Audio Systems: HUMMING IN SPEAKER, mixer gemini, input cables

Question
I ONLY GET A HUMMING SOUND WHEN I HOOK UP MY SOUNDCARD WHICH IS  MAYY44 USB USN VIRUTAL DJ SOFTWARE THE HUMMING COMES FROM ONLY CHANNEL 1 CHANNEL 2 IS FINE AND ALSO I CAN HERE IT IN THE EARPHONES. I HAVE 2 SOUNDTECH TURNTABLES,NUMARK  DM1050 MIXER,GEMINI GXA-750 POWER AMP,AND A GLI EQ PLEASE LET ME KNOW WHAT I CAN DO THANKS

Answer
Constant hum in one channel only is usually due to a bad ground, namely an open ground.  It can be a power supply if the equipment has a dual supply, but in your setup I don't think that is part of the problem.

So, you must find out where the hum is getting into the system.  First, reverse the phono input cables from the TT. If the hum reverses channels it tells you the hum is coming from the TT area or cables or ground.

If the hum goes away when you connect to the pc then tie the grounds together between the TT or preamp/mixer and pc chassis.

You may have to experiment to find out where the disconnect is.

If the hum level goes up and down with the level controls you know the hum is in-front of the level control.

The approach to use for quickest results when chasing hum is to determine if it is an "external" cause or "internal".   By external we mean outside the equipment, such as a grounding or wiring problem.  By internal we mean the hum is coming from within one of the electronic boxes.  Each box can be checked out individually by disconnecting it from the system to determine that no hum is present at the output....  this concept of system checking will lead you to focus on the source of the hum quickly.

Hope you find it and that it turns out, as usual, to be something easy to recover from.

Let me know if you need further suggestions.
C