Methods for Reducing Turntable Motor Vibration

[baiken]

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!

[Conrad Hoffman]

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.

[davidsrsb]

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

[Mark Kelly]

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

[SimonLogan]

Originally Posted by Mark Kelly

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.
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

[bear]

Simon,

Any schematics or other useful information about the AC and the DC supply you cited?

_-_-bear

[SimonLogan]

Originally Posted by bear

Simon,
Any schematics or other useful information about the AC and the DC supply you cited?

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

[christaiwan]

Quick newbie question: Can I fix my motor on a big lead plate to dampen out vibrations? I also have a few shock absorbers lying around from model cars, can I use them too? I am sorry if my questions are kinda stupid.

[BaMorin]

Originally Posted by christaiwan

Quick newbie question: Can I fix my motor on a big lead plate to dampen out vibrations? I also have a few shock absorbers lying around from model cars, can I use them too? I am sorry if my questions are kinda stupid.

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.

[sq225917]

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!