Monday, October 19, 2020

Stomp-Synth Adapter--All Hail Ken Stone!

Simple one this time: I needed to hook up some of my old guitar stomp boxes to my modular rig. How to do that? It's a simple attenuator (to not overload the stomp's audio in) and then op amp boost.  

Two finished stomp adapters.  Front panels are PCB material covered with Mr. Label

You guitarists already have a backlog of stomps right?  I do....some of them dating back to my high school days.....so let's get em hooked into our modular setups.

Yes that's a 1970s era Deluxe MM.  Collectable! Yep, my DIY skills in 2002 were pretty NG, but these old stomps still work. Bonus points: see if you can guess which of the stomps here are hand made?

Sure I could have designed and breadboarded something, but why do that when Ken Stone has already done the heavy lifting?

Enter Mr. Stone: If you've built audio circuits before you might have already run into Ken Stone's work. His designs have been ever present this hobby (?) as long as I can remember. He got out of the PCB shipping business (understandably) and gave it all over to Elby Designs, you can find that site, and his boards for sale, here. I find myself studying Ken's designs all the time; they are practical, the ones I've built always sound good, and well documented. Yeh!


The schematic for this post is very close indeed to Ken's CGS-60 circuit, with a few minor mods to accommodate frac setups--the form factor sold by Elby didn't look like it was going to fit into my rack, but it's easy enough to lay out a new board, then off it goes to the fab shop (here). Gotta love that CGS "floating ground" design. Ken crafted all sorts of cool stuff including some licensed Serge circuits, right? Right! all hail Ken Stone!

As Ken says via the Elby site, if you build this, expect to have to mess with R3/R7 to adjust output gain and maybe screw a bit with the attenuation subcircuit to process input, but really that's easy--it's changing 2 values essentially. I am using 10K now but will probably go for 4.7k soon since right now I'm getting too much gain. UPDATE: for my 10V P/P synthesizer, using 4.7k for R3/R7 and A10K pots for FXLEV1 and FXLEV2 did the trick. Your setup may require different values.

Poof, back from China, let's fab and build it:



Let's forget the rest of the build photos, you've seen this before? Here it is with crap front panel, ready to go into the rack for further refinement:

But does it make balloon animals?




So far so good, but there were enough dumb mistakes on the rev1 pcb that it will have to get a rev2 pcb fabb'd. 

The mock up w kludge fixes works great, no smoke yet, and this basic design opens a whole new world of stomps and synthesizers living together in double happiness. 

Once that's sorted, I'll build the Rev2 PCB up and see if it works. Update 10-28-20: rev2 is back and I am fabbing it now. Stay tuned....UPDATE 11-5-20. REV2 here and works.  See photos of finished units at the top of this post. The schems are pretty much just like Ken Stones, so I don't feel it's OK to post them on my website; go to the Elby site here if you want to build these, otherwise if you want copies of my eagle or gerber files, comment below.

Yep I'm echoing and delaying and will continue to do this over and over. Until next time, Don't breathe the fumes.

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