MXR Blue Box question

[Barcode80]

I'm building the MXR blue box from Tonepad's layout, and I have a 4013BE lying around, which i assume is buffered from the suffix. is this okay for the blue box or do i need the unbuffered version (if there is one)?

[George Giblet]

The buffered device is the correct device.

[Barcode80]

sweet!

[col]

Please post on how you find it when built. Mine is very tempremental but all my voltages compare well with others that have posted. 

[SonicVI]

I just built one a few days ago and I'm having trouble with some background ticking kinda noise.  I haven't been able to figure out the problem yet. Other than that it works and sounds very cool. I had a reissue back in the 90's and it sounds just like I remember it.

[Barcode80]

sonic, check the build reports on tonepad, i think someone may have addressed the ticking problem.

[SonicVI]

I've tried a few different opamps in there and all are noisy. Maybe I'll try a few more.

[Barcode80]

right now i have a 4558 in there, finished the build and it sounds pretty dang good, but it isn't tracking as well as it should. the octave jumps a little every now and then. i'm going to try the tl072 and report back. but the build works great!

[Mark Hammer]

The tracking ought to be better, in principle, with a more consistent signal source.  In theory, the amount of gain applied to IC1a (x471) and IC1b (x100) ought to accomplish that by maxing out the chip, but a little bit of "help" might not be such a bad thing.  I'm wondering if the following might not be such a bad idea to make the circuit behave a little better:

1) Stick a feedback cap in parallel with R6 (Tonepad diagram) to keep the lower harmonics attenuated for octave tracking.  C3 already provides a 6db/oct rolloff above 1129hz, but a second cap/stage might not be such a bad idea.  With R6=1M, try a 120-150pf cap.  Note that this will change the tonal quality of the same-pitch distortion signal.  If the loss of sizzle bugs you, just lift the ground side of C9 or replace it with a different cap value.

2) R7/R9 form a fixed voltage-divider circuit that takes the maxed-out signal from IC1b and drops it down a bit, where it can be suitable buffered by Q1 before going to the 4013 flip-flop.  But how much does it need to be attenuated to provide optimal tracking?  There's the rub.  I'm wondering if replacing R9 with, say, a 6k8 fixed resistor in series with a 10k trimpot might provide for adjusting/fine-tuning the to-be-divided signal used for generating the octaves.

[johngreene]

2) R7/R9 form a fixed voltage-divider circuit that takes the maxed-out signal from IC1b and drops it down a bit, where it can be suitable buffered by Q1 before going to the 4013 flip-flop.  But how much does it need to be attenuated to provide optimal tracking?  There's the rub.  I'm wondering if replacing R9 with, say, a 6k8 fixed resistor in series with a 10k trimpot might provide for adjusting/fine-tuning the to-be-divided signal used for generating the octaves.

Q1 is acting as a switch, so adjusting R7/R9 probably won't have much effect as long as the base is driven above .6V. The divider is more for protecting the base of Q1 than anything else. The 10K to ground probably isn't doing too much since the base will clamp at .6V with the emitter grounded. The 56K is determining how much base current the transistor is driven with. If the transistor being used isn't switching hard enough to provide a nice clock signal, then lowering the 56K will help by driving it harder. Just be careful that you don't go too low and drive the transistor with too much base current (check the data sheet) as you can damage it.

--john

[Mark Hammer]

In which case, then maybe the tracking adjustment to make is the leave R7/R9 as they are and adjust the gain of IC1b by varying R23.  Note that since we are dealing with the multiplicative gain of 1C1a times IC1b, small adjustments of R23 will have large impacts on the resulting signal amplitude reaching Q1.  Again, I wouldn't recommend anything more radical than a 6k8 fixed resistor in series with a 5k or 10k trimpot.

[johngreene]

In which case, then maybe the tracking adjustment to make is the leave R7/R9 as they are and adjust the gain of IC1b by varying R23.  Note that since we are dealing with the multiplicative gain of 1C1a times IC1b, small adjustments of R23 will have large impacts on the resulting signal amplitude reaching Q1.  Again, I wouldn't recommend anything more radical than a 6k8 fixed resistor in series with a 5k or 10k trimpot.

