Motor position

To all you turntable builders out there, I would be interested to know how you choose a motor position. For an onboard motor, the options for the motor spindle position would be on a circle around the subplatter. However, the preferred position on this would seem to be as far away as possible from the cartridge when on the inner grooves to minimise any interference problems. However, using this criteria only, this would seem to put the motor at about 11 o clock, toward the back of the plinth. However, this is now moving the motor closer to the arm pivot which may give rise to induced effects on arm wiring. Perhaps the position is not that critical at all, however, I would be interested to hear whether anyone ever considers this, after all, in many new builds, the motor position is free.
 
To all you turntable builders out there, I would be interested to know how you choose a motor position. For an onboard motor, the options for the motor spindle position would be on a circle around the subplatter. However, the preferred position on this would seem to be as far away as possible from the cartridge when on the inner grooves to minimise any interference problems. However, using this criteria only, this would seem to put the motor at about 11 o clock, toward the back of the plinth. However, this is now moving the motor closer to the arm pivot which may give rise to induced effects on arm wiring. Perhaps the position is not that critical at all, however, I would be interested to hear whether anyone ever considers this, after all, in many new builds, the motor position is free.


If you do have self space place it,as far enough as,you belt allows you to.
If your turntable,is of the suspended kind,the best place is the one,that disturbs the suspension less.On the solid type plinth,I do prefer a 19:00 -21:00hours position ,for reasons of accesibility.
If motor related interference,is your problem,a silent groove disc,will solve your problem. Roll it and change the positions of the motor.Leave it at the position less problematic.If all sound the same,go to phrase one.

B.L.
 

kevinkr

Administrator
Paid Member
Thanks for the replies. My measurements seemed to put the optimum position at about 11 o clock so it's nice to see this backed up. The (possibly also ill fated) turntable I am thinking of building will be a simple solid plinth and decoupled motor affair so no suspension issues to worry about.

When you do please share some photographs.. I always like to see what fellow diyer's are doing. It's often quite inspiring. :D

My one ill-fated (expensive) acrylic plinth project turntable will live on in the hands of a friend who will use it to build himself a TT in the near future.. With 3 TT "projects" and several arms I don't need another.. I'm swapping it for a bunch of NOS octal tubes of various sorts.. :D
 
When you do please share some photographs.. I always like to see what fellow diyer's are doing. It's often quite inspiring. :D

Here are a few shots of mine. The plinth and the platter are acrylic, the whole thing sits on a granite surface plate isolated with some EAR Isodamp grommets. The motor is a surplus 900 RPM hysteresis synchronous capstan motor, for extremely smooth rotation. The spindle is machined from non-magnetic stainless steel.

I built this around 1990, when I had access to a machine shop.
 

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fiddlesticks I tell ya, fiddlesticks!

One argument that you all seem to forget is the potential for lateral vibrations (with respect to the cantilever at any given time) to effect the cantilever while in the groove of the record. I did a little mathematics and the location of the motor does have a preference for the 7-7:30 position, based on analysis of the angle of the drive belt to the driven platter.

Seems the Pink Triangle/Funk Firm folks agree...

Jaques Merde: very good implementation.Looks good. I imagine that it sounds very good.
 
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Nanook,

I have to admit to having not given that much thought. My initial reaction is that it would be a bigger issue for some design directions than for others. For example, a light suspended subchassis design might be particularly sensitive. Noisy motors, like the commonly available steppers and (most) PMBDC types would also be prone to issues.

Doesn't seem like it would be too hard to take some meaningful measurements, though. Hmmm...
 
fellas....

if assuming that the vibrations are random and distributed evenly.

if at the "11 o'clock" position, any vibration will almost always create a much larger vibration (based on vector analysis) in the lateral direction (where it the cantilever and arm can very easily be moved) compared to the dorsal direction (in-line to the tonearm), assuming the arm pivot is firmly located.

If at the "7:00 o'clock" position , assuming the pivot is firmly mounted, then the dorsal movement will be very minimal as the angle will be much smaller, therefore the magnitude of the vibration acting on the cantilever will be reduced.

to me it seems intuitive. Someplace I did all the math, but lost it all in "the crash of summer 2010"