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previous: Mark Hammer Re: MXR Boards???????? -- 7/5/2000 6:44 PM View Thread

Bluebox mods

7/6/2000 5:18 PM
Mark Hammer
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Bluebox mods
Looking at the schematic (http://www.compassnet.com/~cjlandry/schems/bluebox.jpg), there will be a PC trace going between pins 13 and 3 on the 4013 chip. This connects the 2 flip-flop stages so that what was divided by 2 gets divided by 2 again to yield a divide-by-4 function (i.e., 2 octaves down). Tied to pin 1 of that same chip is a 1meg resistor. If you were to cut/break the trace between pins 13 and 3, and do the same to the trace between pin 1 and the end of the 1meg resistor, you could stick a SPDT toggle switch in there and select between one or two octaves down (shunt 1meg to pin 13 directly or to pin 1).  
 
As well, the schematic shows a .01 cap between gnd and the wiper of the LEVEL pot. Presumably, this is to take out some of the objectionable high end that results from using square waves as your octave down signal, although what gets taken out is a function of where the level control is set (less high end the more you turn down). I suspect part of the characteristic sound of the Bluebox is the fact that there is precious little lowpass filtering between the flip-flop and the output jack.  
 
Octave down boxes get more usable with some degree of rounding to those square waves. You can do this two ways: by trimming back high end of everything (octave and straight) as is done in the BB, or by trimming back the high end on the octave alone, so it recedes behind the crisper timbres of the straight sound. Both have their merits.  
 
You can do this in a variable or preset way. If you unsolder the .01 cap from the wiper of the level pot, and solder on to the other (nonground) outside lug, you will get high end rolloff whose frequency is fixed, and largely independent of the level control. This will affect the tone of everything (octave and straight) though. If you were to put a 25k variable resistor in series between the input to the level pot and the .05 cap leading up to it, in addition to the .01uf cap going to gnd, then you could have a variable "Rat" type tone control, so you could dial in amount of treble cut.  
 
If you'd like to retain the clean straight sound and tame the octave tone, a good place to intervene is at the blend control. If you tacked on a small cap, say 6800pf, between the octave side lug of the BLEND pot, and gnd, you would get a rolloff starting a bit over 800hz, without only a small penalty in terms of level drop.  
 
Finally, there may be some issues with respect to triggering and sensitivity. A suitable sensitivity control can be made by replacing the 470k resistor between pins 1 and 2 of the op-amp with a 220k resistor and 500k pot in series. This should provide you with a decent range of sensitivity.  
 
Incidentally, the picture of the board shows two op-amp chips. The schematic show two op-amp sections labelled as 1/2 of a 4558. Clearly, from the picture and your comments, the board actually uses 2 741's. You may want to doublecheck the pin equivalencies between the schematic and your board to avoid confusion (i.e., the 470k resistor WON'T be between pins 1 and 2). You may also want to yank the first and maybe second op-amp and stick in something with better specs. Perhaps a TL071.  
 
Obviously all the mods described here tend to preclude using a 1590B or similar sized chassis. If you had the Sensitivity and variable Tone controls, then you have a 4-knobber, which loudly proclaims 1590BB.