Tonepad's blueBox build report!

[keko]

Hello world!
I hope what has already passed of this new 2004 year has gone smooth and OK for everybody.

About a week ago I built the - to my opinion - underestimated MXR Blue Box....I have to admit I never looked into it either.

The pedal is amazing...again, a fabulous layout by Francisco and/or Matt. Great harmonics, fair enough note tracking, and cool distortion by itself.

The only issue I have to deal with, is that the sound fades out once you pluck a string and let it ring. I'm not sure if it's a build problem, or it's just the way it is.

Could someone confirm this please?
Also, it wouldn't hurt to increase the output volume or reduce the fuzz , or both.

thanks!

[B Tremblay]

My store-bought Blue Box does exhibit both of the characteristics you describe.

There is a "noise gate" present and when not playing, mine is dead quiet.  The notes fade away rather ungracefully, sometimes sputtering, then they completely disappear.

Also, there isn't much volume available.  I keep the Output control at 3:00 just for unity.

I really love the pedal for what it can do.  Placing the Blue Box before a Green Ringer is great.

[Mark Hammer]

The reason for the seeming gating effect became instantly clear once I took a peak at Francisco's infinitely more comprehensible (compared to the factory schem  :?, blecch!) redraw.

Trace the signal path and you will see a classic half-wave rectifier envelope-follower (the two diodes, one going to ground and one in series with the signal give it away).  That envelope follower signal goes to the collectors of the two transistors feeding the blend control.  This acts as a crude sort of VCA, turning the transistors "on" when the signal exceeds a certain level, and off when it dips down.  Note that the envelope follower is tapped off the first op-amp, before the second op-amp which boosts the signal to feed the flip-flop.

Now that I finally "get it", it occurs to me that there are some potential mods in there to better suit one's instrument and playing style, the brunt of them centred around that envelope follower.

The envelope signal turns from bipolar (both positive and negative halfwaves) to unipolar via D2.  C8 charges up when D2 conducts and hangs onto the signal for a wee bit but R15 discharges C8 quickly.  If you stick a small value resistance (1k or so) between D2 and the R15/C8 junction, you can increase the amount of time it takes to charge up C8, so that you won't hear anything until a few milliseconds after the note starts.  That should chop off some of the initial transient, or at least soften it so that the note emerges a little more gracefully.

You can also increase either or both R15/C8 so that the decay is a little slower, but as has been frequently observed, the "sputtering" of flip-flops as the input signal hovers around tracking threshold is a constant source of annoyance about octave dividers so I wouldn't expect such a mod to improve things.  Still, if you have a compressed input signal so that the note is able to stay above threshold longer, you may get a less choppy effect with a little more decay time.  Try it and see.

C7 limits the low end of the spectrum in what the envelope detector receives.  If you have highly discrepant low and mids from your guitar, you may want to chop the value of C7 to tailor the responsiveness of the gate to lower notes.

IC1b further boosts the signal (R6 and R23 yield a gain of 101) and though there is some trimming off of high end earlier in the circuit, via C4 and C3, a little more couldn't hurt.  A cap in parallel with R6 can keep a little more noise out of the direct signal, and also prevent the flip-flop from triggering for non-note-fundamental content.  A value of 82pf will set the rolloff just under 2khz and a 100pf cap will bring it down to just under 1.6khz.  Pick your poison.  If you intend to use it with bass, a 120pf cap may be more suitable.

Finally, the "gate" is applied to both Q2 and Q3 equivalently, since R13=R14.  Of course, as any octave-down user knows, the octave signal craps out long before the "normal" signal does.  So, I'm wondering if a useful mod may involve an additional resistance in series with R14 so that you can make Q3 shut off before Q2.  Anderton tries to accomplish this very goal with the way he uses a compander chip in the Rocktave.  Maybe this is an easier way to do it.

Thanks FP, the redraw is a major breakthrough in understanding on my part.  Sometimes that's worth far more than the convenience of the layout.

[Fp-www.Tonepad.com]

Thank you Mark for the circuit description!

Fp