Copyright 2000 R.G. Keen. All rights reserved.
Module 5 is a feedback diode clipper - that is, it does distortion by putting diodes around the feedback path of an opamp. This is very similar to the clipping stage of Ibanez' Tube Screamer series. For a detailed discussion of the workings of a feedback diode clipper, see "The Technology of the Tube Screamer".
Here's the schematic and layout:
Here's the parts list and what each part does (Note - caps in uF unless noted; pots are audio taper unless noted):
|Create a 4.5V bias supply for U1
|with Pot 1 and C2 determines the gain and rolloff properties of the clipper
|Feeds bias current to the input of U1; must be large to prevent loading signal
|"Pull Down" resistor for output capacitor C5
|The clipping diodes - their conduction voltages determine where positive and negative clipping occur
|Input capacitor; also helps determine the point at which bass input is rolled off
|DC blocking capacitor on feedback loop; with R3 helps determine the bass response of the clipping amplifier U1.
|filters the bias voltage created by R1, R2 to prevent noise.
|feedback capacitor to cut higher harmonics from the clipped sound
|output capcitor, couples the clipped signal to the output pin
|JRC4558 is highly thought of, but LM833, TLC2202, and others work well too. Only one section of the dual opamp is used; this is because duals are almost as cheap as singles, and easier to lay out.