Useful, Easy DIY EuroRack Module (3.5mm to 7mm Converter)

I've been doing a lot of DIY for my modular and semi-modular instruments lately, and recently I decided I wanted a more elegant way of patching my Eurorack system with 3.5 mm sockets to pedal-style effects that have 1/4" ins and outs. The result was something I thought I could share and that could act as a beginner project for anyone thinking about putting some of their own work into their synth racks.

This is pretty simple-there is a little rudimentary metalworking and about 8 to 16 very easy solder points, but it can still be instructional or educational. But mostly, it's FUN!

Step 1: Tools and Materials

The BOM is pretty tight:

4 or 8 3.5mm TS (mono) panel mount sockets (amazon)

4 or 8 1/4" TS (mono) panel mount sockets (amazon-Note:they sent me stereo, but you can still use TRS here)

Make sure that all the sockets are built with sleeves that are for grounding (this means that the common electrical signal is actually passed through the panel from each socket)

Conductive sheet metal (aluminum is best) .064" thickness is best, at least 5 1/8" x .8" (4hp) or 1.6" (8hp)

Hook Up wire (threaded will be most dependable)

Solder

Paint and/or Markers (optional)

Eurorack

Eurorack mounting screws

Tools you'll need:

Metal-capable saw (Table saw is best)

Drill (drill press is best)

Soldering Iron, wire strippers and cutters, "Third hand"

Continuity Checker (usually a multimeter)

Files and/or Rotary tool

Scribe

Nail/Center punch

Steel Wool (optional)

Step 2: Cut and Drill the Plate

Choose the size you want to make. The photos are for the 8hp 8 route converter, but there is also a 4hp 4 route option. Using the PedalMod.pdf as a guide, cut your metal plate out using the saw. In the original Eurorack spec (defined by Dieter Doepfer in 1996) 1hp is .2" wide and the rack "height" is 5 1/8" high. This translates to about 20.25mm for 4hp and 40.5mm for 8hp by 130.1mm.

ALWAYS TAKE SHOP SAFETY PRECAUTIONS WHEN WORKING WITH METAL! Wear protective eyewear when cutting,drilling, filing and/or sanding metals. Be aware of what you're doing! After any metal work, don't touch your face (especially your eyes) or eat anything until you've thoroughly washed your hands. Clean up your work area when you're done, and be aware of any flakes or bits of metal stuck to your clothing.

Begin by scribing a guide grid out onto the plate (using dashed lines on the PDF as a guide) then use a center punch or a nail to divot the intersections of the grid lines at the center of each circle on the PDF.

Next, choose the drill bits closest to the diameters of the threads of the sockets. The 3.5mm sockets correspond to the smaller blue circles and the 1/4" sockets are represented by the orange circles. If you have any "drop" metal, you might want to test fit the jack sockets to the holes created by the chosen bits. You want the tightest fit that you can get the sockets through without having to "screw" them through the holes.

Drill out the holes-It's a good practice to drill pilot holes in metal before drilling the finish holes-it will make drilling out the finish holes faster and the results will be less sloppy.

Once the plate is cut and drilled, file, sand or steel wool down any burrs or rough edges, especially any drill scraps folded into the drill holes.

If you want, you can paint and decorate the top/front side of the plate (so that the painted side is as you're looking at the PDF representations) at this point, but ONLY THE FRONT SIDE! If you get paint on the back of the plate, the unit may not work! Steel wool and sandpaper can both be used to eliminate the scribed guide grid and/or prepare the surface for paint or remove paint spilled onto the back side of the panel. A round file or even a hobby knife can be used for removing any paint that drips into the drill holes.

Step 3: Electronics

If you're not an experienced solderer, don't let this part intimidate you-it's really quite easy!

Using the continuity checker, figure out which of the solder tabs connects to the tip of the cable for each type of jack. The easiest way to do this is to plug a cable into a jack and hold one pole of the continuity checker against the tip of the free end of the cable and use the other pole to probe the solder tabs of the sockets.

