Adaptive limiter clamp circuit for line-level signals driving power amplifiers (Cordell 2011, taken as-is except the R4-D1 polarity is reversed). Clipping level tracks the rail voltage of a power amplifier via a smoothing filter R1C1R2C2. Ideally, the diodes should limit the signal just short of the point when the power stage goes into hard clipping, at any conceivable rail voltage. The forward drop across the diode is fairly large compared to the line-level signal, so the soft-distortion area is soft and wide.
<a href="//commons.wikimedia.org/w/index.php?title=User:Retired_electrician&amp;action=edit&amp;redlink=1" class="new" title="User:Retired electrician (page does not exist)">user:Retired electrician</a> (this drawing), Bod Cordell (the design)
Cordell, Bob (2011). Designing Audio Power Amplifiers. McGraw-Hill. <a href="//commons.wikimedia.org/wiki/Special:BookSources/9780071640251" class="internal mw-magiclink-isbn">ISBN 9780071640251</a> - chapter 17.5