Sound Bar Samsung harman/kardon HW-N950.
TV QA65Q6FNAWXXY.
Pre Amp Selby RIAA-EQ.
Turntable no brand from Target.
Using cable with RCA to HDMI.
What exact cable? RCA to HDMI would need some sort of an active electronic converter. If you don't want to mess with adapters, converters, etc... read the last paragraph.
That preamp is also run on battery only it seems, did you check the battery to make sure it's OK? Would be a good idea to test the setup through the pre-amp to some normal speakers/receiver to make sure that side is all working before trouble-shooting past that link.
That soundbar has an optical input, an RCA to optical adapter may be better to use here than HDMI. Something like this, although I have not used any...