Thu Dec 9 16:05:04 EST 2010

Distortion : Duty Cycle

From [1]: "... if you filter properly, soft clip, and arrange for the
duty cycle of the clipped signal to vary with varying signal levels,
you get something very close to tube distortion."  (John Murphy)

Looking at the pictures, with a sine input, the varying duty cycle
requires a-symmetrical clipping _and_ DC bias before entering the
clipping network.

The DC bias could be fed back from the average of the output,
i.e. asymmetrical clipping of an AC-only sine wave leads to DC output,
but adding proper DC bias to the input can make the output of the
clipper DC free.

Schematic: a typical inverting opamp with 1+2 asummetrical diode
clipping, parallel gain resistor and possibly tone cap.  Filter this
output to get the DC component, i.e. f_c = 10 Hz, and feed this DC
signal back into the circuit.