Here's a cheap and Lo-Fi filter I made in about 6 hours. That includes picking up parts too. I still need a knob for the gain pot.
I got the schematics from here:
As you can see (if you've looked at the Gakken schematic), this is almost identical to the Gakken filter section, with a few resistor sizes altered and different routing for input through that 100k resistor - I experimented reversing the CV and filter inputs, as the Gakken has the filter pot going through the 100k resistor instead of CV, but it worked less well.
This is a proper lo-fi filter, so your not going to get all those big juicy filter sweeps with it. And, unless I wired something wrong in mine, you'll lose the signal (in slightly different ways) at high and low gain settings. It sounds like a bit crusher or decimator effect - which is cool.
You can adjust the input levels along with the gain trimmer to tune in certain sounds. In fact, I found that this can fatten dry character-less sounds quite a bit.
The way I've been using it is somewhere between distortion and tape saturation, but without the hiss. You will get the characteristic 'droning' sound identical to the Gakken SX-150 (so that must also be the source of the drone in that unit too!!!), especially when there's no sound running through it. So gating is in order. But I've made bass-sounds, etc, sound much hotter and oversaturated, and I've run some drums through it - to get that fat borderline-distorted 1997-2000 drum sound.
Will put up sounds later.