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How to isloate turntable motor from making a hum

My turntable has an AC motor that is mounted right at the bottom of the turntable. The turntable itself is placed on top of the vacuum tube preamplifier. This placement makes the AC motor only half inch away from the pre-amp. Spinning at 33 amp produces a barely audible hum. But once it is switched to 45 RPM the hum level increases substantially. This morning I lifted the turntable up holding it about 10 inches above the pre-amp and the hum was gone! Once I lowered the turntable closer to the pre-amp the hum was gradually increasing reaching its max level once the turntable was placed right on top of it - when the distance between the motor and the pre-amp was shortest. Apparently, there is some form of interference going on between the turntable motor and the vacuum tube preamp.
Unfortunately, there is not enough space in my room to position the turntable to the side, next to the preamp. And I can’t raise the turntable 10 inches above it as it would make it hard to reach. I wonder if there is any isolation material I could use to place in-between the turntable and a preamp? Can an acrylic or plastic padding or a metal sheet used to block the interference?
esputnix
mijostyn's avatar
mijostyn
8,570 posts
 
Your motor is not well shielded. As others have suggested trying any grounded metal under the turntable might help as would a high output cartridge. It could also be the reverse. Fields from the Manley could be causing the cartridge to hum. Grado's are notorious for this.  In either event it should not happen. The phono stage is never that far from the turntable.  Mine is also directly under the turntable and the system is dead silent. You might try different grounding schemes. I had run a separate ground from the turntable chassis to the phono stage along with the ground wire from the tonearm. If I disconnect either I get a hum. 
millercarbon's avatar
millercarbon
12,531 posts
 
You know now that I think of it, at the level of these components what I would do is first test with aluminum foil. Wrap the motor, and fold a sheet for between the table and preamp. Then if that works upgrade the whole thing with some Krissy mats. I have never thought of using them for shielding but that is basically what they are doing. Why settle for eliminating hum when you can get an upgrade out of it?
czarivey
5,946 posts
 
copper sheet covering tube preamp??
good luck
russ69's avatar
russ69
3,496 posts
 
Shielding is really just a band-aid. Best to get some distance between components. Electromagnetic fields drop away quickly with distance. 
lewm
12,975 posts
 
Russ, op says he can’t get TT and phono stage any further from each other. Maybe sell some of that expensive gear and get a bigger room. (Joke)

ps. I doubt the Steelhead is the source of the EMI. Steelhead has an outboard PS with a long umbilical. I assume the steelhead PS is out of the way on the floor or on a lower shelf.
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