Adaptive limiter clamp circuit for high-level signals driving power amplifiers - adapted from Cordell 2011 (the original was for line-level signals). The idea is to limit the signal at the input terminal of the output stage, so that the latter never clips. The amp's output stage has a gain of 3, and with rail supply of +/-40V (at idle) it normally clips at exactly 25V RMS into 8 Ohms, or +/- 35V peak (at which point the rails sag to +/-38 V RMS). Thus, the input voltage fed into the output stage must not exceed +/- 11.5 V peak with aforementioned rail voltages; the limit varies with mains voltage. Here, the voltage divider LED1-LED2-R1-R2-R3-R4 tracks the rail voltage; the LEDs account for fixed dead voltage loss in the power output stage (around 3..4 V)<br><ul><li>Since the limit increased tenfold, the number of diodes in each leg of the limiter increased to three (two diodes plus base-emitter junction). Schottky diodes are preferred over basic 1N400x types due to faster recovery</li>
<li>There is no need to wrap the diodes or transistor in feedback loop (which was essential for line-level signal).</li>
<li>The purpose of emitter followers is to divert the clamping currents away from the opamps - so that the latter don't introduce their own ringing into the audio chain.</li>
<li>Time constant in the rail voltage divider match the timing of the Cordell circuit. Two-stage filtering may be excessive; I did not test other configurations.</li>
<li>Any general-purpose, fully compensated, preferably low-current-consumption opamp would do. I used what I had in the bin.</li></ul>
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Own based on Cordell, Bob (2011). Designing Audio Power Amplifiers. McGraw-Hill. <a href="//commons.wikimedia.org/wiki/Special:BookSources/9780071640251" title="Special:BookSources/9780071640251">ISBN 9780071640251</a> - chapter 17.5