smooth overdrive response curve (with graph!) feedback opamp.

[Steben]

After remembering Jack's work on diode clipping theory...
I calculated a response curve for my new take on amp-like distortion. Think of a TS non-inverting opamp stage. The gain pot is doubled (dual gang pot). The first is normal gain pot, the other is in series with a couple of zener diodes. Beyond the zeners, the preset gain is hence divided by two. Parallel to this second pot are anti-parallel silicon diodes. Beyond silicon+zener gain is unity.

The key factor is the series resistor(gain B) in the clipping that follows the gain resistor (gain A). Many other circuits use a preset resistor for this, yet do not achieve a smooth variable curve. By a explicit choice of zener voltage/silicon, the curve is very smoothed out.

-ZENER diodes of 3.6V (giving 4.25V thresholds)
-Silicon diodes of 0.65V



Feedback line:
          |
-------------
|        |        |
|        |        |
|       | |       |
--      | |      Si
Si    pot     --
|        |        |
|        |        |
--------------
          |
         --
        3,6v
          |
        3,6v
         --
          |

SHOULD BE USED with +18V supply (2 batteries) !

[Steben]

This is becoming a nice project. Fetzesr in front of distinctive frequency sensitive Opamp clipper...Schematic and YEs... PCB design are running down the factory soon!...

[Processaurus]

Yes, the zener diodes give very soft clipping, I'm suprised more people don't experiment with them.

[Steben]

As promised:
Note the complex structure of the feedback loop. treble bleed cap on drive pot and treble cut cap in clipping (bass goes through more). This comes closer to the treble cut in overdriven tube amps.

http://www.diystompboxes.com/smfforum/index.php?topic=55352.0