The MXR M-103 Blue Box is a sub-octave fuzz (two octaves down). It's not a clean sub-octave either, pretty dirty really, well it's a fuzz... kind of synthy on some settings.
Might want to check how many 56k resistors you have before starting this.
MXR BLUE BOX - VERO LAYOUT
BLUE BOX OCTAVE MOD
Same again, but with the octave mod - this provides the option to switch between just one octave below or the standard two octaves below. Very handy; I would recommend this over the stock version.
Another mod is replacing the 10n cap to ground on the output with a smaller cap, or remove it altogether, or putting it on a switch for the best of both worlds.
The 10n does shave a lot of high frequencies off (see scope traces below) - even on a switch you may want a smaller cap.
BLUE BOX ON THE SCOPE / FFT
Signal 440hz sine wave, approx 130mv TRMS
Out of the IC, on the 56k / 10k divider to drop the level to a reasonable amount (massive signal straight off the pin) - as you can see, it's pretty much a square wave now.
Blend set to max octave, switch on 2 octaves down, no cap across the output to ground.
Blend set to max octave, switch on 2 octaves down, 10n cap across the output to ground.
Blend set to max octave, switch on 1 octave down, no cap on output to ground.
Blend set to max octave, switch on 1 octave down, 10n cap on output to ground.
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