Help Need to combine L and R feeds for mono amp

I need to combine the L and R channel signals from a stereo CD player, and other stereo sources as well, to feed a mono amp. Can you tell me how to build a bridge device to accomplish this - maybe send me a simple schematic?

All help appreciated.
Cccc

This is for a vintage setup with HH Scott preamp/amp, Klipsch Corner Horn, Thorens TT, etc.