King of the Stone Age? Cloning the Limited Edition Mad Professor Stone Grey

Back in February I built the AionFX clone of Mad Professor's Stone Grey Distortion pedal. It was a neat pedal, but the clone wasn't 100% accurate as I ended up using clipping diodes rather than transistors (essentially used as diodes) in the original. I also became aware of a limited edition version of the pedal and figured if I could figure out what changed in that version, it would make a far more interesting build! So, I did something fairly rare, I actually bought a pedal and set about trying to figure out what made it tick. 

As you can see from the image above, the Stone Grey Distortion isn't terribly complex, and it is very compact! I went through all of the normal components on the main side of the board, and didn't find anything that didn't match the schematics I'd seen previously during my AionFX Protolith build. However the same couldn't be said for the other side of the PCB.

Rather than actually changing out discrete components on the PCB (which is likely mass produced in a set configuration), Mad Professor soldered surface mount devices (SMDs) to the other side of the board in parallel with three of the existing components (one resistor and two capacitors). This has the effect of changing the values of those components giving the Custom version of the pedal a different (and in my humble opinion better) sound. That being said, I wasn't set up to remove SMDs and get a value off of them, so I called in an expert and sent the Pedal to PedalPCB so he could remove the surface mount devices, identify them, and then reattach them to the board.

Armed with that information, you can now buy an updated version of the PedalPCB version of the Stone Grey Distortion - the Graphite Distortion. This version includes a section where you can add two capacitors and one resistor that sit in parallel with the ones in the Custom Modern version of the pedal. AionFX also has updated the documentation for the Protolith to allow for building the Modern version of the pedal as well. 

As with all of my builds of modern circuits, this one uses 1% 1/4W metal film resistors (mostly Yageo and KAO Speer), which are ironically a bit tighter than the ones used in the original. The film capacitors are all WIMA and KEMET, and there are KEMET and other high-end MLCC capacitors as well as a couple of Nichicon electrolytic capacitors. The ICs are genuine and socketed. I'm using a Star Ground per my normal practice, but as I'm using the older AionFX drill pattern, I was able to just use a four-prong pin header to attach the 3PDT daughter board to the main PCB rather than ribbon cable.

One thing that was interesting is that the original pedal appears to make liberal use of carbon film resistors and what may even be standard ceramic capacitors. I may eventually dig a little deeper and try and build one with as many exact to original components as possible.

My previous version of the pedal followed the layout and appearance of the original pedal - with some slight modifications to the art and name of course. That being said, I really liked the name, but wanted to come up with something more fitting the "dino" theme, or at least something fun and prehistoric!

So... I decided to go all out and create a cave painting for the enclosure! I used various bits of licensed art as well as created some of my own in the same style in Photoshop and combined it into a fairly busy, yet fun, whole with some dinos thrown in (tongue firmly in cheek). I went ahead and picked up genuine Davies oxblood knobs from Love My Switches as I thought they'd go best with the overall enclosure. 

When I tested the pedal, it fired up first time without issue. This was one of those rare times when I could actually play my pedal and compare it directly with an original. I'm happy to say that I honestly can't tell the two apart - at least using my Chapman and Boss Katana amp. Needless to say, Steggo and Steggi are very proud of this one. Steggi did the initial tear down of the real thing, and then wisely indicated that she wasn't up to messing with SMDs. The clone is a bit bigger than the original, but pretty much all of my pedals are 125B with top mounted jacks anyway, so having smaller pedals doesn't help much.