Sun Jun 8 17:44:00 EDT 2014

4 pole single ended diode filter on breadboard

Tweaking the resistor values a bit, it seems to do what it's supposed
to.  Verified that a large cap in the feedback circuit is a problem
when stepping the voltage: hits saturation and sound drops for a bit.
Making that cap smaller transforms the drop into an audible click.

Also, there's a lot of bleedthrough from the digital communication to
the analog path.  Maybe best to start separating the analog and
digital supplies and ground.

So, 2 problems:
- digital bleedthrough
- CV bleedthrough due to bias voltage change (diff amp?)

Playing a bit more, it seems that sending a hotter signal is probably
a good idea for this kind of filter - s.t. normal out is just a little
under self-osc amplitude.  Since it's quite noisy and has a lot of
bleedthrough, let's go for "character" instead :)

Some more remarks:

- I'm actually not using a cap from SAW->Filter.  Since the bias is
  constant, this is OK.

- The pop might be mostly due to the coupling cap still used in the
  feedback path.  I wonder if it's ok to leave that one out as well,
  i.e. allow DC to come in but make sure it's much less than the set
  DC.  This would then leave only a single cap in the circuit to set
  the HP pole.

  Basically, if the current is low, the bias voltage at the bottom of
  the ladder is low.  This gets fed back to the top of the ladder
  bringing the voltage down a bit.

  It seems that as long as the gain through the loop is small enough,
  there should be a set point that is stable.  Is there a way to
  completely cancel the voltage drop at the collector?  No, but it's
  possible to spread it out evenly by setting inverting gain to 1/2.

  Trouble seems to be that there is very little voltage headroom to
  add more than 5x gain at that point.  Distortion happens before loop
  gain goes to 1.

- What about placing a cap on the CV?  Making it slower should make it
  easier for the cap inside the feedback loop to follow.

So basically it works when the signal level in the feedback loop is
kept in check.  However, this requires large coupling caps and those
cause bleedthrough.  So all-in-all this circuit is not good for sharp
CV transients.  Might be good for other things though.