Electrical – Remove mains hum out of amplifier

Co-ax and twisted pairs are usually good at removing common mode noise.

Ground loops in coax installations can be a problem as they cause unbalanced noise. This becomes more of a problem if the co-ax shield carries any stray or intended 50/60Hz current.

Make sure signal co-ax screen is not used or involved in current paths that include conductors other than internal core. Running the Co-ax parallel with unbalanced high current 50/60Hz conductors could ealisy cause stray currents to flow in the screen if it is grounded at both ends. Any current on the screen that is not on the core will show up as noise.

Also not that with co-ax capacitive pickup should be minimal if the sensor is shielded. Electromagnetic pickup should be cancelled out except at the terminal points.

You should make sure the grounding at the aquisition end is as good as possible and the co-ax screen is isolated from ground at the sensor end. Grounding the sensor housing locally is usually still a good idea (unless it is small and can be left to float with at co-ax screen potential) but having the co-ax shield grounded to two unequal grounds is always a bad idea.

The toroidal type mains transformers do not have a lot of magnetic leakage.

The hum you hear is more likely at 100Hz.

The hum that remains with no input connected can be reduced by increasing the filter capacitor but adding a proper voltage regulator (linear or switched mode) would help more.

The hum when CD player connected is probably due to differential ground noise from the two separate mains supply points, sometimes if you can use a common power-supply ground you can reduce it sometimes this is NOT possible (such as when analogue and power grounds are not tied together in the devices).