Headphone amp humming because of cheap cables?

This is hopefully something that can be cleared up real quick...

I just finished my diy headphone amp and hooked it up with a cheap cable I found laying around, 3,5mm (1/8"?) -> 2xRCA. The problem being I'm hearing a humming sound when I turn up the volume and turn off the music. It's probably there when I play music as well but I can't hear it (only audible at louder volumes with music muted).

So I'm wondering if the humming comes from the unshielded cable I'm using and if I should go buy a better cable or if it's a grounding loop I'm hearing and what can be done in this case.

The amp consists of a power supply and an amplification section. Basic power supply providing about 15V to Burr-Brown OPA2134 along with an alps pot.

Questions and advice are welcome!

I don't know much about this subject, but the thing is, I get the same amount of humming from any device using the cable. So either it's the cable or it's my heaphone amp. I'm gonna try hooking it up with another connector as well as switching power outlet. Don't know if it'll help, but it won't hurt.

Edit: No success. I switched source, cable (2xRCA-2xRCA, another one just as crappy, came with some electronics equipment I bet), power cord and power outlet. It's a grounding issue inside my amp? Should I be posting this in the DIY forum or somewhere else?

The humming isn't very bad at all, it's barely audible at around 50-60%, which is quite loud anyway and goes louder as I raise the volume.

With/Without power cord and power turned off it's silent.

With power cord connected and power turned on I get a very slight humm.

With RCA connected without power cord connected it's silent.

With power on and one RCA connected the humming is quite loud

With power on and both RCA connected I get a humm.

All at max volume.