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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
 
After doing much research on Audiogon and the web in general, I am beginning to realize the problem is a ground loop hum. I have a rooftop FM antenna that is grounded to a single rebar spike. This apparently is not a good setup - too much resistance. It should be two copper clad steel rods, 5/8" x 8' long, 8' apart driven below grade, with the two ground rods connected via buried heavy gage stranded copper wire. The loop is formed by the potential difference between the ground of the FM coax (steel rod) and that of the main electrical service panel (water pipe). It seems that I should also add two ground stakes outside the main panel and connect all ground stakes with the heavy buried copper wire.
Off to the electrical supply store and not the needle shop!
atmasphere's avatar
atmasphere
11,936 posts
 
Grados will hum with certain 'tables, like the Rega. Usually this comes from the table not being sufficiently grounded. For example, the Rega needs to have its motor and platter bearing both grounded to the arm ground. I suspect the same is true in this case, based on the fact that if you move the arm, the hum changes. That's not a ground loop!

So this might be easy to fix- have a wire installed that goes from the motor chassis to the platter bearing and then to the base of the arm- make sure that every bit of metal on the 'table is grounded.
stringreen's avatar
stringreen
6,483 posts
 
There is a fix for the Grado hum... Its a grounding issue - I don't remember what the proceedure is, however if you call Grado, they should know.
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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