I fertilize my berries with a fish emulsion and they have flourished! If you are going to reach for a conventional fertilizer - choose one made for tomatoes or one with a higher percentage of K (the last number). This will allow your plant to make more flowers. And more flowers mean more berries!
Nitrogen is great for keeping it green and Phosphorous helps it develop roots and spread new suckers. It doesn't take a lot for raspberries to do both of these. But you DO want a high percentage of flowers (aka berries.)
After this initial feed, you shouldn't have to feed them again until next spring. I occasionally give them a little fish when I'm feeding other plants (usually toward the end of the first fruiting - to gear up for the second harvest.)
Either way, if you are itching to get out there and garden, you may be able to get your fix in the berry patch. Feed them, cut out any dead or broken stems, then wait.....