Build this inexpensive, rack-mounted power supply unit, capable of delivering 12 volts at up to one amp and suitable for powering many rackmounting projects, including the Syndrom. Design and description by Paul White.

Most audio signal processors make extensive use of operational amplifiers which, thankfully, have fairly standard power supply requirements. The RackPack provides a split rail, regulated supply of ±12 volts and features several power outlet sockets so that more than one processor may be powered simultaneously, the maximum available current being one amp. The Syndrom project, although specified as requiring a nine volt supply, will run happily from the RackPack, and it should be possible to run up to six Syndroms from a single unit.

Circuitry

The output from the transformer is full wave rectified and smoothed, giving an unregulated voltage of roughly ±18 volts (12x√2) which provides sufficient headroom for the regulators to operate reliably. A toroidal transformer was chosen for its low stray electromagnetic field but, as it takes a high switch on current, an anti-surge mains fuse is required.

The regulation is performed by the industry standard 78 and 79 series monolithic regulator ICs and, as can be seen from the circuit diagram, this facilitates a simple design which enables the component count to be kept to an absolute minimum. Two LEDs are incorporated which monitor the output voltage so that an overloaded or shorted power rail is instantly evident.

Construction

If the rack case design is to be used, drill the metalwork first (Figure 2) and then paint the front and rear panels. The dimensions given are those used on the prototype, but positioning of switches and sockets may be altered if required and more than four output sockets may be fitted if needed. It is also possible to fit two PCBs into the box and in this case, a larger transformer will be required (see parts list), the transformer is fitted by means of a central bolt and a set of mounting washers is provided as part of the mounting kit (Figure 3). Wire up the DIN sockets as shown in Figure 1 and insulate all exposed mains connections with rubber sleeving or PVC tape.

The PCB presents no assembly problems (Figure 4), but as always, check the polarity of electrolytic capacitors and semiconductors before applying power. As the regulators are capable of supplying up to one amp, heatsinks must be fitted to RG1 and RG2. It's necessary to mount the PCB on spacers, and short lengths of ballpoint pen serve well in this capacity, four Ba bolts being used to secure the PCB to the case. Internally, the wiring layout is not critical and the photograph should give you a general idea.

Testing

No sandbags are required for this one. Simply turn on the mains with a long stick and see if the LEDs come on. If not, put the fuse in and try again! Check that the polarity of the output sockets matches the device(s) that you're going to run from the RackPack; even a momentary reversal of polarity can have serious consequences.

There should be few or no problems encountered in building the RackPack, and it should work as a useful power supply for some years to come.