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.
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.
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.
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.
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.
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.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.
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.
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.
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.
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.
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
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
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.
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.
I was initially trying to do a 'motor drop mod' in which the motor is detached from the plinth and attached at the bottom to a base of some kind, so that it's no longer touching the plinth, but still sticking through the 20mm pulley hole, possibly a mm or two lower, but the pulley on a stock rega is towards the top end of the subplatter, so you have 3 or 4 mm to spare going lower. However, once I got the motor compartment case off and started gently trying to detach the motor I decided that that sucker must be superglued on there or something. I don't know how other people I've read about have unattached it. If anyone has done it on a P1 maybe they can clue me in on how to do it.
But instead of giving up completely, I went halfway and I think may have come up with an easy and cheap way to seriously dampen motor vibration and thereby drastically improve the sound of your P1 or other modern Rega table:
I bought a $7 sheet of acryllic (20" x 24") from Home Depot, along with a $3 roll of Rubber Splicing Tape.
Turn off and Unplug the table and the rca outs. TRY NOT to turn your Rega upside down, because the subplatter will ooze out and not want to go back down, although eventually it does. (much to my relief!) If you can manage to unscrew the two screws holding the motor cover while keeping the table upright, that'll be much easier and safe.
I cut 2-3" strips of tape off and made a little mound in the spot of the acryllic sheet the motor would likely be above. I checked for height periodically, and when I was close made two small stacks in the direction of the foam pads already on the bottom of the motor, so they would make contact, not the actual bottom of the motor.
Then place the table on your acryllic sheet so that the two foam pads on the bottom of the motor touch your top two stacks of rubber tape. Plug and play. The difference in sound quality is huge! So quiet, great imaging, soundstage, pretty much better in every way. Even non-audio friends of mine notice the difference and are blown away by how good it sounds. I'd still like to do the 'motor drop mod' someday, but I'm happy to have gotten the improvement I have and still have a working table!
But instead of giving up completely, I went halfway and I think may have come up with an easy and cheap way to seriously dampen motor vibration and thereby drastically improve the sound of your P1 or other modern Rega table:
I bought a $7 sheet of acryllic (20" x 24") from Home Depot, along with a $3 roll of Rubber Splicing Tape.
Turn off and Unplug the table and the rca outs. TRY NOT to turn your Rega upside down, because the subplatter will ooze out and not want to go back down, although eventually it does. (much to my relief!) If you can manage to unscrew the two screws holding the motor cover while keeping the table upright, that'll be much easier and safe.
I cut 2-3" strips of tape off and made a little mound in the spot of the acryllic sheet the motor would likely be above. I checked for height periodically, and when I was close made two small stacks in the direction of the foam pads already on the bottom of the motor, so they would make contact, not the actual bottom of the motor.
Then place the table on your acryllic sheet so that the two foam pads on the bottom of the motor touch your top two stacks of rubber tape. Plug and play. The difference in sound quality is huge! So quiet, great imaging, soundstage, pretty much better in every way. Even non-audio friends of mine notice the difference and are blown away by how good it sounds. I'd still like to do the 'motor drop mod' someday, but I'm happy to have gotten the improvement I have and still have a working table!
FWIW, with the Hurst or Haydon motors used in AR turntables, you need to use the right value capacitor or the vibration will increase. Ideally you can use a cap box to choose the optimum cap by monitoring any convenient turntable surface with an accelerometer, and tuning for a minimum. Next, don't use more power than you need. Reduce the voltage to the motor as much as practical while still starting and holding speed properly. A series capacitor can be selected to give the necessary voltage drop without any dissipation. These are easy things; obviously if you generate your own phases all sorts of things are possible.
It is critical to balance the voltages to the two phases. The simple R-C dropper and phase shifter commonly used in 240V countries does not do this.
I found that wiring a small transformer primaries and secondaries in series to get an auto transformer with suitable taps for the direct winding and the capacitor coupled winding allowed me to get accurate voltage balance and 90 degree phase shift.
