buying a amp and dac

[physiongaming]

i don't know how the dac and amp thing works do i need a dac and amp or just a dac or just an amp and is it really worth it i have 70 dollars left for a dac/amp or just an amp or a dac this is my motherboard https://www.msi.com/Motherboard/Z170A-GAMING-PRO.html#hero-overview and the headphones im going to be using are the sennheiser hd598

 

[physiongaming]

and there is almost no noise from my internal dac (onboard sound) from my motherboard