Overdrive pedals were invented in order to allow the user to simulate an overdriven amplifier without the need to crank the amplifier into natural tube distortion. These pedals can be classified into families.
- Soft Clipping
- FET Clipping
- Simulator Circuit
In this circuit diodes, leds, or mosfets are used in a feedback loop between the input and output of an opamp in order to use the threshold of the clipping diode to control its own gain. Somewhat like a compressor but much more aggressive.
This circuit is quit similar to the soft clipping circuit except that it uses the clipper at the ouput of an opamp and shunting the output to the 4.5v rail (VR on some schematics).
In this particular type of circuit soft clipping and hard clipping are used to either create a high gain pedal circuit or to create a more versatile pedal.
In this circuit instead of op-amps, FETS or Mosfets are are over-driven to produce these desirable tones.
Amplifiers can be simulated by using an FET per tube clone with favorable results. These are very similar to FET Circuits except that it is a copy of an amp in a pedal and there is no generalization or ambiguity on the circuit design. Most times a cab simulator is included in this design.