Eurorack Synthesizer Power Supply

Welcome to my Instructable on how to make a DIY power supply for a Eurorack synthesizer.

Please be aware that my knowledge of power supply design and Eurorack synthesizer is second to none. Take my advices carefully. I will not be held responsible for your expensive modules getting destroyed or worst...

Anyway If want to learn more about the process that I followed to get to the finished product, I invite you to further read this Instructable.

Step 1: What Is a Eurorack Synthesizer Power Supply ?

First what is a Eurorack Synthesizer ?
Eurorack is a type of modular system used to create very personalized Synthesizer. In this system sound synthesis is achieved by interconnecting analog signals between the different modules. Eurorack was first introduced by the German brand Doepfer, it has now outgrown all the other modular synth systems.

The "patching" (a.k.a wiring) of the modules allows for very complex voicing, sound modulation and sequencing.

This kind of modular synth is widely used in ambiance music ( Listen Stranger Things soundtrack), in electronic music and in generative electronic (Listen Colin Benders).

What about the power supply ?

In the Eurorack system the sound and modulation patching is done on the front panel of the unit using mono or stereo 3.5mm jacks. In order to keep the user interface from being too cluttered all the power delivery is done through a connector at the back of the module.

Usually the power is delivered through a bus that supplies each module in parallel with -12/+12 V, 5V and ground. The bus carries also control signals also known as CV (control voltages) but those signal are rarely used.
The Eurorack modules are widely based on analog electronics thus the -12/+12 V voltage rails. But as modern DSP modules are getting more and more popular (See Mutable Instruments) the need for a good 5V supply rail is very important.

Why make a DIY power supply ?

The DIY community around Eurorack Synth is huge. Which makes the designing resources available for anyone wanting to find them.

First I am a complete newbie into modular synth and I wanted to learn more about it. I figured designing a piece of my synthesizer myself would teach me more than scrolling Youtube videos for ages but I was not ready to take my chances at designing a sound processing module yet. That is why I went for the "easy" part: the power supply.

Second I wanted a custom case with the power supply nicely integrated in it, thus making both the case and the PSU would ensure I would get the result I wanted (most likely not as good as off the shelves solutions, but I don't care too much I am not a professional musician anyway). I will make the case as soon as I get a chance (lookout for a future instructable).

Third I have a license for Altium designer through my work and I have made PCBs in the past. This shouldn't be too hard to make this board work.

Fourth and finally, I am a cheap guy, and I figured why not save a bunch on the expensive power supply modules. That being said I think it might end up being more expensive than some of the most popular Eurorack power supply. Too bad for me.

Step 2: Requirements

As we saw in the previous section the power supply consists of 3 voltage rails -12/+12V, 5V and of course ground (or 0V).

There are two strategies that are popular to achieve the same result:

  1. a transformer based design with a center tap to get a floating ground, both secondary get rectified and regulated to create the dual voltage rail
  2. a DC power supply design which is then converted to -12/+12 V with switch-mode regulators

The first solution is great since everything is build into the power supply. It takes 115/230V AC as an input. But you need to be willing to mess around with main AC, and you will need a bulky transformer inside the Eurorack case. Or you would need an AC wall adapter, which are not widely available as soon as you want more power out of your system.

The second solution is great since it can use powerful laptop like power brick. But the switching regulator will unfortunately be a little more noisy than its linear counterpart that can be used in the previous solution. Also OEM power supply brick can be quite expensive.

In any case I don't want to deal with sourcing the proper transformer nor do I want to be involved in fusing mains voltage. Thus the power supply that we are going to make will be a switch-mode based one.

The requirements:

  • output around 1A @ -12V, 1A @ +12V and 2A @ 5V for a whopping grand total of around 34W plus some heat (we will probably save on the household heating)
  • be daisy chainable so that a single power brick can be used for multiple power supply
  • be compatible as a front based unit or mounted inside the synthesizer case
  • have a personalized design all made of circuit boards
  • on-off switch
  • LED voltage rails status
  • low noise output voltage if possible

.

Step 3: Schematics

In order to create the schematic for this power supply we need to find switch-mode regulators capable of delivering +12V and -12V from a single supply. We can have two different IC (integrated circuits) for the positive and negative voltage rail but having the same would simplify the design.

I usually search for electronic components using either Digikey or Mouser. Both offer really powerful filtering system to find specific parts.

My choice of regulator for the -12/+12 V is the LM2576S-12 from TI.

Generally speaking if an IC manufacturer wants you to use their part they will have a nice schematic with all you need to get the schematic right. In the schematic for this component is described a positive supply wiring, a negative supply wiring and even an output filter implementation.

