FXBus 1 Module 4 - a feedback diode clipper 

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):

Part Value Notes
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.