' DC motors as found in portable cassette decks, windshield wiper motors etc.
They are actually AC motors with integral electronics that generates the correct AC waveforms, using a specialist chip.

If you remove all of that electronics and speed control servo system, you'd end up with a motor that has really terrible speed regulation.
 
#5
Hmm, thats really too bad.

So what are my options. Can I still use the computers PSU to power an ordinary DC motor or will I run into the same speed issues?

Am I better off robbing the motor/drive system off of a cheaper TT (like the Audio Technical PL-50)?
 
#7
grimberg said:
I am not sure if you have seen the project described at http://www.altmann.haan.de/turntable. The author uses an inexpensive surplus stepper motor, treating it as AC synchronous, feeding it an adjustable sine wave.
Click to expand...


I did actually see that and it was one of my inspirations!
I was trying to avoid going that route due to the extra steps of generating the sine wave as well as needing an extra amp and source, but if that seems to be the best way than I can certainly make it work.

I lack the electrical experience most around here have, so it needs to make sense in my (begginer) head. I had experience with RC cars in the past and was originally trying to set the platter up using that technology and that working knowledge, but it seems as though the 540 or even 370 motors I am familiar with would have the power to turn ANY platter and run smoothly (bearing shafts) but may be too noisy or unstable.

Back to the drawing board I guess...
 
#8
Use the Altmann ideas! That's a superb piece of cheap but very effective engineering. I'd (almost) be prepared to buy an MP3 player to produce the sine waves if it weren't for the fact that I can make an oscillator perfectly well myself.
 
#9
Very creative idea! The only question I have would be to the vibration (or not) of the motor. I know you can drive steppers with sine waves which is called microstepping, but I always thought that there was a basic vibration related to the number of pole pairs (which is 48 I guess for a 7.5 degree motor) and the rotational speed. Here you do the 48 steps in 20mS, about 2400Hz?

But I'm not sure. Anybody has more experience with this?

Jan Didden
 
#10
Many (most?) turntables and tape decks with synchronous motors use a hysteresis-type motor, which has no permanent magnets and provides very smooth operation.
I've never used a stepper motor in an application where vibration and noise was critical, but they are generally noisy motors because they contain very strong permanent magnet rotors.

The thing is, NO motor will give you stable speed - with the ability to trim it to the required value - without some sort of electrical/electronic control system.

DC motors as used in car cassette decks, and many mid-range home cassette decks have inbuilt speed regulation: a tiny hole in the bottom for a screwdriver. One of those is probably the easiest route - and they aren't *bad* performers either. And yes, you can run it off a computer power supply, since they need around 9 to 12v unregulated.
 
#11
There's precious little difference between a stepper motor and a permanent magnet synchronous motor. When there's a difference, it's that the electromagnet pole pieces on a stepper are rectangular whereas a permanent magnet synchronous motor has triangular or at least trapezoidal pole pieces to reduce the cogging. The picture is of a Philips/Maclennan/Premotec motor
 

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#12
morepower4me said:

...I was trying to avoid going that route due to the extra steps of generating the sine wave as well as needing an extra amp and source...
Click to expand...

There are many different ways to generate sine waves, including a function generator chip that I have used in the past and seems to still be available, the XR2206. The amplifier can be a chip amp and there is no need to get fancy with boards and premium components, as you will not be playing music through it.
 
#14
Wow, that breaks it down to where even I can read/understand it. Perfect!

Now, to circle it all back to the beginning... Can I power the floppy motor with the computers PSU. I don't mind the extra space, and it would seem the speed control aspect should still work. Any other issues? I think there is a 5v DC output. That seems to be what was used int he PDF.

I'd certainly prefer to not have to charge batteries, but if thats the best way, I am sure I can manage.
 
#15
morepower4me said:
Now, to circle it all back to the beginning... Can I power the floppy motor with the computers PSU.
Click to expand...

EC answered this in post #2. The issue at hand is: do you have enough pulleys and belts or can fabricate them such that you can be sure to run the motor at the original fixed rpm? If you are reusing what you have on hand you will need to vary the motor's speed, a predetermined voltage will not work for you.
 
#16
leadbelly said:


EC answered this in post #2. The issue at hand is: do you have enough pulleys and belts or can fabricate them such that you can be sure to run the motor at the original fixed rpm? If you are reusing what you have on hand you will need to vary the motor's speed, a predetermined voltage will not work for you.
Click to expand...


Very good point, I am afraid I missed that rather obvious reply!!

I have almost nothing on hand, so I guess the advantage is that I don't need to make anything else work with it!!

I am planning on using Altmann's bearing/platter/plinth idea (modified to look a little different) and then using the floppy motor mounted on its own detached base (to control tension) and driving it with sewing thread. The hardest part (IMO) will be pulley size.

What I am hoping to do, is mock up the motor pulley in wood, with a 'skewer' sticking out of the top and the motor mounting on the bottom. I am hope to bring it down to size by sticking the extended 'skewer' portion into a drill and using sandpaper to decrease the size of the pully until it spins the platter at the appropriate RPM. Once I have the size down, I'll just have the pulley machined from Aluminum.
I don't know how well that will all work... but it sounds good on paper!

If it all works (and decently well) I am planning to do a step by step pictorial to share my new knowledge with others (as you all have with me.)
 
#20
Hi,

I got another formula from this site. Please have a look:

Platter Diameter = PD in mm
Platter RPM = PRPM
Main Pulley Diameter = MPD in mm
Motor RPM = MRPM

Motor Pulley Diameter (MPD) = PD x PRPM / MRPM

-----------------------------------------------------------------------------------
Example:

PD = 12 inch = 304.8 mm
PRPM = 33.33
MRPM = 300
MPD = (304.8 * 33.33)/300
= 33.86328 mm
= 1.3332 inch
-----------------------------------------------------------------------------------
While using the first formula, I am getting another value (32.97438 mm). Can someone tell me which formula is more correct. The first one or the above one. In the first one, the belt thickness is also getting into consideration. Do we need to take that value also into account ?

Regards,
Bins.
 
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