Typically (actual physical) preamps are necessary for LP turntables, not for tape decks. If you have one of the old style amplifiers into which you plug everything, the amplifier has LP inputs which take care of the pre-amp for you, you can take your output off of the headphone jack and adjust it for balance and gain with the amplifier controls. This of course is before you do any computer modifications to the signal, which should be fed into the 'line-in' port of the soundcard. Also make sure your patch cord is 'stereo' as those mini-jacks are often mono.

As artbrooks said above, a feed directly from a tape deck to the 'line-in' should work. Listen to the music off the computer speakers.