I mean on record one of the channels L or R is inverted (I forget which) before it’s cut. The phono pre-amp will re-inver it as part of its function. When you

I mean on record one of the channels L or R is inverted (I forget which) before it’s cut. The phono pre-amp will re-inver it as part of its function. When you combine L+R after preamp to make mono the feedback signal from one channel will be combined with inverted feedback signal from the other channel and cancel out. Try it and be amazed. Audio engineers have known this trick since the dawn of disco.