The mixer/pre-amp should output 500mA line level (It amplifies the gramophone input to that level). Try going from the mixer output to the sound card input.
I've done it thusly:
Turntable connected to amp. Tape rec output into soundcard input via 2 x phono to 3.5mm stereo cable. Can't remember exactly which Audacity setting but fiddle with it's under "devices".