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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
esputnix OP
19 posts
 
The turntable is Oracle AC and the pre-amp is Manley Steelhead. 
russ69's avatar
russ69
3,497 posts
 
You need to give your turntable it's proper space and support. Stacking a turntable is not going to work.
vinylzone
288 posts
 
You can try placing a grounded copper sheet between the TT and preamp, on the shelf where the turntable sits.

dekay's avatar
dekay
5,541 posts
 
Not certain if the hum is originating from the cartridge and/or the motor.

Anyway, there is a product called Mu-Metal (sold in sheets) that you might try placing over the transformer(s) in the Manley.

You could place the sheet on top of the preamp case.

DeKay
millercarbon's avatar
millercarbon
12,531 posts
 
Never a good idea to stack like that, but we gotta do what we gotta do. Electric motors make electric fields. Preamps are especially vulnerable, phono stages even more so. Distance is your best protection. If you can rearrange to move the preamp lower and farther away that will help even if stuff winds up being stacked simply because it will move the phono stage further away.  

Next thing you can do is shield. Vinylzone has one way with the copper sheet. Another is to shield the motor itself. Another is to shield the phono section, or even the whole preamp. Shielding can be anything conductive doesn't have to be copper sheet doesn't even have to be solid. Sheet metal is used inside a lot of preamps to shield the phono section. You can also try battery grounding strap, which is braided tinned copper. Battery strap is fairly cheap and can be cut up like fabric. You could also search for Faraday fabric, more expensive but a lot better looking.    

Keith Herron suggested aluminum foil as a cheap and easy test. Never tried it myself but, Keith Herron, worth a shot. 

Any of these whatever you try will work better grounded to earth ground- your AC ground will do fine. Just not grounded to the preamp.
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