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Hurst motor rumble?

I have an HW19-IV with the stand alone motor.
When I drop the stylus onto a stationary object next to the platter, there is rumble as soon as the motor is turned on. Turn it off, the rumble goes away.

I removed the drive belt and tried again. No rumble. This means the noise is not coming through the platform on which everything sits.

Next, drive belt still disconnected,I dropped the stylus and spun the platter by hand. No rumble. (The bearing appears to me to be tight and well lubed so I don't think the noise can stem from disaligning of the platter bearing by the pressure of the belt.)

It seems obvious that the noise is coming from the motor through the drive belt to the platter. I am using a standard Hurst 600 rpm motor.

I always thought, apparently erroneously, that the drive belt itself damped motor noise. If any of you has a belt drive turntable, I would like to know what happens when you perform the aforementioned test. You need deep bass and may have to turn up the gain.
Can anyone verify that a "quieter" motor might reduce or even eliminate this rumble?
Has anyone done actual measurements of this effect and what are they?
rpfef
bpoletti
1,552 posts
 
I've never thought about "dropping the needle" onto anything that didn't have some kind of spiraling groove toward the center of the turntable. My tastes in music don't venture out that far.

If the noise doesn't happen during playback, who cares?
terry9
2,333 posts
 
I for one commend RP for using the highly sensitive instrument he already had (his music system), rather than renting a vastly expensive instrument to do the same thing. Identifying the source of noise is pre-requisite to eliminating it, and that is always worth doing in a decent system.

RP, you might find that you prefer the Premotec 9904 111 31813 low torque motor, made expressly for audio turntables. It is 300 RPM AC.
rodman99999
6,126 posts
 
Have you ever lubricated the bearings on your SAMA? If you only hear the rumble, when the belt is on and spinning the platter; it's obviously not being transmitted through the platform. If you can hear the rumble; a subsonic filter is not the answer. Those roll off frequencies below about 20Hz. Personally; I have a hard time imagining motor vibrations being transmitted through the drive belt, unless you have it stretched really tight. You might try moving the SAMA a bit closer to the platter, and see if it helps. You'd be unlikely to hear your platter's bearing, if dry or defective, without it being spun at playback speed. The platter bearing would be my first suspect. How long has it been since you thoroughly cleaned and lubed your 19's bearing?
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