Sometimes when listening to classic recordings from the 78 age, a little surface noise just adds to the authenticity of the experience. I have Benny Goodman recordings that were made from "Air Checks" of his AM radio broadcasts. These are some of his very best performances, that would have been lost forever if these checks were not done, and the audio quality is OK, AM radio and all.
As Dopogue suggests, the stereo pickup Left and Right signals should be shorted together right at the pickup pins, or at the preamp input. If you wait until the two signals have been amplified and equalized (RIAA) before mixing them, slight inequalities of gains and RIAA equalization will result in some sensitivity to vertical groove modulation, which, of course, is pure noise and/or mistracking.
As Dopogue suggests, the stereo pickup Left and Right signals should be shorted together right at the pickup pins, or at the preamp input. If you wait until the two signals have been amplified and equalized (RIAA) before mixing them, slight inequalities of gains and RIAA equalization will result in some sensitivity to vertical groove modulation, which, of course, is pure noise and/or mistracking.