Tone arm Hum Fixing
Some Ideas

Aures habent et non audient
Psalms 114

2/10/2016
D Vautier


Grounding

A cartridge carries two types of grounds.  The first one is the signal ground, or actually two signal grounds for stereo which come from the negative end of the windings on the magnetic coils in the cartridge, the green and blue wires.  These are unique connections that probably should remain isolated from the field ground, the black wire, until it all gets into the pre-amp stage.  

The field ground connects from the shell of a cartridge to the tone arm and the metal body of the preamp.  If you have a plastic head shell there is no connection between the two grounds. The field ground has one purpose, to shield everything from EMI (electrometric interference).  It is not designed to play a part in signal generation, only to keep out noise. The field ground should not be connected to any part of the turntable other than the tone arm.  It should only isolate the four signal and ground wires from cartridge to pre-amp. 

Cartridge manufacturers sometimes tie the grounds together or give the user an option.  Shure has a little tab where you could experiment, but now companies sometimes choose to do the grounding inside the cartridge by tying the right or left signal ground directly to the field ground and the casing is bolted to the arm.

 

 

On my old Technics head shell I noticed that the green wire (right signal ground) was soldered to the field ground.  I removed this ground which eliminated some of my low level hum.

I don't think that there should be any grounding done between the two signal grounds and the field ground.  This should be up to the cartridge manufacturer but some of the earlier head shells did this.

 

The signal produced was less hummy as shown below.  It is probably because I replaced the older head shell with an all plastic one.  

 


Rewiring Tone Arm

This is perhaps an extreme move but it can reduce noise.  There are two major parts to a tone arm, the arm itself that moves up and down and the plinth or base that moves horizontally and rides on bearings.  The arm is held to the plinth by two needle screws that are secured on each side with two locking nuts or with rubber bushings.  to replace the wires you just about have to back off the pin bearings and slide the tube out.

Your basic material are a replacement set of wires, solder and gun, a small screw driver and spanner wrench.  Additional materials may be tape and feeder wire.  It is always good to braid or plat the wire into 5 strands if you can if you go to all this trouble.

The spanner wrench can be built by using an old screw driver and a grinding blade to make a gap in the center so it looks like a fork.  The set screws come loose easily with a spanning wrench and do not need to be very tight.  Take pictures of the position of the set screws before disassembling the tone arm.  The plinth does not have to be removed, but the arm does because it has locking bolts underneath that remove the little spring loaded contacts.  It's smart to de-solder the wires at the breakout board so you can just braid the wires and replace if you want to.

Braiding

I think it is always a good idea to braid tone arm wires but not too tightly.  There are five wires and the braiding process is pretty simple for odd numbered strands.  De-solder the wires at the  breakout and remove from the tone arm.  Braid and reinstall.  To braid, go two on one side and three on other side.  leave lots of extra wire in front of the breakout board.

 

 

Covering Head Shell

I like to shield any exposed signal wire.  That especially goes for the cartridge head which can pick up EMI.  I took some aluminum tape and wrapped the head.  It increases the weight but can usually result in noise reduction.  Here I have an ADC stylus in a standard head shell.

 

 

 


Fade

One thing I did notice was fading, that's what I call it anyway.  Notice below that a strong signal is recording and then the stylus is lifted.  The immediate result is good quiet but then a gradual 60 cycle hum appears after one second.  The wav file is here.  This was my first indication I was getting feedback.


This had to lead me to suspect a noisy sound card which was introducing a slight 60 cycle spike so I replaced it. The hum was reduced noticeably.

I did further tests on my pro-ject.