Every modular synth needs a reliable, stable power supply and mine is no exception. I’ve recently been building my first home-made modular synth case; the design and integration of the power supply unit went hand-in-hand with the case build.
I chose Ray Wilson’s Adjustable LM317/LM337 1.5A Supply as the basic design for my supply, as the parts were readily available and very cost-effective. To this design I added a couple of modifications of my own (entirely outside of the core circuit design):
- Indicator LEDs for +12, -12 and +5 volt supplies
- A supply to my MIDI and 5V distribution board
- A single toggle switch to interrupt the +12 and -12 volt outputs
- An isolator switch and fuse on the mains voltage input
- A ground loop breaker
- An optional route for the +12 and -12 volt outputs, controlled by a toggle switch, that interrupts the output with 2 pairs of banana jacks; this allows me to measure total current consumption by my modules by inserting a multi-meter into the supply line
The PSU is built on stripboard, which is then mounted onto a “sled” made from 2 bits of MakerBeam. The toroidal transformer (which gets the input voltage down from mains 240V) is bolted to a piece of aluminium strip which is then mounted on the same sled. Connections to the mains inlet/switch/fuse assembly are via a screw-terminal block. The sled (which is physically connected to the rest of the metal frame of the case) connects mains earth and circuit ground. The MIDI/5V board is mounted on the same sled, and the 5 power distribution boards are mounted on more MakerBeam that runs the full width of the rear of the case.
The PSU sled is mounted vertically inside the bottom-right sidewall of the case. This keeps all of the power connections nice and short (everything is, essentially, contained within the last 1U of volume on the bottom row of my case).
Calibration was really easy – just a case of hooking up a meter and adjusting the trim pots until +12 and -12 volts were read at the outputs. I’m really happy to say that, through being extremely cautious and double-double-checking everything at every stage of the build, this supply worked perfectly and safely first time (which is good because, as can’t be stressed enough, mains voltage is really, really dangerous!)
The front panel was made via my usual approach and holds the main output switch (under a totally unnecessary but nevertheless very cool “missile switch” cover), indicator LEDs, “test” switch and banana-plug sockets. I initially went for blue LEDs, but switched to green as the blue ones seemed a bit too glaring.
Everything you need to build the basic circuit can be found on the Music From Outer Space website.
My schematics contain Ray’s original circuit diagram, plus all of my modifications. Please note that just because my build went well and safely, that doesn’t guarantee that yours will! Mains voltages can kill and you should exercise all necessary caution if embarking on this or a similar project!