For the 5V rail we will go with the LM2576-5 from TI. This not the most economical option but it saves time during the design of the board since all the passives are the same as for the 12V version. Also the footprints are the same which saves a bunch of time.

There are 3 status LED which are each connected to a voltage rail in order to report to the user wether everything is working or not.

Please look at the schematic attached for more details.

Step 4: Layout

Creating the layout is not easy, but it is not hard either. It always depend on the constraints.

My main contraint this time was to keep everything inside a rectangle of 100mm by 60mm. The reasons for this specific dimensions are the following:

  • less than 100mm length because of the PCB manufacturing quotation system for low volume that favors hugely sub 100mm (width or length) designs, remember, I am a cheap guy :P
  • less than 60mm width since I want the module to fit inside a "skiff" Eurorack case that usually have very limited depth.

Width that in mind I could trace the outline of my PCB, populate it and make all the required traces.

Notice that it is a dual sided board, having 4 layer would make the layout process easier but would cost more. In order to still get a nice ground I kept most of my traces on the top layer and laid out a ground pour on both sides. Both pours are stitched with many vias to ensure optimal grounding, thus minimizing noise (hopefully).

It is always useful to keep in mind the 3D footprint of your component since when dealing with power supplies those components are usually pretty big.

Once the layout is done we cans now output the gerber files and send those to our favorite manufacturer. In my case I send the files to PCBWay (I am not affiliated, but I have always had good luck with them in the past). The ordering process is dead simple.

Gerbers are attached bellow.

Step 5: Front Panel

In order to be a proper Eurorack module, the power supply needs a front panel. And since I am already ordering a PCB why not make it has a PCB itself as well...

Thus I created a new project an layed out my artwork. It is not super great but it will do the job.

You will notice here that I had to drop the daisy-chainability of my system because of a lack of space. All the artwork were created in inkscape then converted to dxf and imported into altium. This made quick work of it.

For the led I decided to try to make them shine-through the front panel. To do so I poured entirely the top layer and made circle openings with little artwork inside. I have never tried this before, it might not work.

Since this power supply is for a musical instrument, I tried to have a little music theme going. It might be lame, but it was easy to make with a music symbol font.

Gerber for the front panel can be found bellow

Step 6: Components

Now that we have our boards on their way we need to worry about purchasing the remaining components. And for that you need to choose a distributor. In my case Mouser had the best selection of component for what I needed:

  • AC adapter (418-TRH100A13502E126)
  • DC barrel connector hopefully compatible with the item above (502-721AFMS)
  • Rocker switch (691-651122-BB-1V)
  • Reg LM2576-12 (926-LM2576S-12/NOPB)
  • Diode 1N5822 (511-1N5822)
  • Inductor (673-PF0382.223NLT)
  • Capacitor (661-APSG160E222MJ20S)
  • Capacitor (661-APXG250A101MHA0G)
  • Inductor (994-MSS1583-683MED)
  • Capacitor (80-A750MS108M1CAAE13)
  • Header (517-30316-6002)
  • Terminal (571-624091)
  • Crimp connector (571-6409051)
  • Reg LM2576-5 (998-LM2576-5.0WU)
  • Led (710-155124VS73200A)
  • Led (710-155124RS73200A)
  • Led (710-155124YS73200A)
  • Screw terminal (534-7689-3)

The rest are jelly bean components:

  • 250 Ohm 0603 resistor
  • 100 Ohm 0603 resistor
  • 330 Ohm 0603 resistor

See the pdf attached for more details (adjust quantity according to the schematic)

Step 7: Assembly

The assembly process fo this is plain old boring soldering.

I will not go into to much detail on this since there are so many instructables dedicated on this topic.

All components should be soldered in the right spot and everything should work fine.

Step 8: Done

Here we are the moment of truth !
Does it power up ?

You should check everything with a multimeter before powering any modules obviously.
I am very happy with the LED that shines through the music keys ! It is gorgeous.

Overall this project was a great success, but an update should be coming soon in order to correct some issues that should improve the fit on the Eurorack rails. But as a side mounted power supply it is perfect.

Here is a tune made with my freshly built instrument. If you wonder what a modular synth might sound like, have a listen. The concept of modular is to create an instrument that is tailored for you. Which makes is pretty much unique, and I love the idea of having a uniquely sounding instrument.

I wasn't very confident going into this modular trend since I have no prior experience in synthesizers. But I have no regrets. And if you are hesitating, I would encourage you to try it out. It is quite easy to resell modules, and if you build it yourself as much as possible, it is not more expensive that any other good quality instrument.

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