EURO SEM style Attenuverter

I already have a DIY attenuverter in my rack, in FRAC format (+/- 15V). My Modular Synth geek group have been telling me: stop it, the only format left is Euro and large format. 

I have a bit of space in my Eurorack setup, so, let's get building--let's port last post's design to Eurorack.

Previous posts for this SEM style attenuverter are here (prototyping), here (Skiff depth attenuverter) and improving the module's bias control--here.  

To summarize those posts, because, who reads blog posts anyway? I always liked the sound/feel/response of the attenuverters found in the Oberheim SEM. 

The legendary Tom Oberheim confirmed via email that his design used a center tap pot. 

I bought a few ( ALPS RK11K114) and built a few.

Finished module

Boards were already laid out in Eagle; I only needed to change power connectors and add a  few 100 mil test points to gang 2 attenuverters in series.  

I also needed to design a Eurorack sized front panel:

I used the previous board and paper templates to create a new front panel.  But--would everything fit?

 

Next I sent the gerbers off to the blog's trusty sponsor, PCBway.  

BAM!! they are back:

Happy weekend--new boards from PCBWAY.  Thanks to those guys for helping out this blog.

Next I got soldering. 

The design had 2 SOIC op amps, but those aren't too hard to solder. Use a fine tipped solder pen, flux, and patience.

Beyond that, this is all through-hole. Happy day.

The design required a standoff board for the center tap pot. Otherwise its threads won't stick up far enough through the front panel. I included this small standoff in the gerbers and sch/brd files you can get here.

I had 2 pulled standoff boards from earlier projects so i reused those......

Only a couple mistakes, which i corrected in the gerbers uploaded to the PCBWAY project page (here); I forget to silk "red stripe" on the back of the PCB (oops) corrected below with a p-touch label. Wow, dumb error.

Beyond that everything worked first time!  Front panel fit fine, everything lined up.

The CON xxx connector sets from All Electronics are inexpensive handy for connecting signals between modules.  2.54mm spacing...nice.

As far as new features: after having this module (I have built two) in my rack for several months, I found I was patching the output of module 1 to the input of module 2 frequently to get complex inverted CVs, audio phase shifts and whatnot.  

Using CON-242's from All Electronics I wired two Euro attenuverters  together from the back, so the output of 1 hit the switched input of module 2:

Rear view--two attenuverters chained together

I also added another CON-242 male to each module's output.  This will allow me to wire up something new to the module's output in the future.  I will build an output expander for this next month--let's hope it works.

Update 11-4-22: I have created an expander module for 2x of these attenuverters, adding simultaneous inverted outputs with level and bias control. Project here. Blog post here.

All in all this was a fast and fun build.  

If you want to play along, get BOM, gerbers, PDFs of schematics, eagle files, from PCBway's community page here. 

2x Attenuverters with Expander (right)

Remember: If you have to vert, go up and down not back and forth.  See ya next time.