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):
|R1,R2||10K||Create a 4.5V bias supply for U1|
|R3||4.7K||with Pot 1 and C2 determines the gain and rolloff properties of the clipper|
|R4||510K||Feeds bias current to the input of U1; must be large to prevent loading signal|
|R5||510K||"Pull Down" resistor for output capacitor C5|
|D1, D2||The clipping diodes - their conduction voltages determine where positive and negative clipping occur|
|C1||0.022||Input capacitor; also helps determine the point at which bass input is rolled off|
|C2||0.047||DC blocking capacitor on feedback loop; with R3 helps determine the bass response of the clipping amplifier U1.|
|C3||10||filters the bias voltage created by R1, R2 to prevent noise.|
|C4||47pF||feedback capacitor to cut higher harmonics from the clipped sound|
|C5||0.022||output capcitor, couples the clipped signal to the output pin|
|U1||Dual opamp||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.|