A 4558 is probably not the best choice for this application. I have seen 4558's actually 'fold-over' the output when the input is driven below ground. Get some assymmetry going on the first opamp output causing a DC shift negative on the other side of C2 and you will see an 'extra' transistion on the output of IC1B.

I haven't built this circuit, but I have seen this happen with 4558's.

--john

[Mark Hammer]

I couldn't tell you about any "folding over", but I've built it successfully several times and the unit has a pleasant sustained distortion sound quite apart from the sub-octave, that comes purely from driving the bejeezus out of the dual op-amp.  Part of the "trick" of the circuit lies in:

a) achieving a sort of compressed sustaining tone/signal by driving the op-amp hard
b) dividing up all that gain across two stages such that the envelope-follower section also has enough signal to work with for gating purposes

[Barcode80]

well, i have since switched to a tl072 and the sound is a bit more synthy, and it seems to track a bit better, but still not as well as i would expect. i wonder if the opamp is being overdriven and causing some overtones making it difficult to track. i only say this because if i pluck lightly, it tracks dead on almost all over the fretboard, but if i play with any actual amplitude or force (and trust me i'm a light picker anyway, so i'm not talking banging) it tends to jump some from up an octave to down an octave and so on.

i'll try everything listed here and report back.

[SonicVI]

I couldn't really tell a significant difference between a 4558 and TL072.   Originals used two 741's.  There's one on ebay right now with a really good photo of the component side of the board.   I find picking fairly lightly gives best tracking as well, maybe since when you pick harder you move the string a little more making the pitch go slightly more sharp on the attack than the sustain than when you pick more lightly.

[Barcode80]

I couldn't really tell a significant difference between a 4558 and TL072.   Originals used two 741's.  There's one on ebay right now with a really good photo of the component side of the board.   I find picking fairly lightly gives best tracking as well, maybe since when you pick harder you move the string a little more making the pitch go slightly more sharp on the attack than the sustain than when you pick more lightly.

good point, but i notice that the EH octave multiplex doesn't suffer from this much. might be my next build

[SonicVI]

Compared to other octave dividers like the EH or MuTron or Rocktave, the Blue Box seems pretty simple and probably just wasn't designed to operate flawlessly as much as it was to fit inside a small MXR sized box.  Unless you mod the crap out of it it may just be a box you have to take for what it is or else use something else.

[Barcode80]

Compared to other octave dividers like the EH or MuTron or Rocktave, the Blue Box seems pretty simple and probably just wasn't designed to operate flawlessly as much as it was to fit inside a small MXR sized box.  Unless you mod the crap out of it it may just be a box you have to take for what it is or else use something else.

i thought that may be the case, but i think it is a promising mod candidate, as considering parts count and circuit size, it does things fairly well. i can't shake the feeling that this circuit is capable of more in terms of tracking, and i think some of these suggestions are the right track, just gotta try and see!!

ps - i connected pin 1 and 3 of the 4013 to switch between 2 octaves and one, and i heard no difference. anyone else here done this mod? which state is 2 octaves, open or closed connection?

[oskar]

ps - i connected pin 1 and 3 of the 4013 to switch between 2 octaves and one, and i heard no difference. anyone else here done this mod? which state is 2 octaves, open or closed connection?

Hi! You tap the proper octave from either pin 1 or 13 (which connects to 3), you don't close the circuit between them because they are outputs!

[Barcode80]

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ps - i connected pin 1 and 3 of the 4013 to switch between 2 octaves and one, and i heard no difference. anyone else here done this mod? which state is 2 octaves, open or closed connection?

Hi! You tap the proper octave from either pin 1 or 13 (which connects to 3), you don't close the circuit between them because they are outputs!

strange, there are a bunch of build reports on the tonepad site that say to connect them. here is a quote from the most recent build report:

5) I added a switch to connect 1st and 3rd pins of flip-flop, the pedal it self gives you 2 octaves down, when they are connected it's 1 octave down

i really see no use for a pitch 2 octaves down. especially since ultimately this will be used with bass which will render the octave subsonic.