This is a remix of the excellent Compact Bowden extruder, direct drive, bearing bloc, auto load 1.75mm filament by scarou, http://www.thingiverse.com/thing:669459 . I used his design for a long time with my bowden setup but I was having problems with my stepper motor "thumping" during fast extruder moves. Also, I was having the usual "ringing" and "moire" on my prints. Of course this wasn't caused by his model. So I ordered a Bulldog XL clone from Robotdigg and found their extruder lacking in a lot of ways. They don't use a hobbed gear and bearing like the actual Bulldog XL does, but rather, a cylinder type mk7 gear and grooved bearing. This proved to be an expensive lesson in futility as I had to exert a crazy amount of spring pressure on the filament to get it to work. I tried a hobbed gear and bearing setup, but still had to tighten the springs way too much. This brings me to scarou's design. I decided to try importing it into Tinkercad and possibly moving the filament guide to the right 3mm among other mods I did so it would fit on a 5:1 geared stepper. After about 6 or 7 prototypes, I think i have it. I've been using the new extruder for a couple of weeks and it works like a charm. I think I'd put it up against an actual Bulldog XL, it works so well. The only difference in hardware needed is (4) m3x35mm bolts to mount the extruder to the stepper motor because of the gearbox mount. Also needed is a 688zz bearing to support the 8mm shaft on the gearbox. This is very important. (2) m3x8mm bolts for the shaft bearing retainer. You'll also need a hobbed mk7 drive gear around 12mm in diameter. something like this: http://www.aliexpress.com/item/3D-printer-parts-Makerbot-Reprap-mendel-DIY-MK7-extruder-drive-gear-hobbed-gear-8-mm-bore/32244205970.html?spm=2114.40010408.3.7.1w2AHc and of course, a 5:1 geared stepper motor like this one http://www.omc-stepperonline.com/gear-ratio-51-planetary-gearbox-with-nema-17-stepper-motor-17hs130404spg5-p-140.html You will have to shorten the tensioner springs by 5mm (if you're using the kind used for print beds). So far, I've tried PLA and ABS thru it with no problem. Going to try PETG and Nylon next. BTW, the ringing and moire have all but gone away! If you're using a bowden setup, I think a geared extruder is the way to go. Anyway, here is my Pitbull XL extruder and thanks again to scarou fopr the awesome design.

Note: There is a slight flaw with my design. The mounting bolt on the upper right under the levers is hard to get to. On scarou's original design, you could poke a 3mm allen wrench thru the mounting hole for the levers to tighten the mounting bolt for the extruder. With mine, the hole has moved, so, I ground down a 3mm allen wrench (as pictured) and slowly turned and tightened the mounting bolt. Sorry if this is an inconvenience.

Update: 7/13/16 - added recesses for left side mounting bolts on extruder. Update: 7/14/16 - added link to 5:1 stepper motor

Extra hardware needed (besides the hardware for scarou's original design): (1) 5:1 Geared stepper motor (4) m3x35mm bolts (2) m3x8mm bolts (1) 688zz bearing (without flange) (1) mk7 hobbed gear with 8mm bore (~12mm dia.)