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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
 
Moving the turntable away from the preamp solved the hum issue. There is no hum regardless of the rpm - it is dead quiet now. While shuffling the gear components around, I've placed a monoblock power amp onto the preamp.  That made a preamp generate a different kind of hum. The bottom line, the preamp doesn't like any gear around, otherwise it picks an interference from a nearby component. It's best when it is positioned away from anything else, as far from turntable, a power supply or an amplifier as possible. 
lewm
12,975 posts
 
QED
millercarbon's avatar
millercarbon
12,531 posts
 
One thing vexing me this whole time is referring to the Manley Steelhead phono stage as a preamp. You would not be having these problems if this was a preamp. Your problems stem from the tremendous phono stage gain, something no preamp ever has or does.   

These distinctions are important because it helps a lot to understand this stuff. A phono stage includes RIAA equalization. Since the lowest bass on a record is -20dB the phono stage has to equalize this 20dB. That is a lot of amplification, and it comes on top of the 40 to 65dB gain needed simply because cartridge output is so low.

This is the real reason hum is so prevalent in situations like this. Once you understand it you would never in your life even think about putting something like a motor or amp on top of a phono stage. Preamp maybe. Phono stage, never.

If you look inside one of these things you will see the circuit is laid out or organized so that incoming power and transformers are kept as far away as possible from the amplification circuits. Usually both physical distance and physical shielding are used. This is the case with all components. But with phono stages it is taken to extremes, because the phono stage takes everything to the extreme. So it is worth it to call it what it is, and it is not a preamp. It is a phono stage.
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