Tonearm Hum

2/10/2016
D Vautier

This documents my torturous efforts to reduce noise and hum from my system.  I was successful.  A more general discussion of hum can be found at this location  


When I was measuring for hum I had to find a record that was hum free or almost hum free.  Some records do have hum.  I have noticed that there can be hum to different degrees in any record which may have resulted from the recording or cutting process.  I did finally select a pretty good older record ie. (America Hearts BS2852 Warner Brothers 1975, side two between track one and two) that sounded clean. I wanted to use an actual record, not just a test record.


General

I attempted to isolate and correct a slight hum that started in my system several months ago.  I suspected EMI (electromagnetic interference). It is everywhere and it comes in more and more shapes and sizes these days; cell phones, microwaves, bluetooth, security systems, wi-fi, and may effect audio components.  The most sensative area for EMI is the weak signal from cartridge to preamp and it seems that one generator of EMI could be the turntable motor itself.

I have two turntables to test with, two preamps and a number of cartridges so by switching these around I was somewhat able to find out where my personal hum was coming from.

Here is a typical wave chart.  A represents a signal.  B is where I lifted the stylus to show hum, later found to be the head shell.  The hum doesn't start right away but is delayed, later determined to be some kind of feedback form the sound card. At point C I replaced stylus at record lead-in.  At D there is soft music.  F begins regular music.  Hum can be tolerated I suppose at higher signal but it does contaminate the signal.  The above hum was totally unacceptable to me, and did not seem to be coming from the usual suspects.

Below is the wave pattern of soft music, raised stylus, music, raised stylus and turn off.  there is hum even at turn off and disconnect. for this test I used an older shure M97E, which is noisy anyway so not much to say here.

Observation

 

I switched out the turntables.  My Sony has slight rumble (-70db) but the Technics has less rumble advertised at (-78db).  I still had the hum.  I then began changing cartridges and pre-amps, all to no avail.  I replaced and soldered new lead-in cables.  No improvement. When I disconnected the TT from the pre-amp there was no hum, just white noise at full volume way below signal. I tested the lead-in by connecting it directly to a cartridge and there was no hum at all.

 

No Power at all

I also ran both turntables with no power whatsoever by just turning the platter with my finger.  I still recorded a hum, small but nonetheless present and audiable.  No electrical motor was anywhere near the turntables so I think that turning the platter by hand may have generated EMI from the hysteresis synchronous motor but then again it was probably the noisy cartridge.

 

This is the wave pattern generated with no power to the turntable.

Just the cable

I decided to connect just the lead-in cables directly to a cartridge from the preamp. I was able to get  excellent wave pattern with very little hum.  At this time it still did not point to a noisy sound card.

 

 

 

So How did I fix this Mess