Next, pair each 3.5mm socket with a 1/4" socket and solder each of the tip connectors in a pair to either end of a short length of wire (2-3cm). The best practice here is to "tin" both the tip connector and the wire (which means to melt some solder directly on the tip of the iron, then touch the iron to the connector and touch more solder to where the iron is touching the connector/wire so that the solder flows onto the connector enough to break its own surface tension or uniformly coat the wire. Then when both are tinned, again touch the iron tip to the tinned connector and when the connector solder melts, press the tinned wire against the melted solder on the connector. The wire should sink below the surface of the liquified solder on the connector and more or less disappear. When this happens, remove the soldering iron tip and hold the wire in place for a few moments until the solder solidifies.

(An excellent demonstration of this method is presented by Sam Sam at Look Mum No Computer in this project at right about 3:05 in the video. He does it to some potentiometers and uses some diodes on his connections, but you can see the technique.)

When you're done, you'll have 4 to 8 jack-pairs that look something like the second photo for this step.

Next, remove the mounting hardware from each socket, then one jack-pair at a time, insert the 1/4" side in a larger hole and the 3.5mm side in a smaller hole adjacent to it. Reattach the nuts and any washers to the jacks from the front side of the panel and tighten them (if you've spray painted the front of the panel, do this delicately as it will be really easy to scuff the paint.) It's best to be consistent here, so if the top 1/4" jack goes to the left 3.5mm jack beneath it, then mirror that on the bottom half of the panel.

EDIT: If your sockets have tabs of the right length, you may be able to do away with the wire and soldering ahead of time. See the last photo on this step (added with edit, of the 4 hp 4 route version)-all you have to do is line up the sockets as you mount their hardware in such a way that each jack-pair's tip connectors are touching, then solder them together. In this case you should be able to simply tin the tip of the soldering iron, touch it to each of the tip connector junctions and touch the solder to the tip of the iron until solder flows to join the connectors. Just don't use so much that it touches the metal panel! Continue reading the original text to see why!

The reason that I specified that the metal used for the panel should be conductive and the jacks had to have conductive sleeves as we're using the panel to complete the circuit for each of these jack-pairs. The sleeve connector for Eurorack and PA or effects is ground, so we can simply solder the tips together and let the connection to the panel do the rest of the work. That's why you don't want to get any paint on the back or in the "walls" of the drill-holes:paint is an insulator and can keep the circuit from completing.

As you insert the jack-pairs and tighten the hardware down, make sure that none of the tabs (at least the ones that aren't soldered to each other) are touching. This could short the transition you're trying to make and defeat the purpose of sending your signal through a pedal or effect or out to a PA.

Once all the jack pairs are inserted and tightened, get out the continuity tester again and make sure that a) none of the soldered tabs connect to ground and b)none of the soldered tabs connect to another soldered tab jack pair. (The last picture is how I did that. All the multimeters I've ever had have had an audible continuity detector built in, so I attached a probe clamp to one of the free ground tabs, which is connected to all the other grounds through the panel instead of solder, and probed each tab point with the free end to test the ground.)

Step 4: Mount and Use

If you have a Eurorack then unless this is your first and only module, this part should be familiar. Simply slip the module into the channel formed by two paired rack rails, (if necessary) slide a couple rail nuts behind the mounting holes of the units, and screw the mounting screw through the front of the unit into the nuts or threaded strips.

Using it is just a matter of taking a signal from a point in the modular signal path and patching it into the module at a 3.5mm socket, continuing that path out of the corresponding (jack-paired) 1/4" socket to the destination and if necessary, inverting the process to bring the signal back in to the modular signal path through a separate jack pair.

The embedded video demonstrates how to use the module as a stereo return to an Eventide H9. (The process is at about 1:40 in though.) I hope you don't find the extreme wave folding on the lead too annoying, and that you learned from or got something useful out of this simple project.