I am using pitch analyzer app to write down notes of singers voice in a song but background music (instruments) makes it difficult. I am sure there is some Eq, filters setting that can make only vocal frequencies loud making rest as silence as possible. Sound quality is not important, but what matters is only pitch of a singers voice.
Todd is right about the frequency overlap in his first OP reply comment, meaning simple EQing may not get you the results you require.
However, something else to note is that the main vocals are commonly centered, spacially. So extracting this 'centre channel' will remove the unwanted side channels from the stereo file. The bass is also commonly centred, so will remain, but lower frequencies will be easier to control on the mono output signal that is returned from this process.
You should Search this site (SSD) for 'Remove vocals'(very similar process) and 'Extract vocals' to get some other ideas. This subject is like a holy grail for audio professionals. It has widespread implications, so it is a common question on this site.