Sun Dec 12 21:06:33 EST 2010

Breadboarding duty cycle circuit (a-sym clip + DC feedback)

This works as expected.  Some notes:

  - Using 45x gain (100k / 2k2) to drive the diodes.

  - Used a 1N4148 // red led 1.87V.  The led seems to have quite a
    large capacitance as it made the circuit less noisy.  I tried with
    other led (a bit noisier) and 2 x 1N4148 (quite noisy).

  - Using 1M + 100n for DC, with 10k feedback (next to 2k2 guitar
    input).  Using smaller feedback resistor gives more noise.
    Changing the 1M to 1k increases bass attenuation as expected.
    How to explain the value of the 10k feedback resistor?

  - Used a 4k7 pullup for the LM358N.  At higher gain earlier I did
    see some severe crossover distortion which disappeared after
    adding a small enoug pullup resistor.

To try / to do:

  - Make some EQs.

  - Figure out ways to get a smaller duty cycle.

  - Change the bias such that there is more headroom for the larger

EDIT: changing the DC feedback resistor to 2k2 lowers the error
signal.  It probably also has some effect on the bandwidth, as some
lower AC frequencies get fed in (RC network behaves as integrator at
audio frequencies).  Maybe the best thing is to make the resistor
variable?  Let's try again when the pots arrive.