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-   -   Methods for Reducing Turntable Motor Vibration (http://www.diyaudio.com/forums/analogue-source/94463-methods-reducing-turntable-motor-vibration.html)

pjpoes 17th January 2007 04:05 PM

Methods for Reducing Turntable Motor Vibration
 
I was looking for some idea's on how to reduce the vibration made by an AC Syncronous motor. I have some of my own, but I'm not sure I have a good enough understanding of what causes a motor to vibrate in order to do this.

So, I have read about things like Rega developing a new motor and controller that allowed some adjustments to be made which made the motor vibration completely go away. My assumption is that this was some sort of motor drive that synthesized the drive oscillation and could have the phase tuned in order to minimize vibration, and thats what the adjustment was.

My thorens had such adjustments, and again, vibration could be reduced, though never eliminated by getting those adjustments just so.

I also found that in the case of my new turntable, with a seperate motor bod, that having too much tension on the string causes motor vibration problems.

My motor is controlled currently directly by the wall current, with a transformer used to reduce the voltage from 120 to I believe 12, and I assume that a capacitor is used to "fake" the phase shift. One idea I had was to use an adjustable capacitor to reduce vibration by finding the exact right value. Does that seem like a good or bad idea?

I know that some say reducing the voltage some, and thus torque, can reduce vibration, but at 12 volts, I believe they have already done this enough. However, it may be a motor that works with quite a bit of torque at 12 volts, and so maybe reducing it to 11 or 11.5 would be a way to further reduce it.

Another idea I had was to by a project speed controller and modify its output to 12 volts. It talked about being regulated at 16 volts, so my assumption is that somehow I could reduce the output to 12, if nothing else through resistors. However I believe I could reduce the supply voltage at the regulators to something lower, and that would reduce output. This will still rely on the motor to "fake" the phase shift with a capacitor, but will ensure a perfect 60hz sinewave at a better regulated voltage.

hacknet 17th January 2007 04:15 PM

i've been working on a ac regneration system for a linn lp12's motor. i find that the noise in the mains does affect the sound alot.

the other think i find extremely important is the voltage that the motor is driven at. a lower voltage directly translates to much 'blacker' background but it tends to sacrifice on the prat alittle.

sreten 17th January 2007 08:07 PM

Hi,

Using a capacitor causes enough phase shift for the motor to work
but it seems its never achieves the full 90 degrees phase lead. At
a cost it seems by introducing a relatively small amount of lag you
can achieve the ideal 90 degree relationship, i.e. put an inductor
in the other phase and then adjust the capacitor value.

Perhaps it this these "special" circuits implement, especially 240V ones.

:)/sreten.

pjpoes 18th January 2007 06:35 PM

Sreten you now have my very intriqued, anywhere I can read more about this implimentation. How is this not more common, or is it, is this the miracle cuircits they are using now with Rega and what not. I always assumed it was simply a an amplifier that either through a wein bridge or ic created the oscillation and then was shift 90 degrees on the second phase, through whatever method was possible, given an analogue or digital cuircit. Or is this one of the methods they shift that phase, even in those cuircits. Thanks.

Listens2tubes 28th January 2007 01:05 AM

Can you wrap the motor with Plast-i-clay? This would physically damp the vibration. Even a lump on one side should have some effect. My thinking is cumulative in nature. Enjoy!

pjpoes 28th January 2007 03:51 AM

I actually intend to experiment with various methods to decrease the effects of motor vibration. Recording 1khz tones directly to my computer, and looking at an FFT graph shows what I believe to be artifacts of that vibration, so I think I may have a somewhat objective way of measuring the difference. I'm somewhat unsure at the moment and I base this on John Atkinsons method of measuring turntables, but I note that he hasn't used the same method in some time, and question its validity.

By the way, in experimenting with different turntables to get different graphs I found a similar thing to Mr. Atkinsons. Direct Drive quartz locked tables like an older Pioneer model and the famous Technics model measure with a much smoother tone, less side bands, but its much wider too. It also danced more I thought, but that is really hard to tell. My Acoustic Solid and Thorens TD125MKII both measure with a much narrower band, but with some sidebands. However, they are far down in level, and I don't know if they would be audible. Anyone know why that is? Are the sidebands caused by the belt drive? Motor vibration?

Anyway, as to how I want to try and stop these vibrations. First thing I was thinking of was to invert the pulley, so the larger pulley is at the bottom. It will lower the center of gravity. Also, it attaches with one set screw I think, instead of two or three, which I think is a mistake as you can't center it. I will see if a shop can't drill and tap two more holes for me. I want to place a piece of dampening material inside of it, and on the bottom. I may try making some sleeves for the pod to fit into, which may dampen vibrations as well. I also want to try using different platforms that might help isolate the motor better, spikes, brass weights-footers, etc. Also, as I have already mentioned, I want to try constructing a motor drive to improve motor speed stability and reduce vibration.

EC8010 28th January 2007 04:51 PM

First, let me apologise in advance for not telling you the whole story, but I did the work as a commercial project, so I can't give out a circuit diagram...

Yes, it is possible to make a driver that almost eliminates synchronous motor vibration. A Wien bridge is not the way to do it. You need to amplify your two phases separately - motor phasing capacitors (or inductors) are out.

khush 28th January 2007 08:30 PM

It was said that minimizing vibration depended on be able to adjust the phase difference of the two driving voltages.
The way I tried it was to have a wien oscillator driving and old stereo amp with two step up transformers on the output.
In one channel there was an all pass filter set to provide 90deg phase shift at the oscillator frequency, by making the resistance variable you can alter the phase shift. look at http://www.werewoolf.org.je/apf.htm for details of all pass filter.
As your motor is 12v you won't need step up transformers a couple of chip amplifiers could be used.
I used this set up with a motor from a dual turntable, never got a really definitive nulling of vibration but ymmv. never tested it on a turntable to check in improvements in SQ.
There are all sorts of possibilities for a power supplies like this filtered divided down xtal oscillators, using micro controller generated sine waves. You could vary the motor voltage to minimize vibration.
good luck
Khush

rabidlistener 5th February 2007 07:55 AM

Damped the Airpax motor I use on my much modified Thorens TD125 by embedding it a jelly like substance (actually a jelly aliens head for kids!! LOL). This really works!!

Zero One 9th February 2007 01:01 PM

On my highly modded and ancient Pioneer TT I araldited a small plastic bottle to the bottom of the motor, this pokes out through a hole cut in the base of the TT. The TT is on some home made damping legs that support it 3 inches or so above the concrete block support.

The bottle is half filled with oil, which sloshes around enough to cancel out quite a bit of vibration, in addition the bottle has a cut down brick with non slip rubber matting on it, this presses lightly against the outboard side of the bottle and counteracts the pull of the belt on the top of the motor.

It sounds rough but I swear it works very well. The brick will be replaced by a custom shaped painted steel block when I get a chance (this thread has reminded me I must do that). I have also added damping to the motor (ie blutac) but the difference was pretty minimal ,but it can't hurt.


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