MXR Blue Box : Q2 & Q3 ?

[ricothetroll]

Hi !
Looking again at the schematic of this great pedal, I was wondering about the role of those transistors : how are those polarised ? It looks like it is the positive part of the input signal (rectified by D2/D3) that does it, but why did they do such a thing ? That's confusing...
http://www.tonepad.com/getFile.asp?id=73
Best regards.
Eric

[Mark Hammer]

Q2 and Q3 serve as gates.  If the input signal (as detected by the rectifier circuit, partly formed via the diodes) is strong enough to generate triggering of the flip-flop (and a stable sub-octave), then you are "allowed" to hear the output.  If the input signal amplitude falls low enough, then Q2/Q3 don't turn on.

R15 and C8 provided a reasonably fast decay of the rectified signal, so that you get clean gating with minimum sputter.  Keep in mind that there are TWO potential sources of "sputter" in the circuit: the flip-flop itself, and the rectifier/gate combo.  In principle, you can play with the time constants of the rectified and gating action via R15/C8.

You will note that the values of R8/R5 in the first op-amp stage are to bring the signal level high enough to provide a usable rectifier signal output.

[ricothetroll]

Thanx a lot Mark !
Best regards.
Eric