Just put together my first setup. Im running a vintage 1970s Pioneer PL-12D turntable. through an inexpensive phono pre-amp, into a Sony Amp/Reciever into speakers.

When playing back stereo records (from 1970s and records of modern bands as well) im getting playback through both speakers however only one channel is coming through.

Taking for example the Beatles album "Rubber Soul" - track "Girl", both speakers are only playing one of two channels and the vocals are barely audible with the instrumentation clearly in the forefront. One of but many examples.

Where should I begin in my troubleshooting?