Speaker Buzz on Mixer

[_JohnB330ci]

Hello there, recently I obtained a Mixer, to be precise the Behringer Xenyx Q1002USB. On this mixer I have connected up my PC Audio, Tablet, Guitar (Through an amp) and a 2Track input. My outputs are through Phones (To my speaker) and Main out (To my headphones) When using my headphones, If I zero the output on my main mix, there is no background hum, however on the speaker there is, I have switched the headphones to Phones and Speaker to main out and it makes no difference there is still a background hum on the speaker. I believe people would suspect it to be a grounding issue, however I would've thought my headphones would have also have the hum? The speaker is going out using a 1/4 Inch to 3.5mm adapter, my headphones are the same, but L/R 1/4 inch to 3.5mm adapter. My 2Track comes in using RCA, and the rest come in using L/R 1/4 Jacks. To output to my speaker, I increase the FX to send a particular channel to the Phones/Control Room, then increase that main knob (Not the main mix) to increase and decrease the audio on my speaker, there still remains no hum on my headphones. I don't overpower any sound go into or out of the mixer (Making it go between 0 - +15) as I know this can make a hum, but I never do this. Does anyone have any suggestions on a fix?

[_JohnB330ci]

Bump.