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Hum from Grado on LP12.. Better carts to consider?

I've had a problem with hum on my LP12 for as long as I can remember. The table (bought used) came with a Grado Signature 8MZ and I've since tried a Grado Z+ that I had from another table. Both have the same amount of hum. I have a lot of RF and noisy AC in my house. The hum is noticeable when I switch to phono and gets louder as I move the cart over the rim of the platter. From there it decreases as I move the arm towards the spindle. (The table is 80's vintage with Ittok and Circus/Lingo upgrades).

A (non-Linn) dealer said 'oh Grados do that on LP12s - lack of shielding'. My Linn dealer didn't nave a strong opinion. He suggested a Benz Ace H given my under- $700 budget. From reading the Agon forums, some think an AT150 M series has better sound for less $. I've also heard good things about the Dynavector DV-20X. Does MM/MI vs. HOMC matter in terms of hum or is the degree of 'shielding' a variable from cartridge to cartridge no matter which type? Does a good review on a MC necessarily extend to the HO version, as opposed to the LO/MO versions?

My system: ARC PH1 (48 dB gain, variable loading via soldering iron)
ARC LS2BII
PSE Studio IV/V's
Audio Physic Tempo III's
FWIW: Meridian 508 CD player is dead quiet in this system.

Music: from Joni Mitchell to Ian Dury and the Blockheads, Monk to Mozart.

Thanks for any knowledge you can impart!
geor
geor OP
37 posts
 
Thanks Atmasphere. I'm pretty sure there is a ground wire going from the arm base to the bearing sub-chassis. The motor is attached directly to the top stainless steel suspension plate. Is there a missing connection between the top plate and sub-chassis? I thought the springs made this connection, but there are rubber washers on the springs, so I think your idea makes sense. One would think that Linn would have considered this in their design of the LP12.
snfrosten's avatar
snfrosten
3 posts
 
Its not a grounding problem. The motor is not shielded and most MI will hum a bit. The only way to remove this hum is to shield the motor with some type of material like Mu-metal or add a shied on the platter, between the mat and platter, to shield the cartridge from the magnetic interference from the motor. The Grados are excellent sounding cartridges and worth the effort to fix this problem with the table.
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rauliruegas
12,781 posts
 
Dear Geor: I agree with Snfrosten about the Grado cartridges performance.

Regards and enjoy the music,
Raul.
atmasphere's avatar
atmasphere
11,575 posts
 
Geor, they may have springs but that cannot be considered an adequate ground. The motor must have a wire that does the job, so plinth, motor, platter bearing and arm have the same ground.

The motor may not be shielded, but if you ground it that may not be important! Certainly an ungrounded, unshielded motor *will* be a problem for an unshielded moving magnet cartridge!! So get it grounded right and let's see how it works.
geor OP
37 posts
 
Thanks everyone. Does anyone know how to attach a ground wire to a Linn motor? I'll ask my Linn 'mechanic' but I can already see the look on his face.
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