Hi guys,
as I already stated in my
post at
pix of Custom built guitar amps/cabs board, to be able to use my Micro Cube with
looper pedal, I have designed and built in effect loop circuit. In principle it is nothing special, biggest challenge was to actually get inside my MC

(there's a rubber around chassis and it sticked to the cabinet over the years), but it had few specifics. First was that MC guts are quite complex and I could not find schematic. Luckily
Cube 30 schematic is available on-line and all the critical parts of the circuitry turned to be almost identical. Second was, that basically all except the most low level functions is handled digitally inside DSP, including Volume control, so add-on also has to feature master volume pot

But it is actually a good thing, as I realised later, because the original Volume control is quite useful when using looper, as it allows to adjust levels of different setups when layering loops. Third sort-of-challenge was placing add-on physically to the MC chassis, as there is not that much space, but it was not that bad either. I have found nice place for the Master Volume pot at the top panel (see
pics) and there is some space to squeeze send/return jack(s) at the back side. For now to save some drilling I used stereo jack for both signals (requires special cable), but two jacks would fit either.
TL'DR:
Basic topology (blue is MC, green is add-on):

Add-on replaces decoupling capacitor C32 (this capacitor has the same part name on MC and Cube 30 btw.

), power is taken from C107 pads (- is
AGND, + is
A+8), be careful not to connect audio ground (AGND) with chassis. To be able to easily find right spots, check my
pics (wire colors:
⬤ A,
⬤ B,
⬤ A+8,
⬤ AGND,
◐ FX-IN /SEND/,
◐ FX-OUT /RETURN/,
◐ FX-GND).
Add-on schematic:

Circuit is quite simple, could be easily soldered on universal PCB or whatever technique you prefer, but in case you are interested, my PCB layout and Eagle files are also available at my
page. Btw. for master volume linear (not audio) taper pot is used. With combination of 10k load at C32- input it works
better
Attenuation and Gain are set to 3x (so the TRIM_ATT is set to one third and TRIM_GAIN is adjusted to unity gain of the add-on with POT_MASTER at maximum), but this may yet change in the future, as I was not sure what would be the best value TBH. It works fine with my Boss RC-3 looper, but it would be maybe worthy to try with your pedals before putting your MC back together for good.
Sample Loop Good luck, T.