This also has the benefit of not boosting mains harmonics as the standard circuit does
I found that wiring a small transformer primaries and secondaries in series to get an auto transformer with suitable taps for the direct winding and the capacitor coupled winding allowed me to get accurate voltage balance and 90 degree phase shift.
This also has the benefit of not boosting mains harmonics as the standard circuit does
It all rather depends on the actual motor in use. Assuming that phase angle and voltage balance are correct (the optimal values for each of these depend on the actual motor and are often not exactly 90 degrees and 1:1 respectively) there are two major factors to address.
The first is the variation in magnetic reluctance with rotor position which causes cogging due to the rotor accelerating as it is pulled into the preferred rotor position and then decelerating as it attempts to stay there as the field sweeps past. You can tell how susceptible a motor is to this effect by rotating the shaft gently by hand - on a bad motor like a Hurst it will rotate in palpable steps. If it isn't too bad this can be cured by introducing third harmonic into each of the quadrature waveforms - the kits I sold a while back allowed this to be tuned and it could be very effective with the right motor.
The second is a resonance between the moment of inertia of the rotor and the electromagnetic torque "spring" caused by the drive current being greater than the stall value at the output torque, which causes the angle between rotor and stator field to reduce. Since this is related to the difference between drive voltage and "generator voltage", the higher the drive voltage the worse this effect. It isn't as easily solved as the above effects and my first attempts were either abject failures or impossibly complex to implement. I'm still searching for a good simple solution to this one.
Mark Kelly
The first is the variation in magnetic reluctance with rotor position which causes cogging due to the rotor accelerating as it is pulled into the preferred rotor position and then decelerating as it attempts to stay there as the field sweeps past. You can tell how susceptible a motor is to this effect by rotating the shaft gently by hand - on a bad motor like a Hurst it will rotate in palpable steps. If it isn't too bad this can be cured by introducing third harmonic into each of the quadrature waveforms - the kits I sold a while back allowed this to be tuned and it could be very effective with the right motor.
The second is a resonance between the moment of inertia of the rotor and the electromagnetic torque "spring" caused by the drive current being greater than the stall value at the output torque, which causes the angle between rotor and stator field to reduce. Since this is related to the difference between drive voltage and "generator voltage", the higher the drive voltage the worse this effect. It isn't as easily solved as the above effects and my first attempts were either abject failures or impossibly complex to implement. I'm still searching for a good simple solution to this one.
Mark Kelly
Hello everyone,
I recently did some tests on adjusting the drive voltage of the phases independently and achieved a significant improvement in sound quality. I made some recordings for anyone who wants to hear what kind of difference is achieveable for a fairly strightforward modification. Please see Optimising the PT Too power supply to reduce AC motor vibration | diyaudiotest
Simon
Hi Bear,
The AC supply is the AC synthesizing supply that came as standard with the pink triangle pt too : you occasionally see them for sale on the well known auction site. There's one (a different version from mine) for sale on the UK site at present - search for "pink triangle turntable".
The DC supply I tested in the other blog post was an aftermarket upgrade called the v drive, sold by the funk firm (set up by the original designer of the pink triangle). It uses the back-EMF speed regulation method, using a standard dc motor servo chip when I examined the circuit board.
I don't know if you would find the circuit diagram of the AC power supply on the web but there are several manufacturers of similar products (for example heed in the UK).
Btw, that's a nice looking amp on your web site
Good luck,
Simon
That is one way, another is to use the energy of the motor (vibration) to counter it thus draining it. You need to set a tensioned mass that's tunable into motion.
Adjusting the phase offset gives you a real and meaningful improvement in vibration. Fiddling with motor drive voltage largely just reduces torque and is a crutch for incorrect phase offset.
Alternatively you can load the motor with external drag, either viscous at the bearing or something like an eddy brake and you'll find that vibration drops a lot, an awful lot. Pick your motor up while its running, now place your finger on the pulley and press down a little- feel that vibration drop to almost nothing!
Alternatively you can load the motor with external drag, either viscous at the bearing or something like an eddy brake and you'll find that vibration drops a lot, an awful lot. Pick your motor up while its running, now place your finger on the pulley and press down a little- feel that vibration drop to almost nothing!
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