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TT Hum

So... I have not noticed this through my speakers, but in headphones; I get a low hum depending on the location of my tonearm. When it’s resting it’s noticeable, but when I move it to cue the hum reduces. Basically, the closer to the center of the platter the less the hum. I have an SL1200, and the power supply is offboard so it shouldn’t be anything under the platter, there is no transformer, etc. It’s not the ground wire from the arm (Jelco SA750D) either. My PSU and everything else is on another shelf, so I can’t see it being interference, and when moving things around it doesn’t change. The only things I can think of could be some kind of weird interference from the pitch fader (that’s the only thing even near the arm's resting position) or I’ve yet to try yet another cart/headshell. Stumped. Thoughts?
au_lait
mijostyn's avatar
mijostyn
8,570 posts
 
au_lait, when it talks like a duck and walks like a duck it is usually a duck.
The Cartridge is not well shielded and there are electronics at the right side of the table that are emitting. Make sure the cartridge body is getting grounded. You may need a cartridge with better shielding. It is not too bad and you do not hear it through your speakers so you may elect to just live with it. 
lewm
13,266 posts
 
Mijo, I ask this out of curiosity, because you raise an interesting question: how many cartridge bodies are grounded or even can be grounded, unless there is continuity between the cartridge body and one of the  audio grounds available at the pins?  As you know, many cartridges are made of plastic or other non-conductive materials.  Also, many headshells are non-conductive (made of wood or carbon fiber, for example), so grounding through the headshell is often not feasible.  For that matter, the only cartridges that would be shielded would be those with conductive metal bodies that are then grounded preferably to the chassis.  I am not implying that your advice is not good.  I am just curious whether I am missing the point.  Thanks.
This is not at all to say that you are not correct in guessing that the OP's cartridge is picking up some radiation from the electronic panel under the tonearm.
chakster's avatar
chakster
6,144 posts
 
@au_lait

-What is the cartridge?

-If you have a few different cartridges do you have a hum with all of them?

-Any proper platter mat on your Technics SL1200 or just a felt slipmat or stock rubber mat ?

-Hum is noticeable on normal level or on high volume only ?

-Can you hear the hum only using your headphones and its amp or you can hear it in the speakers?

P.S. First thing to check is ground wire from your Jelco tonearm to your phono stage. You can disconnect it and try again to see is the hum is gone or increased. You have to check headshell leadwires and headshell connection (lock) with the arm. DIN connection of the phono cable to the tonearm base. This is basics. Put some nice metal mat on Technics platter and try again.




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