Modular synthesizers are extremely attractive objects. With the right selection of modules, they give musicians the possibility to express themself with new and very personal sounds, but also give electronics tinkerers the opportunity to deal with the most famous circuits of the old glories of the past, or completely brand-new solutions.

An adeguate power supply is the first piece of equipment we need to start a modular synthesizer project and it should come as no surprise that it is the first "modular" circuit I have approached.

In this instructables I will show you the power supply solution that charmed me the most and a simple but effective auxiliary device to feed our modules.

Linear Regulated Power Supply - What and Why

There are two main types of power supply units (PSU) we can choose from: linear and switching PSUs.

Switching power supplies features high availability, higher efficiencies, lighter weight, longer hold up times and the ability to handle a wider input voltage range. A well designed linear PSU, on the other side, is less noisy than a switching unit,and this is the main advantage one could ask for when dealing with audio. Thry also have the advantage of being easier to realize, and this could be even more important point for a tinkerer :)

There are various linear power supply circuits around the net, mainly based on fixed LM78xx and LM79xx, or adjustable LM317 and LM337 regulators. These are excellent starting point to develop a (even slightly) new solution (tinker to the rescue!).

These at least are the main reasons I choose a linear, regulated solution for my personal use.

The Hardware

The PSU circuit I opted for is a single half wave rectifier with LM7812 and LM7912 voltage regulators.

It starts with a 12VA transformer with single winding at the secondary output that isolates all other circuits from being mains referenced. I wanted to get an AC-AC wall socket transformer to reduce the use of precious PCB space, but they are rare as unicorn poop, then I had to source a readily available PCB mount transformer (Indel TSZZ12/007M).

A couple of big 4700 uF filter capacitors and center tapped diodes bridges linearize the current before entering the two voltage regulators.

Two diodes across input and output of both regulators protect them in case of a short at their input.

Both primary and secondary parts of the circuit are fuse protected.

Notice that the Eurorack standard asks also for a +5V line. This PSU doesn't produce +5V but only +12V and -12V, so another PSU could be needed for some module to work.

 Warnings and Limits

I tested this PSU with 15W ceramic resistors, and at 660 mA of total current the transformer was getting too hot to be touched. This is the demonstration (was it really necessary?) that the rating of the power transformer cannot be transposed to the whole PSU circuit directly.

During operation the transformer can go hot. Durning my testings in a ventialted open space at less than 350 mA total, transformer temperature was moderate (felt hot to the touch, but I could stay in contact without burning my skin), but if you plan to place this PSU in a closed space (your modular's chassis) it is well likely that temperature will rise, possibly to failure levels.

Without further testing then, I would not load a design like this more than 400 mA total.

This project asks to deal with HIGH VOLTAGE! The danger of injury or death is HIGH if you don't know what you are doing. You have been warned!

Capacitors are polarized components and explode if installed with the wrong orientation. Take due precautions to avoid injuries to people or damage to things

The circuit design adopted here is not mine, but mostly based on resources I found online. I have put the maximum effort I was capable of on this (I really want to use it in my system!), but I cannot be considered an expert at any level when dealing with high voltage - high current units. Your equipment has value and should be powered by certified products and fully tested by qualified individuals. The test specification should include all acceptable operating limits and the various operating conditions (temperature, line conditions, and so forth), under which the system must operate. No one of these operations has been performed on the design here presented.

Even if doing things yourself is great (I mean: GREAT!), please consider to adopt an already built, tested and certificated switching power supply from a trustable manufacturer. You (and your modules) are valued!

Power Bus Bar

While designing the Linear Power Supply PCB, I forced myself to keep dimensions within the 10 cm x10 cm limit in order to keep board costs low. When I realized that only 4 power connectors could be accomodated in the remaining space (3 in the final design, see next step), I decided to draw an auxiliary board: a power BUS bar.

This is a very simple design, made of 4x10 pins and 4x16 pins IDC connectors, all respecting the eurorack pinout.

The BUS board also has solder pads for each required voltage (+12V, -12V, +5V, GND), so that you can use it with any external power supply of your choice.

Look at my shared projects for the poer Bus Bar files!

Design Optimizations

As always, despite the time you can spend over the PCB design before triggering your order, some rethinking is inevitable (at least for me!).

In this case, the Power Supply prototype pictured misses a screw terminal to adeguately bring power to the Power Bus Bar. I have anyway soldered one on an IDC slot (IDC spacing and Eurorack pinout combined allows this), but is not nice looking obviously. The PCB version I shared in one of the previous steps accomodate a 3 pins screw terminal and 3 2x5 pins IDC connector. 

I also increased the distance between the on-board transformer and other components, in particular those big 4700uF capacitors. During operation the transformer could go hot, so even if the distance has been increased in the final design, use 105°C rated electrolitic capacitors.

Some improvement also affected the Power Bus Bar: even if the layout is essentially unmodified, there was room for three LEDs and three corresponding current limiting resistors. This gives the user a visual feedback of the presence of the three DC voltages (+5V, +12V and -12V), a welcome addition especially if the Power Supply doesn't have any.

Despite the changes, don't worry: both PCB files I have shared in previous steps already adopt all this improvements ;)

Please notice that I also changed the name of the Power Supply board from "ALR1000" to "ALR350" because that "1000" (referring to the current carry capability of the plain transformer) was misleading.