Pre-workout supplements?

Had RNY done last year, and got into serious lifting/cardio to finish up this weight loss and was looking into some pre-workout supplements.  I've been using C4 so far without any side effects, but since with RNY I already feel the effects within 10 minutes. Anyone with any experience on the matter? And yes, I have tried carbing up and there wasn't really much of a difference performance wise.