Circuits for Soft Clipping
Soft clipping (also known as overdrive) is achieved either by silicon diodes in the feedback path of an op amp (variation 1) or by using Germanium diodes as shown in circuit 2.
Instead of Germanium diodes some people use LEDs.
Assymetric clipping is produced by a variation of circuit 1 with more diodes in one direction than in the other. Sometimes the additional diodes are Germanium types.