opamp would fall to 1 (buffer), and thus tension would certainly fall under their
threshold (+/-2V) bringing back the GAIN pot in action. So to simplify, what will
dynamically happen is that the signal will never raise above 2V or fell under -2V.
That's smooth clipping.
Note that if one of them leds is suppressed the output will be able to rise until it
reaches the opamp voltage rail value (if there's enough gain). This will provide
asymmetrical clipping, as one side of the signal will stop at 2V and the other at -9V.
When connecting the clippers at the output, we have the following schematic:
This one as a much simplifier explanation: the opamp will amplify the signal up to
it's voltage rail value +/-9v. Then the leds will shortcut the signal to ground when
the signal is higher than their threshold of +/-2V.
In this clipping schematic, the signal is clipped twice: first with opamp and then
with the leds. This provides a much more aggressive and modern harmonic content
and thus is called hard mode.
Switch SW3 selects symmetrical and asymmetrical mode. Note that in case the
mosfets transistors are used, SW3 will just remove any clipper from the gain stage,
making it a booster stage.
Clippers choices are made according to switch SW1:
–
Mosfet clipping, with 2x 2N7000. Threshold is around 1,8v
–
4 x 1N4148 clipping. Threshold is around 1,4v.
–
2 x Green leds clipping. Threshold is around 2v.
–
3 x 1N34 germanium diode clipping. With 1v and 2v thresholds.
All these clippers have different properties and frequencies responses which will
provide different overdrives tones. For some reason, Germanium diode clipping in
the feedback loop don't work very well...
One last thing: if a TL072 is used for opamp, it's gain/bandwidth value 3MHz. So
with x500 gain it would cut at 6kHz. This would be added to the filtering done by
C8. Some would think it's ok. I don't. I prefer to use OPA2604 with 20MHz
gain/bandwidth which will cut at 40kHz. It gives a more bite, more aggressive
harmonic content when overdriven hard and better attack.
Copyright Zorg Effects G.Denneulin december 2015 – For non commercial use only.
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