Any Advice About Eliminating Induced Hum in Turntable?

hirscwi

Member
Paid Member
2014-03-06 2:18 am
Indianapolis, IN, USA
I have a modified VPI HW19 turntable with two arms. The first is in the normal spot on the right and it is quiet. The second arm is mounted in the left rear corner of the table - within 2 inches of the motor. I'm getting a hum only when the motor is running. If I remove the arm from the table, the hum is dependent upon how close the arm is to the motor. At about 9 inches away from the motor the hum is gone. No combination of grounding connections has any impact on this hum.

Are there any recommendations to eliminate this hum? They will be most appreciated.
 
Your only defence is mu-metal screening around the cartridge (weight an issue) and around the motor (more practical).

This is magnetic induction, not capacitive coupling - mag field falls off as the inverse-cube of distance so usually distance is the cheapest way to reduce it.

Mu metal is a pain to work with as it needs re-annealing after any deformation to restore the high mu. Adding a flat layer to the turntable frame or even platter might be simplest, but isn't complete screening.

You might be able to substitute a more shielded substitude motor - you'd need to know it was significantly less leaky magnetically though.
 

hirscwi

Member
Paid Member
2014-03-06 2:18 am
Indianapolis, IN, USA
Thanks, Mark. This makes perfect sense to me. The hum does appear to reduce exponentially as distance is increased.

I am not terribly familiar with magnetic shielding. Does it need to be enclosed around the motor or is a simple plate over the top of the motor and between motor and arm pillar sufficient? I suppose the answer will be to try it and see. Also can you recommend a source for mu metal? I assume thicker is better than just foil that I've seen on eBay. Does the shiled get connected to the preamp ground?

Your only defence is mu-metal screening around the cartridge (weight an issue) and around the motor (more practical).

This is magnetic induction, not capacitive coupling - mag field falls off as the inverse-cube of distance so usually distance is the cheapest way to reduce it.

Mu metal is a pain to work with as it needs re-annealing after any deformation to restore the high mu. Adding a flat layer to the turntable frame or even platter might be simplest, but isn't complete screening.

You might be able to substitute a more shielded substitude motor - you'd need to know it was significantly less leaky magnetically though.
 
Implement differential 3 wire connection to your stylus with a differential input to the preamp. This ofcourse eliminates common node induction noise. Remember that one leg of the stylus output is grounded at the tone arm out. Simply remove that grounding.

Its a bit of a hassle but worked well for me.
Just used a single dual op amp in unity gain diff mode on matrix board in the input to the preamp and replaced the RCA connectors with the 3 pin type for each channel.
 

hirscwi

Member
Paid Member
2014-03-06 2:18 am
Indianapolis, IN, USA
Problem solved

To close the loop. I believe the problem is solved. I purchase a piece of shielding material and surrounded the motor. I used a product called "GIRON" - which is a woven product that isn't supposed to ne re-annealing after bending. I put a piece around the motor and over the top. This appears to have completely eliminated the problem. Thanks to all for their suggestions.