Macbook pro 15 inch 2019 - stereo line in problem when trying to rip vinyl.

I have an macbook pro 15 inch 2019, and am trying to covert some old music using Audacity. I'm using a line out from my amp, but can't seem to find a converter that will convert the stereo RCA out from the amp, to USB-C stereo line in to Audacity. I've tried a phono to 3.5mm jack cable, with a jack to usb-c converter, but it creates a mono signal. All phono to usb-c cables seem to only work with he input on the usb-c end (for plating music from phones etc) Please help!