Some Bluebox tweaks

[Mark Hammer]

Once again, digging into the bin-of-boards for unboxed circuit boards to finish up and install into enclosures.  Oner of today'a was a Tonepad Caja Azul Bluebox clone.

While folks like the tone, the tracking is a constant source of grumbling from people.  Apart from installing a toggle to select between 1 or 2 octaves down, I did a few mods to improve tracking a bit.

First, I replaced the silicon diode going to ground in the envelope follower portion with a Schottky diode.  I've replaced diodes with similar functions in other pedals recently, and been pleased with the improved reduction in envelope ripple.  That diode serves to shunt the half wave you don't want to ground.  The 500-600mv forward voltage of the silicon diode means that up to a half volt of the unwanted half-wave still gets preserved.  The 190-240mv Vf of the Schottky reduces that further.  True, what's left still has to meet the threshold of the series Si diode, but this helps a little.  Because the Bluebox doesn't do much to smooth out the envelope, I thought this might be even more useful to intervene with.  So far, so good.

The other thing I did was replace the 1k ground-leg resistor in the first op-amp stage with a 5k pot in series with an 820R fixed resistor, to provide variable gain in that first op-amp stage, and better match it to the properties of the input signal.  The stock unit has a fixed gain in that stage of 471x, which may or may not be optimal for every input.  This gives a range of gain from about 82x to just under 575x, which oughta do ya.  The caveat to consider is that, since the amount of gain in that stage also dictates the level of the non-octave signal, you might need to adjust the Balance control a bit as you tweak the sensitivity.