Alright, so I didn't think things would be so complicated connecting headphones to my PC. I have a pair of Sennheiser HD 558 Headphones with 2 rings on the plug I'm trying to use with my ASUS Xonar Phoebus ROG Gaming Soundcard that has this control box thing you connect the headphones to and it accepts a 3 ring plug. When I connected the headphones with the 1/8 adapter I had sound only out of one side. After searching online I thought I needed a different adapter so I ordered one that specified stereo-to-stereo assuming only hearing one channel meant mono. Nope, same thing. Now I've been reading about TS v TRS, balanced v unbalanced, mono v stereo.... yeah it's safe to say I'm confused as to what I need.
I'd agree with the poster above, check that your speaker/headphone balance is set in the middle. Also I'd check that the 'control box' is properly plugged at the PC. Mono sound can often result from cables not being plugged in all the way. Maybe just to be sure you should unplugged and plugged the control box back in to make sure of a firm connection.
As for TRS it's just Tip, Ring, Sleeve it's just the technical name for a jack connector describing the points of contact.
Pretty sure TS is just a phono/RCA connector (like the ones that most composite and component cables use), so it's not relevant here.
Balanced and unbalanced cables are really more for speakers where you're buying separate leads. Your headphone cable should be balanced.
I've tried connecting the headphones directly into the soundcard as well as the control box, firmly, and either way I only hear out of one side. I've also messed around with the software to confirm both sides were balanced as well as making sure the headphoes work on a different device. Still nothing.
After more googling I discover another connector, TRRS. Turns out the other ring is used for mics, which makes sense if this control box thing for gamers caters to headphones/mics. Still, why would using a common TRS headphone plug inserted into the control box or directly into the soundcard's headphone input only produce sound from one side? I plugged my earbuds w/mic, which has what appears to be a TRRS plug, into the control box and can hear from both buds but one side was louder and crackled.
So unless anyone can confirm/deny my new findings then I will probably purchase this adapter to connect to my headphones. I'll report back if I do.