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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
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.




au_lait OP
353 posts
 
@chakster Yeah I have done/checked every one of those basic tests and have narrowed it down. It's not a ground issue - my phono cable grounded to either phonostage or preamp is all good. Its a very quiet hum (not buzz) and it really only happens when the arm is resting. Not hearing through the speakers unless its really loud, BUT I am digitizing lots of vinyl and I hear it a bit through the ADC on headpones. It's not the end of the world, of course, and I will try another cart tonight. Just curious if anyone had experienced this before. Maybe I lowered my noise floor to the point that I've uncovered something that was masked before?

Cartridge: Nagaoka MP150 (Will try with Zu DL103 tonight)
Mats: Nagaoka Crystal, will try with Herbie's, TMM Monitor, Rubber/Cork mix tonight
lewm
13,386 posts
 
Almost any cartridge will produce a faint noise if it is in space, not touching the LP surface, and if you then turn up the gain high enough. That is more a function of the phono stage signal to noise ratio and/or ambient electrical noise, than it is a function of the cartridge.  But on the one hand you do say it only happens when the arm is resting, which fits my theory.  Then you imply you hear the faint noise on your digitized copies of LPs, where the stylus must have been tracing a groove, which contradicts my theory.  Which is it?  The cartridge hanging in space is like an antenna which picks up stuff and then delivers it to your phono stage, which has a lot of inherent gain.
mijostyn's avatar
mijostyn
8,570 posts
 
Lewm, many cartridges have a shield inside that is grounded. Some may not. There is the noise floor of the phono section which you will hear if you turn it all the way up. But at any reasonable volume my ARC phono stage is dead quiet even with your ear right up next to the speaker.
I think the principle defect here is probably in the turntable as I have never heard of hum problems with a Nagaoka especially since it is an MM cartridge. The only cartridge I have ever had a noise problem with was a Grado which hummed as the cartridge got closer to the motor. They are notorious for this problem. 
au_lait OP
353 posts
 
No I don't hear it on the recordings really, it's far too faint for that, or I haven't tried any music quiet enough to notice. I'm talking solely about when there is no music playing (so maybe it just doesn't matter) ad it's loudest while resting. As I pivot the arm closer to the center of the platter, the hum lessens, almost as if it's on a fader. Will try new styles this eve and see. Thank you all for the advice!
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