Is this normal Blue Box Behavour?

[jpm83]

I just finished making a MXR Blue Box for my friend. Is this normal that when you play a note effect on it gates the note off when the sound starts fading away? I can sustain a note when using a vibrato, but when the signal level from guitar drops below certain point the sound gets gated off.

Janne

[captntasty]

It is normal for there to be a gating effect.  I've built 3 and all exhibit this.  If it gates off immediately then something is wrong and needs adjustment.  You might post some voltages so others can get an idea of whether or not it is functioning correctly...

[jpm83]

Thanks for answering. It doesn't gate immediately it just cuts the very end of the sustain of guitar, so I would think it works as good as it can. BTW the sound of this thing is wicked.

Janne

[Mark Hammer]

In fact, the gating behaviour is intentional and a fundamental part of how the Blue Box is supposed to behave.  If you look at the Tonepad layout for the Blue Box, you'll see R13 and R14 on the collectors of the two transistors Q2 and Q3.  Those two resistors are tied to D2, which rectifies the input signal and provides a positive-going voltage that shuts the transistors off when the signal dips below a certain level.  The 1uf cap and 56k resistor that are also tied to diode D2 and R13/R14 provide a wee bit of lag so that the "control signal" from D2 takes a couple of msec to totally fade out, providing a not-totally abrupt shut-off.  If you want to be able to adjust the shut-off speed to taste, consider replacing that 56k resistor with a 250k-500k variable resistance (pot) and 33k fixed resistor in series.  This will let you achieve different decay times.  Alternatively, you can stick a 3-position toggle in there to select between stock (56k) and two longer decay times (say, an additional 100k and 220k in series with the 56k).

You don't want the decay time of the gating to be too slow because then you'll start to hear the sputtering that is characteristic of a divider circuit that is receiving an input signal that randomly falls just above and just below the critical threshold for detection.  That sputtering is the very reason for the gating circuit.  So, while you can theoretically extend the decay time of the gate, you may find that it produces undesirable side-effects.  Let your ears decide if longer decay is a blessing or curse.

[jpm83]

Thanks Mark. I had to print your answer immediately since there was so much valuable info there. I think I will give that pedal for my friend to test and let him decide can he live with that decay and if he can't I will mod the pedal to more of his liking.

Janne